A marked sexual dimorphism exists in healthy individuals in the pattern of blunted neuroendocrine and metabolic responses following antecedent stress. It is unknown whether significant sex-related counterregulatory differences occur during prolonged moderate exercise after antecedent hypoglycemia in type 1 diabetes mellitus (T1DM). Fourteen patients with T1DM (7 women and 7 men) were studied during 90 min of euglycemic exercise at 50% maximal O2 consumption after two 2-h episodes of previous-day euglycemia (5.0 mmol/l) or hypoglycemia of 2.9 mmol/l. Men and women were matched for age, glycemic control, duration of diabetes, and exercise fitness and had no history or evidence of autonomic neuropathy. Exercise was performed during constant “basal” intravenous infusion of regular insulin (1 U/h) and a 20% dextrose infusion, as needed to maintain euglycemia. Plasma glucose and insulin levels were equivalent in men and women during all exercise and glucose clamp studies. Antecedent hypoglycemia produced a relatively greater (P < 0.05) reduction of glucagon, epinephrine, norepinephrine, growth hormone, and metabolic (glucose kinetics) responses in men compared with women during next-day exercise. After antecedent hypoglycemia, endogenous glucose production (EGP) was significantly reduced in men only, paralleling a reduction in the glucagon-to-insulin ratio and catecholamine responses. In conclusion, a marked sexual dimorphism exists in a wide spectrum of blunted counterregulatory responses to exercise in T1DM after prior hypoglycemia. Key neuroendocrine (glucagon, catecholamines) and metabolic (EGP) homeostatic responses were better preserved during exercise in T1DM women after antecedent hypoglycemia. Preserved counterregulatory responses during exercise in T1DM women may confer greater protection against hypoglycemia than in men with T1DM.
- sexual dimorphism
- glucose clamp
the human body reacts to physical stress by eliciting a series of metabolic, neuroendocrine, and autonomic nervous system homeostatic mechanisms, defined as counterregulatory responses. During stresses such as hypoglycemia and exercise, these responses are qualitatively similar and have the common goal to increase glucose availability to restore euglycemia (during hypoglycemia) or to prevent hypoglycemia (during exercise).
Counterregulatory failure during physical stress may result in increased susceptibility to hypoglycemia, an issue particularly relevant for patients with type 1 diabetes mellitus (T1DM), in which a high incidence of hypoglycemia is a common complication of therapeutic regimens. Certain aspects of counterregulatory failure may be irreversible (such as the loss of the glucagon responses to hypoglycemia after a few years of T1DM) (14) or may be induced acutely and reversibly (i.e., epinephrine) by an antecedent episode of stress (2).
Previous studies from our laboratory have demonstrated that antecedent hypoglycemia and exercise reciprocally blunt subsequent counterregulatory responses. Our recent studies reported that, after antecedent hypoglycemia, counterregulatory responses to next-day exercise were significantly blunted in both healthy subjects (8) and patients with T1DM (13). In a separate study performed in healthy subjects, exercise was used as the antecedent stimulus, and hypoglycemia was performed on the following day. Again, a widespread blunting of neuroendocrine and metabolic responses to second-day hypoglycemia was observed (11). A more recent study from McGregor et al. (19) also reported that antecedent exercise can blunt some (epinephrine, growth hormone) but not all neuroendocrine responses during subsequent hypoglycemia in healthy individuals.
In healthy subjects and T1DM, there is a clear sexually dimorphic pattern of neuroendocrine and metabolic responses to differing stimuli such as prolonged hypoglycemia or physical exercise (7, 9). During these conditions, most counterregulatory responses are enhanced in men compared with women (7, 9). Despite this, data from the Diabetes Control and Complications Trial demonstrate that intensively treated women with T1DM have in fact a lowered risk of severe hypoglycemia compared with men (21). Our working hypothesis to answer this apparent clinical paradox is that T1DM women are more resistant to the blunting effects of antecedent hypoglycemia on subsequent counterregulatory responses. Previous studies in healthy humans provide some support for this premise. Episodes of antecedent hypoglycemia or exercise (12) have been demonstrated to blunt counterregulatory responses to subsequent hypoglycemia by a significantly greater extent in healthy men compared with healthy women. The clinical relevance of these findings in healthy subjects, however, has been questioned in a recent editorial (6). This is because, to date, it is unknown whether women with T1DM are also resistant to the blunting effects of antecedent hypoglycemia on counterregulatory responses to subsequent stress.
