Peptide YY (PYY)3–36 has been shown to produce dramatic reductions in energy intake (EI), but no human data exist regarding energy expenditure (EE), glucose and fat metabolism. Nothing is known regarding PYY1-36. To compare effects of PYY1–36 and PYY3–36 on appetite, EI, EE, insulin, glucose and free fatty acids (FFA) concentrations, 12 lean and 12 obese males participated in a blinded, randomized, crossover study with 90-min infusions of saline, 0.8 pmol·kg−1·min−1 PYY1–36 and PYY3–36. Only four participants completed PYY3–36 infusions because of nausea. Subsequently, six lean and eight obese participants completed 0.2 pmol·kg−1·min−1 PYY3–36 and 1.6 pmol·kg−1·min−1 PYY1–36 infusions. PYY3–36 at 0.8 pmol·kg−1·min−1 produced reduced EI, lower ratings of well-being, increases in FFA, postprandial glucose (only 0.8 pmol·kg−1·min−1 PYY3–36) and insulin concentrations, as well as heart rate and EE (only 0.8 pmol·kg−1·min−1 PYY3–36). PYY1–36 at 1.6 pmol·kg−1·min−1 produced increased heart rate and postprandial insulin response. Ratings of appetite were opposite with infusions of 0.8 and 1.6 pmol·kg−1·min−1 PYY1–36 and seemed to depend on subjects being lean or obese. PYY3–36 caused increased thermogenesis, lipolysis, postprandial insulin and glucose responses, suggestive of increased sympathoadrenal activity. PYY1–36 had no effect on EI and no clear effects on appetite but resulted in increased heart rate and postprandial insulin response. However, highest tolerable dose of PYY1–36 was probably not reached in the present study.
- Peptide YY
- free fatty acids
- insulin sensitivity
- arcuate nucleus
- neuropeptide Y receptors
the imbalance between energy intake (EI) and energy expenditure (EE) that produces the obese state may partly be a consequence of an inadequate appetite regulation. Hence, scientific interest in recent years has concentrated on both the secretion and function of different gastrointestinal hormones, and on the appetite regulatory centers of the brain, including the arcuate nucleus which is considered a key brain area (22).
Peptide YY (PYY) is a 36-amino acid peptide released postprandially from the endocrine L cells in the distal gastrointestinal tract in proportion to the calorie and fat content of the ingested meal (1, 8, 18). PYY is part of the neuropeptide Y (NPY) protein family and exists in human blood in two forms: the intact PYY1–36, and PYY3–36, in which the NH2-terminal Tyr-Pro dipeptide has been removed by dipeptidyl aminopeptidase IV (DPP IV) cleavage (12, 13). PYY1–36 binds to and activates the Y1, Y2, and Y5 NPY receptor subtypes, whereas PYY3–36 preferentially binds to the inhibitory presynaptic Y2 receptor, which is highly expressed in NPY neurons in the appetite regulatory center in the arcuate nucleus (27).
Infusion of PYY3–36 at doses producing supraphysiological plasma concentrations has been shown to result in a dose-dependent decrease in subsequent food intake (8, 18). The highest suppression of food intake has been demonstrated with doses of 0.8 pmol·kg−1·min−1 and amounted to a decrease of between 25 and 34%, with the highest reduction in lean subjects (4, 5, 8, 18) and a somewhat lower reduction in obese subjects (4). The differences in EI were observed either during the subsequent ad libitum meal or during the first 12 h after the infusion. In the context of appetite-suppressant effect in humans, the quantitative effect of PYY is of a magnitude not previously reported in connection with anorectic drugs (sibutramine etc.), and PYY receptors therefore constitute a very promising drug target for obesity management. Stimulation of PYY receptors may cause nausea, and it is possible that the reduction in EI is not solely due to a reduction in hunger or increased satiety but that it might be confounded by nausea.
Under normal physiological circumstances, both PYY1–36 and PYY3–36 are found in the circulation and both hormone moieties would therefore be expected to be part of the physiological satiety signal, but so far the effects of PYY1–36 on appetite have not been investigated. Neither have the effects of PYY1–36 and PYY3–36 on EE, glucose and fat metabolism in humans been investigated. This study aimed to compare the effects of peripheral infusions of saline, PYY1–36 or PYY3–36 on appetite, EI, EE, glucose and fat metabolism in obese and normal weight males.
SUBJECTS AND METHODS
Twelve normal-weight (means ± SD: BMI: 22.7 ± 1.4 kg/m2; age: 25.9 ± 2.5 yr) and 12 overweight/obese (BMI: 31.6 ± 2.8 kg/m2; age: 34.9 ± 8.7 yr) healthy, weight-stable, Caucasian men participated in a randomized, blinded, placebo-controlled, crossover study with infusion of saline, 0.8 pmol·kg−1·min−1 PYY1–36, or PYY3–36. The choice of dosages was based on findings in previous human studies (4, 5). Minimum washout period was 3 wk. Five of the first nine participants did not complete their PYY3–36 infusion day due to severe malaise and the PYY3–36 infusions had to be terminated. Therefore, only data from the four completers (2 lean and 2 obese subjects) are available for 0.8 pmol·kg−1·min−1 PYY3–36 infusions, whereas all 24 subjects completed the saline and 0.8 pmol·kg−1·min−1 PYY1–36 infusions. Due to the side effects of PYY3–36 infusions and the lack of effect of PYY1–36 infusions, it was decided to add two more arms to the study with a lower dose of PYY3–36 (0.2 pmol·kg−1·min−1) and a higher dose of PYY1–36 (1.6 pmol·kg−1·min−1). Of the total 24 participants, six lean and eight obese participants completed these two arms of the study. For comparison, the data on PYY1–36 (0.8 pmol·kg−1·min−1) infusions already presented for the 24 subjects were included again in the data analysis for the 14 subjects. The study was approved by the Municipal Ethics Committee of Copenhagen, Frederiksberg and Zealand (Registration no. 01-091/04) and was carried out in accordance with the Helsinki-II declaration. Informed consent was obtained from all participants.
Subjects received a standardized meal between 1900 and 2000 the night before each test day and they fasted thereafter (including no water intake after midnight). Subjects were instructed to refrain from drinking alcohol and from strenuous exercise 48 h before each test day. On each test day, subjects arrived at the department at 0800 having used nonstrenuous transportation. Two venflon cathethers were inserted into opposite antecubital veins, one for infusion and one for blood sampling. The preinfusion measurements and blood sampling were performed after a minimum of 15 min of rest. Two hours after termination of the 90-min infusion, an ad libitum, homogeneous, casserole lunch meal was served. An ad libitum buffet dinner was served 5 h later. One-hundred millimeters of visual analog scales (VAS) (10) were employed for ratings of well-being and appetite sensations. EE was measured by indirect calorimetry using a ventilated hood system [described in detail elsewhere (3)]. EE was calculated using a formula assuming a fixed protein catabolism (26) as the error introduced by omitting the exact correction from urinary nitrogen is negligible and EE cannot be reliably estimated from protein combustion on the basis of short-term urinary nitrogen outputs. The precision of the ventilated hood system was validated by an alcohol burning test on a weekly basis; CV% was <1.5. The measurements were performed with intervals of ∼25-min measurement and 5-min break for blood sampling and VAS questionnaires. Heart rate was monitored by a digital wrist blood pressure monitor (UB-401, A&D Instruments LTD, Oxon, UK).
The PYY peptides were purchased from Polypeptide (Wolfenbüttel, Germany), dissolved to a concentration of 8 nmol/ml saline (9 g/l sodium chloride; 2.3 g/l acetic acid), and filled into 2R glass vials (Apodan 712026, ApodanNordic, Copenhagen, Denmark) with bromobutyl rubber stoppers (Apodan 712638, ApodanNordic) after sterile filtration under aseptic conditions at the Central Pharmacy, Herlev Hospital, Denmark. Vial content was tested for sterility and bacterial endotoxins (Ph.Eur. 2.6.14, Method C. Turbidimetric kinetic method), and content was verified by sequence, mass, and amino acid analysis. Vials were kept at −20°C until preparation of the infusion, where the peptide solution was diluted further with saline (9 g/l sodium chloride) to which was added the subjects' own plasma in a concentration of 1 vol%. Final peptide concentration in the infusion fluid was 32 pmol/ml, which was confirmed by radioimmunoassay analysis.
Blood samples and analysis.
Venous blood was drawn without stasis through an indwelling antecubital cannula into iced chilled syringes (Vacuette, Greiner Labortechnic, Austria). Syringes for PYY and FFA contained EDTA. All samples were centrifuged for 10 min at 2,800 g at 4°C within 30–60 min of sampling and stored at −20°C until analysis.