Therefore, to test the hypothesis that antecedent hypoglycemia may result in less blunting of counterregulatory responses during subsequent exercise in T1DM women compared with T1DM men, 14 patients with T1DM (7 female/7 male) were studied during prolonged, moderate exercise after either previous-day euglycemia or two 2-h periods of hypoglycemia of ∼2.9 mmol/l.
RESEARCH DESIGN AND METHODS
We studied 14 patients (Table 1) with T1DM, seven males and seven females aged 28 ± 2 yr, with a mean body mass index of 25 ± 1 kg/m2 and glycosylated hemoglobin, or Hb A1c, of 8.1 ± 0.5% (normal range 4.0–6.5%). Patients had been diagnosed with T1DM 13 ± 2 yr before recruitment and had no evidence of tissue complications of the disease (retinopathy, renal impairment, hypertension) or of diabetic autonomic neuropathy. Each subject had normal blood count, plasma electrolytes, and liver function. All gave written informed consent. Studies were approved by the Vanderbilt University Human Subjects Institutional Review Board. Some of the subjects in this report also participated in a larger study investigating the effects of prior hypoglycemia on subsequent counterregulatory responses during exercise (13).
At least 2 wk before the initial study, patients' body composition was assessed by skinfold caliper technique and whole body plethysmography (Bod-Pod, Life Measurement, Concord, CA). At this time, patients also performed an incremental work test on a stationary cycle ergometer to determine maximal O2 consumption (V̇o2 max) and anaerobic threshold (AT). Air flow and O2 and CO2 concentrations in inspired and expired air were measured by a computerized open-circuit indirect calorimetry cart (Medical Graphics Cardio2 cycle) with a mouthpiece and nose-clip system. The AT was determined by the V-slope method (3). The AT determined by gas exchange corresponds to the onset of an increased lactate-to-pyruvate ratio in blood and indicates the level of exercise above which anaerobic mechanisms supplement aerobic energy production (25). At workloads below the AT, exercise can be continued for a prolonged period, whereas above the AT, fatigue will occur considerably faster (24). The experimental work rate was established by calculating 80% AT. The AT was detected at 60 ± 3% of V̇o2 max, and 80% AT corresponded to 48 ± 2% of the subjects' V̇o2 max. This workload was chosen because it is close enough to the AT to produce a physically challenging stress (i.e., large experimental signal) but is sustainable for a prolonged period of time. Subjects studied ranged from sedentary to regularly exercising, although not actively participating in competitive sports. Men and women were matched for physical fitness, number of subjects regularly exercising, and baseline levels of prior exercise. The mean V̇o2 max was similar for men and women. The mean V̇o2 max for the group was 31 ± 2 ml·kg−1·min−1 (range 21–43 ml·kg−1·min−1).
Each patient was then studied during two separate visits, each lasting two consecutive days and including two overnight stays at our Clinical Research Center (CRC). During day 1 of one of the two visits patients underwent two 2-h morning and afternoon hypoglycemic clamps (Ante Hypo), and during day 1 of the other visit they were maintained euglycemic (Ante Eugly). During day 2 of both visits, all patients performed a similar 90-min exercise protocol under euglycemic conditions. The sequence of Ante Hypo and Ante Eugly studies was randomized, and ≥6 wk were allowed to elapse between the two visits.
Patients were asked to avoid hypoglycemia during the 7 days preceding each visit. Patients checked their blood glucose four times/day preprandially, before bed, and at any times that they felt a low blood glucose. Plasma glucose values were reported to the investigators so that adjustments of insulin doses and increases in carbohydrate intake could be implemented to avoid hypoglycemia. Detection of any value <70 mg/dl resulted in rescheduling of the study. Patients were also asked to avoid any exercise and consume their usual weight-maintaining diet for 3 days before each study. Each subject was admitted to the Vanderbilt CRC at 4:00 PM on the afternoon before an experiment. Upon admission, patients were asked to discontinue their usual insulin therapy, and two intravenous cannulas were inserted under 1% lidocaine local anesthesia. One cannula was placed in a retrograde fashion into a vein on the back of the left hand. This hand was placed in a heated box (55–60°C) so that arterialized blood could be obtained (23). The other cannula was placed in the contralateral arm so that insulin and 20% glucose (when needed) could be infused via a variable-rate volumetric infusion pump (IMED, San Diego, CA). An insulin infusion was immediately started at a basal rate. Patients then consumed an evening meal and a 7:30 PM snack and were requested not to ingest any food after 10:00 PM. The insulin infusion rate was increased during meal consumption. Through the night, blood glucose was measured every 30 min, and the insulin infusion rate was constantly adjusted to maintain glycemic levels of 4.4–6.6 mmol/l.
Day 1 procedures.
Day 1 procedures started at 8:00 AM after a 10-h overnight fast and lasted 480 min, divided into an equilibration period (0–120 min), a morning hyperinsulinemic clamp period (120–240 min), a rest period (240–360 min), and an afternoon hyperinsulinemic clamp period (360–480 min). At t = 120 min, in all studies a primed continuous infusion of insulin (9 pmol·kg−1·min−1) was started (20). In prior hypoglycemia studies, plasma glucose was allowed to fall over a 30-min period to a target hypoglycemic plateau of ∼2.9 mM. Plasma glucose was measured every 5 min and maintained at the desired level via a variable-rate infusion of 20% dextrose (2). In prior euglycemia studies, plasma glucose was held constant at ≃5.0 mM by a similar technique (10). At t = 240 min, the insulin infusion was decreased to morning basal rate; euglycemia was restored in prior hypoglycemia studies and maintained in the prior euglycemia studies. At t = 360 min, euglycemic or hypoglycemic clamps identical to those performed in the morning were repeated. At t = 480 min, the insulin infusion was decreased to morning basal rate, euglycemia was restored in prior hypoglycemia studies, and all patients were allowed to consume a standardized meal. Afternoon and night procedures were then identical to those of admission night.
Day 2 procedures.
Day 2 procedures started at 8:00 AM after a 10-h overnight fast and lasted 210 min (t = −120 min to 90 min), divided into an equilibration period (−120 to −30 min), a basal period (−30 to 0 min), and an exercise period (0 to 90 min). A primed (18 μCi) constant infusion (0.18 μCi/min) of [3-3H]glucose was started at t = −120 min and continued throughout the experiment.
Exercise consisted of 90 min of continuous pedaling (at 60–70 rpm) on an upright cycle ergometer (Medical Graphics, Yorba Linda, CA) at 80% of the individual's AT (∼50% V̇o2 max). Plasma glucose was measured every 5 min and maintained equivalent to baseline levels throughout the study via variable-rate infusion of 20% dextrose. In an attempt to reproduce the drop in insulin levels that physiologically occurs with exercise of this intensity, the basal insulin infusion rate was decreased by 40% after the first 30 min of exercise, provided that the resulting reduced rate was ≥1 U/h. In cases in which a 40% reduction of the basal rate would have resulted in an insulin infusion rate of <1 U/h, a minimum rate of 1 U/h was maintained. Potassium chloride was also infused (5 mmol/h) during exercise. After completion of the exercise protocol, patients consumed a meal and were discharged.
Rates of glucose appearance (Ra), endogenous glucose production (EGP), and glucose utilization were calculated according to the methods of Wall et al. (22). EGP was calculated by determining the total rate of Ra (which comprises both EGP and any exogenous glucose infused to maintain euglycemia) and subtracting from it the amount of exogenous glucose infused. It is now recognized that this approach is not fully quantitative, as underestimates of total Ra and glucose disposal (Rd) can be obtained. This underestimate can be largely overcome by use of an HPLC-purified tracer and by measurements taken under steady-state conditions (i.e., constant specific activity). Infusion rates of [3-3H]glucose were tripled during the first 30 min of exercise to minimize changes in glucose specific activity (26). In this study, only data recorded at baseline and during the last 30 min of exercise, when a steady state existed, were used in calculating glucose turnover.
The collection and processing of blood samples have been described elsewhere (5). Plasma glucose concentrations were measured in triplicate by the glucose oxidase method with a glucose analyzer (Beckman, Fullerton, CA). Glucagon was measured according to a modification of the method of Aguilar-Parada et al. (1) with an interassay coefficient of variation (CV) of 12%. Free insulin was measured as previously described (28), with an interassay CV of 9%. Catecholamines were determined by HPLC (4), with an interassay CV of 12% for epinephrine and 8% for norepinephrine. We made two modifications to the procedure for catecholamine determination: 1) we used a five-point rather than a one-point standard calibration curve; and 2) we spiked the initial and final samples of plasma with known amounts of epinephrine and norepinephrine so that accurate identification of the relevant respective catecholamine peaks could be made. Cortisol was assayed using the Clinical Assays Gamma Coat Radioimmunoassay (RIA) kit with an interassay CV of 6%. Growth hormone was determined by RIA (17) with a CV of 8.6%. Pancreatic polypeptide was measured by RIA with the method of Hagopian et al. (15), with an interassay CV of 8%. Lactate, glycerol, alanine, and β-hydroxybutyrate were measured in deproteinized whole blood by the method of Lloyd et al. (18). Free fatty acids (FFA) were measured using the WAKO kit adapted for use on a centrifugal analyzer (16).