Plasma concentrations of FFA, glucose, and TG were all determined by use of enzymatic colorimetric method using a Cobas Mira Plus spectrophotometer (Roche Diagnostic Systems, Basel, Switzerland). Plasma FFA concentrations were determined using NEFA-C test kit (ACS-ACOD method; Wako Chemicals, Neuss, Germany) with intra- and interassay CV <4.5 and <4.2%, respectively. Plasma glucose concentrations were determined using gluco-quant Glucose/HK kit (Roche Diagnostics, Mannheim, Germany) (7) with intra- and interassay CV <0.7 and <1.5%, respectively. Serum TG concentrations were determined by use of Triglycerides GPO-PAP kit (Roche Diagnostics) (25) with intra- and interassay CV <0.6 and <3.3%, respectively.
Serum insulin concentrations were determined by chemiluminescence immunometric assay (IMMULITE 1000 Insulin kit, Diagnostic Products, Los Angeles, CA) using Immulite 1,000 analyzers (Diagnostic Products) (2) with intra- and interassay CV <2.5 and <4.9%, respectively.
Radioimmunoassay of PYY in plasma was performed using antiserum code no. 8412-2II (9), which reacts equally with PYY1–36 and PYY3–36. Synthetic human PYY1–36 or PYY3–36 (Peninsula Laboratories) were used as standards where appropriate. 125I-PYY1–36 (code no. IM259) was from Amersham Biosciences (UK). Assay buffer was 0.05 mol/l sodium phosphate, pH 7.5, containing in addition 400 KIE/ml Trasylol-aprotinin, 0.1 mol/l sodium chloride, 10 mmol/l EDTA, 0.6 mmol/l merthiolate. One-hundred fifty microliters of unknown plasma samples plus 150 μl of assay buffer or 150 μl of charcoal-treated plasma + 150 μl standards were preincubated with antiserum, 100 μl, diluted 1:20,000 (final concentration), for 48 h at 4°C. Then, 100 μl of tracer (5 fmol, specific activity 70 MBq/nmol) was added and the mixture incubated for 24 h before bound and free peptide moieties were separated by plasma-coated charcoal (15). Detection limit of the assay was below 2.5 pmol/l and 50% inhibition was obtained with 23 pmol/l PYY. Recovery of PYY added to plasma in concentrations between 5 and 50 pmol/l deviated <15% from expected values, with intra-assay CV <5%. The antiserum showed no cross-reaction with human NPY or human pancreatic peptide (PP) in concentrations up to 500 pmol/l.
Data were analyzed using repeated-measurements testing the effect of group (lean vs. obese, only for analysis with n = 24 and n = 14), time, and infusion, as well as interactions of these factors with baseline value as cofactor and subject as random factor. When interactions were not significant, the model was reduced successively. When a significant interaction between group and infusion was found, separate analyses of lean and obese participants were performed. For n = 14 VAS data, the analyses were divided into pre- and postmeal data, due to opposite effects in these two periods. Nonrepeated data were analyzed by one-way ANOVA with subject as random factor. Normal distribution of data was assessed by Shapiro-Wilk test, box plot, and normal probability plot. Where necessary, data were log transformed. All statistical analyses were performed using Statistical Analysis Package version 9.1 (SAS Institute, Cary, NC). Data are reported as means ± SE.
Saline, 0.8 pmol·kg−1·min−1 PYY1–36 and PYY3–36 infusions (n = 4).
The PYY peak value was reached at 60 min and was 214 ± 48 pmol/l after PYY1–36 infusion and 190 ± 27 pmol/l after PYY3–36 infusion, compared with a baseline value of 28 ± 5 pmol/l. PYY levels were back to normal levels within 30–60 min after termination of the infusion. Five of the nine subjects did not complete their PYY3–36 infusion day due to severe malaise or nausea. The nausea was in some instances accompanied by abdominal pain and heat flushes. The four subjects who were able to complete the PYY3–36 infusions showed a decrease in subjective ratings of well-being progressing from the beginning toward the end of the infusion (Ptime × infusion = 0.02; data not shown). After termination of the infusion, the ratings of well-being gradually recovered and were back to normal before the ad libitum lunch. PYY3–36 infusion resulted in decreased EI in all four subjects, with mean decreases in EI at lunch of 19.4 and 22.4% compared with saline and PYY1–36 infusions, respectively (placebo: 4,189 ± 478 kJ; PYY1–36: 4,348 ± 186 kJ; PYY3–36: 3,375 ± 341 kJ; P = 0.03). No effects of infusions were seen on EI, macronutrient composition, or energy density at the ad libitum buffet dinner (data not shown). There was a significantly lower rating of perceived ability to eat after both PYY1–36 and PYY3–36 compared with saline infusions (P < 0.0001), but no significant differences were found in other appetite ratings (data not shown).