On day 2, blood samples for glucose flux were taken every 10 min throughout the basal period and every 15 min during exercise. Blood for hormones and intermediary metabolites was drawn twice during the basal period and every 15 min during the exercise period. Cardiovascular parameters (pulse and systolic and diastolic arterial pressures) were measured every 10 min from t = −30 min to t = 90 min. Gas exchange measurements were performed during the basal period and during the final 10 min of exercise.
HPLC-purified [3-3H]glucose (New England Nuclear, Boston, MA) was used as the glucose tracer (11.5 mCi/mM). Human regular insulin was purchased from Eli Lilly (Indianapolis, IN). The insulin infusion solution was prepared with normal saline and contained 3% (vol/vol) of the subject's own plasma.
Data are expressed as means ± SE, unless otherwise stated, and were analyzed by use of standard, parametric, two-way analysis of variance (ANOVA) with repeated-measures design. This was coupled with Duncan's post hoc test to delineate at which time points statistical significance was reached. A value of P < 0.05 indicated significant difference.
Day 1: plasma glucose and insulin levels.
Glycemic profiles for the four experimental groups are shown in Fig. 1. Basal plasma glucose levels were comparable in all subjects in both the morning and the afternoon. During hyperinsulinemic clamps, glucose levels remained at baseline in the Ante Eugly subjects and decreased similarly in the Ante Hypo groups (women: morning, 2.9 ± 0.01 mM; afternoon, 2.9 ± 0.01; men: morning, 2.9 ± 0.01; afternoon, 2.9 ± 0.01). Basal plasma insulin levels were similar in all subjects in both the morning (women: Ante Eugly, 60 ± 12 pM; women: Ante Hypo, 60 ± 12 pM; men: Ante Eugly, 55 ± 6 pM; men: Ante Hypo, 60 ± 12 pM) and the afternoon (women: Ante Eugly, 84 ± 18 pM; women: Ante Hypo, 72 ± 18 pM; men: Ante Eugly, 78 ± 24 pM; men: Ante Hypo, 72 ± 18 pM). During hyperinsulinemic clamps, insulin levels increased similarly in all groups of subjects in both the morning (women: Ante Eugly, 570 ± 102 pM; women: Ante Hypo, 540 ± 60 pM; men: Ante Eugly, 624 ± 66 pM; men: Ante Hypo, 546 ± 42 pM) and the afternoon (women: Ante Eugly, 570 ± 114 pM; women: Ante Hypo, 552 ± 42 pM; men: Ante Eugly, 612 ± 60 pM; men: Ante Hypo, 618 ± 72 pM).
Day 2: insulin, glucose, and counterregulatory hormone levels.
Plasma glucose and insulin (Fig. 1) levels were similar in the four experimental groups before exercise was started, and they remained at basal levels throughout exercise in all subjects.
In men, plasma glucagon (Fig. 2, Table 2) increased above basal levels during exercise by 10 ± 2 ng/l after Ante Eugly, whereas after Ante Hypo, glucagon slightly decreased during exercise (−1 ± 2 ng/l). In women, on the other hand, glucagon increased during exercise by 9 ± 4 ng/l after Ante Eugly and by 4 ± 2 ng/l after Ante Hypo. Therefore, exposure to antecedent hypoglycemia reduced the glucagon response to exercise by 11 ± 2 ng/l in men but only by 5 ± 2 ng/l in women (P < 0.05 vs. men).
After Ante Eugly, epinephrine increased in men by 639 ± 131 pM during exercise; this increase was reduced to 251 ± 49 pM after Ante Hypo (Fig. 2, Table 2). In women, epinephrine increased by 360 ± 136 pM after Ante Eugly and by 186 ± 60 pM after Ante Hypo. Ante Hypo therefore blunted the epinephrine response to exercise by 393 ± 49 pM in men but only by 175 ± 60 pM in women (P < 0.01). The norepinephrine response to exercise followed a somewhat similar pattern. In men, the exercise-induced increase in norepinephrine was 3.3 ± 0.6 nM after Ante Eugly and 2.2 ± 0.4 nM after Ante Hypo. In women, norepinephrine increased similarly during exercise in both experimental conditions (by 2.0 ± 0.7 nM after Ante Eugly and by 2.0 ± 1.2 nM after Ante Hypo). Ante Hypo therefore induced a significant (P < 0.05) blunting of the norepinephrine response to exercise in men but not in women.
In men, the exercise-induced increase in growth hormone was 22 ± 11 ng/ml after Ante Eugly and was reduced to 15 ± 6 ng/ml after Ante Hypo (Fig. 3, Table 2). In women, growth hormone increased during exercise by 10 ± 4 ng/ml after Ante Eugly, and this response was moderately increased by Ante Hypo (14 ± 5 ng/ml). The effect of Ante Hypo on the growth hormone response to exercise (a reduction of 7 ± 6 ng/ml in men and an increase of 5 ± 4 μg/l in women) was significantly different between sexes (P < 0.05).
After Ante Eugly, the exercise-induced increase in cortisol was similar in men (303 ± 83 nM) and women (248 ± 83 nM). After Ante Hypo, the cortisol responses were similarly blunted in men (166 ± 110 nM) and women (110 ± 80 nM; Fig. 3). Pancreatic polypeptide responses to exercise were also similar between sexes after Ante Eugly (27 ± 11 pM in women and 22 ± 9 pM in men). The pancreatic polypeptide response to exercise was not affected by Ante Hypo in either sex.
Day 2: glucose kinetics and gas exchange measurements.
After day 1 euglycemia, the rate of exogenous glucose infusion (Fig. 4) required to maintain euglycemia during day 2 exercise in men was 9 ± 4 μmol·kg−1·min−1 and increased to 22 ± 3 μmol·kg−1·min−1 after Ante Hypo (P < 0.05 vs. Ante Eugly). In women, rates of exogenous glucose infusion during the last 30 min were similar after Ante Eugly and after Ante Hypo (12 ± 3 and 16 ± 3 μmol·kg−1·min−1), respectively.
Glucose specific activity was stable (CV <4.0%) at the start and at the final 30 min of the day 2 exercise studies (Table 3). Basal rates of EGP were similar in all experimental conditions (Fig. 4). After Ante Eugly, EGP during exercise increased to 29 ± 6 μmol·kg−1·min−1 in men and to 25 ± 3 μmol·kg−1·min−1 in women. After Ante Hypo, the EGP response was reduced to 17 ± 9 μmol·kg−1·m−1 in men (P < 0.05 vs. Ante Eugly), whereas it remained unchanged in women (23 ± 6 μmol·kg−1·min−1). Glucose Rd during the last 30 min of exercise was not altered in men between experimental conditions (Ante Eugly, 38 ± 4 and Ante Hypo, 39 ± 4 μmol·kg−1·min−1). Glucose Rd was similar in women after Ante Hypo (38 ± 7 μmol·kg−1·min−1) and after Ante Eugly (31 ± 5 μmol·kg−1·min−1).
Basal rates of RER, lipid oxidation, and carbohydrate oxidation were similar in all experimental groups (Table 4). Rates of lipid and carbohydrate oxidation during exercise in women were unaffected by day 1 hypoglycemia or euglycemia. Conversely, rates of lipid oxidation in men were reduced during the final 15 min of exercise after Ante Hypo (0.9 ± 0.3 mg·kg−1·min−1) compared with Ante Eugly (1.6 ± 0.1 mg·kg−1·min−1).
Day 2: Intermediary metabolism.
Blood lactate levels were similar at baseline during all studies (Table 5). The lactate response was significantly reduced by Ante Hypo, and the magnitude of this reduction was similar between sexes. FFA and glycerol basal levels were similar in men and women after both Ante Eugly and Ante Hypo (Table 5). There were no differences in the FFA responses to exercise in men and women after either Ante Eugly or Ante Hypo. No differences in the circulating levels of alanine, glycerol, or the ketone body β-hydroxybutyrate were measured between men and women at baseline or during exercise after Ante Eugly or Ante Hypo.
Day 2: Cardiovascular parameters.
Systolic blood pressure values were higher in men, compared with women, throughout the study (Table 6). Exercise-induced changes in heart rate and systolic, diastolic, and mean arterial pressures, however, were similar between sexes after both Ante Eugly and Ante Hypo.
This study was designed to determine whether neuroendocrine and metabolic responses to moderate, prolonged exercise are altered by a sexually dimorphic pattern after antecedent hypoglycemia in patients with T1DM. Our findings show that prior prolonged hypoglycemia (two 2-h bouts, morning and afternoon, at ∼2.9 mmol/l) reduce glucagon, epinephrine, norepinephrine, growth hormone, and glucose kinetic responses to next day exercise more significantly in T1DM men compared with T1DM women.
Experimental conditions were carefully controlled during our 2-day studies. Insulin and glucose levels, relative exercise intensity, and degree of individual physical fitness were equated in men and women. During day 1 and overnight stays, hypoglycemia was carefully avoided (except for the designated hypoglycemic clamp periods) by constant adjustments of exogenous insulin and/or glucose. Additionally, euglycemia was strictly maintained during day 2 exercise. During exercise, hyperglycemia inhibits neuroendocrine responses, whereas hypoglycemia would have induced counterregulatory responses independent of those induced by exercise per se. Furthermore, insulin levels were maintained at basal levels during exercise in all subjects and were identical between sexes. Circulating levels of insulin physiologically decrease during exercise, and the magnitude of this insulin drop may be affected by antecedent stress (8). Preventing insulin differences between experimental groups eliminated an important confounding factor in metabolic (glucose kinetics, fat metabolism) data interpretation.
In nondiabetic subjects, the hormonal responses that regulate nutrient flux during submaximal exercise are arranged in hierarchical order, with glucagon, insulin, and epinephrine being the most important. In T1DM, the importance of the glucagon response to exercise is further underscored by the fact that these patients are unable to secrete glucagon in response to insulin-induced hypoglycemia after a disease duration of only a few years (14). In T1DM, secretion of glucagon during exercise is preserved, indicating the stimulus-specific nature of the pancreatic α-cell defect. Indeed, in the present study, glucagon increased similarly during exercise in male and female patients after prior euglycemia. After day 1 hypoglycemia, however, the glucagon response to exercise was only moderately attenuated in women, whereas it was completely abolished in men.
Epinephrine levels increased similarly in both sexes during exercise after antecedent hypoglycemia. This apparent similarity, however, is in open contrast with the full, nonattenuated response to exercise observed after prior euglycemia, when epinephrine rose significantly more in men than in women. Again, antecedent hypoglycemia exerted a significantly greater blunting effect on epinephrine responses in men than in women. Interestingly, a similar pattern of greater blunting in male patients also occurred with the norepinephrine response to exercise. This parallel reduction in catecholamine responses is consistent with a reduced sympathetic drive in men after prior hypoglycemia, a finding consistent with previous observations following antecedent stress (9, 12).
Similar to the response pattern of catecholamine and glucagon to day 2 exercise was the finding that growth hormone responses were more blunted in men compared with women. The cortisol and pancreatic polypeptide responses, on the other hand, were not affected by sex (the cortisol response was similarly blunted in men and women, and the pancreatic polypeptide response was unaffected by antecedent hypoglycemia in either sex). These present results in T1DM are similar to the pattern of neuroendocrine responses occurring after antecedent stress in nondiabetic individuals (5, 10). Pancreatic polypeptide responses reported in this study after antecedent euglycemia (i.e., without the blunting effect of prior hypoglycemia) were not different between sexes and were lower than in nondiabetic subjects exercising under similar experimental conditions (7). Pancreatic polypeptide is considered a marker of vagal efferent input to the pancreas. It has been previously shown that diabetes per se may reduce the pancreatic polypeptide response to stress (hypoglycemia) (27). It is therefore conceivable that prolonged diabetes may also induce a partial inability to appropriately increase pancreatic polypeptide during exercise. In our patients, this may have impaired detection of an effect of sex or of antecedent hypoglycemia on the day 2 pancreatic polypeptide responses.
In this present study, differences in neuroendocrine responses after Ante Eugly and Ante Hypo have been presented as absolute values. This convention was used because we believe it is the level of a hormone that elicits a specific physiological response. However, an alternative method of analyzing the data could include computing percentage changes from baseline of the neuroendocrine response during exercise after the differing day 1 challenges. In general, both methods produce similar results. Thus interpretation of the glucagon, norepinephrine, growth hormone, cortisol, and pancreatic polypeptide responses is unaffected by use of either method. The responses of epinephrine and glucagon are more complex and deserve further discussion. In women, epinephrine levels increased by 360 pM after Ante Eugly but only ∼186 pM after Ante Hypo. Thus there was a relative blunting of ∼48% of epinephrine during exercise after Ante Hypo in T1DM women. In men, epinephrine increased by ∼640 pM after Ante Eugly but only ∼360 pM after Ante Hypo. This represented a relative blunting of ∼61% of epinephrine levels after Ante Hypo in T1DM men. Thus, on one hand, the relative differences in percentage blunting of epinephrine after Ante Hypo in the T1DM men and women was relatively small at ∼13%. However, the relative reduction in absolute levels of epinephrine in women after Ante Hypo was only ∼174 pM compared with the much larger ∼389 pM in T1DM men.
Glucagon levels in women increased by ∼9 ng/l after Ante Eugly but only ∼5 ng/l after Ante Hypo. This represents a 43% relative blunting of plasma glucagon during exercise after Ante Hypo in women. In the T1DM men, glucagon increased by ∼10 ng/l after Ante Eugly but did not increase at all after Ante Hypo. This represents a relative blunting of 100% after Ante Hypo compared with Ante Eugly. At first glance, it may appear that the changes in plasma glucagon values in this study are relatively small. However, it should be pointed out that the peripheral plasma values of glucagon during exercise do not accurately reflect the physiologically relevant portal vein levels of the hormone. Thus, due to hepatic extraction, portal vein levels of glucagon during exercise are approximately threefold higher than peripheral values (23). Therefore, the relatively small changes in peripheral values of glucagon would be greatly amplified at the hepatic sinusoid and produce meaningful physiological effects.
A significant sexual dimorphism was also present in the metabolic responses to exercise. In T1DM men, the rate of endogenous glucose production during exercise was ∼11 μmol·kg−1·min−1 lower following prior hypoglycemia compared with prior euglycemia. This means that prior hypoglycemia obliterated two-thirds of the usual exercise-induced increase in EGP in men, whereas in women this difference was <15% and nonsignificant. As a result, the rates of exogenous glucose infusion required to maintain euglycemia during exercise, which had been similar between sexes after prior euglycemia, became significantly greater in men after hypoglycemia. Consistent with, and a potential mechanism for, the increased glucose infusion rates was the finding that fat oxidation was blunted during exercise after day 1 hypoglycemia in men. On the other hand, fat oxidation during exercise in women was unaffected by day 1 hypoglycemia. Because insulin levels were equivalent during exercise in both groups, the pronounced metabolic changes observed in men after day 1 hypoglycemia are most likely the result of the blunted glucagon and sympathetic nervous system responses.
The mechanism(s) responsible for the sexual dimorphism in neuroendocrine responses during exercise after hypoglycemia in T1DM is unknown. Possibilities include “inherent” differences in men and women (such as sex hormones), differences in body weight, or differential neuroendocrine responses to the antecedent (day 1) hypoglycemia.
In summary, two antecedent episodes of hypoglycemia of 2.9 mmol/l induced a clear sexually dimorphic pattern of blunted neuroendocrine and metabolic counterregulatory responses to next-day exercise in patients with T1DM. Glucagon, catecholamines, growth hormone, lipid oxidation, and glucose kinetic responses were more blunted in men than in women.
We conclude that women with T1DM are more resistant to the blunting effects of antecedent hypoglycemia on neuroendocrine and metabolic responses to subsequent moderate exercise compared with men. These results may also help to explain the apparent clinical paradox of why intensively treated women with T1DM have a reduced risk of severe hypoglycemia despite inherently reduced neuroendocrine counterregulatory responses compared with men.
This work is supported by grants from the Juvenile Diabetes Foundation International (JDFI), National Institute of Diabetes and Digestive and Kidney Diseases (R01-DK-45369), the Diabetes Research and Training Center (5P60-AM-20593), the Clinical Research Center (M01-RR-00095), and the Veterans Affairs/JDFI Diabetes Research Center. P. Galassetti was supported by a JDFI research fellowship grant.
We thank Eric Allen, Angelina Penalosa, and Wanda Snead for expert technical assistance. We also appreciate the skill and help of the nurses of Vanderbilt General Clinical Research Center in the performance of the studies included in this report.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
- Copyright © 2004 by American Physiological Society