In the fasting state, PYY3–36 infusions produced significantly higher FFA concentrations compared with saline and PYY1–36 infusions (Fig. 1A). Significantly higher postprandial glucose and insulin responses were produced by PYY3–36 infusions compared with PYY1–36 and saline (Fig. 1, B and C). No significant differences were observed in TG concentrations (Pinfusion = 0.58; data not shown). Heart rate was significantly higher with PYY3–36 compared with PYY1–36 and placebo (Fig. 1D).
There was a near-significant infusion effect (P = 0.056) on EE, with ∼292 kJ/day higher EE following PYY3–36 compared with PYY1–36 infusion, and ∼154 kJ/day higher EE following PYY3–36 compared with saline infusion, where only data for three subjects exist (Fig. 2A). There was a significant time × infusion effect in RQ with lower levels, indicating greater fat oxidation, following PYY3–36 compared with saline infusion, especially toward the end of the measuring period (Fig. 2B).
Saline and 0.8 pmol·kg−1·min−1 PYY1–36 (n = 24).
Means ± SE PYY peak value was 152 ± 14 pmol/l after PYY1–36 infusion compared with a baseline value of 22 ± 3 pmol/l. No side effects were reported with the PYY1–36 infusions and there was no difference in ratings of well-being between the PYY1–36 and placebo test days (data not shown). The infusion of PYY1–36 compared with placebo did not result in any reduction in EI at the ad libitum lunch (placebo: 3,998 ± 284 kJ; PYY1–36: 3,822 ± 259 kJ; P = 0.30) or at the ad libitum buffet dinner (placebo: 6,934 ± 306 kJ; PYY1–36: 7,332 ± 283 kJ; P = 0.17).
The analysis of ratings of hunger resulted in a significant group × infusion interaction, meaning that the lean group tended to be less hungry during the morning until the ad libitum lunch after the PYY1–36 infusions compared with saline infusions, whereas there was no difference in the obese group (Fig. 3A). When analyzing the data from the obese and lean group separately, the lean group (Pinfusion = 0.0003) showed a significantly lower rating of hunger following PYY1–36 compared with saline infusion, whereas no difference was seen in the obese subjects (Pinfusion = 0.87).
The same pattern was observed for ratings of satiety, where a significant group × infusion × time interaction was found, with the lean group feeling more satiated during the mornings until the ad libitum lunch and vice versa for the obese group in the same time period (Fig. 3B). Analyzing the data from the obese (Pinfusion = 0.81) and lean groups (Pinfusion = 0.68) separately, no significant difference between PYY1–36 and saline infusion could be found. Although a similar picture as that for hunger and satiety ratings was seen for perceived ability to eat, these ratings resulted in a nonsignificant infusion × group interaction (P = 0.15; data not shown), but in significant main effects with lower ratings in the obese group compared with the lean and after PYY1–36 compared with placebo infusion. Analyzing the data from the obese and lean groups separately, the lean group showed a significantly lower rating of perceived ability to eat following PYY1–36 compared with saline infusion (Pinfusion = 0.001), whereas no difference was seen in the obese group (Pinfusion = 0.64). No significant difference between PYY1–36 and saline infusions in ratings of fullness was found, neither when analyzing the lean and obese subjects together (Pgroup × infusion = 0.93; data not shown), nor when doing separate within-group analysis (Lean: Pinfusion = 0.55; Obese: Pinfusion = 0.58).
FFA, glucose, insulin, and TG concentrations over time were not different comparing PYY1–36 infusions with saline infusions (data not shown). A significant difference (Ptime × infusion = 0.003; data not shown) was found in heart rate, with the post hoc test showing significantly higher mean value at min 90, and lower mean value at minute 300 and 480, following PYY1–36 compared with saline infusion. Neither EE (Pinfusion = 0.08) nor RQ (Pinfusion = 0.49) differed following PYY1–36 compared with saline infusion (data not shown).
Saline, 0.2 pmol·kg−1·min−1 PYY3–36, 0.8 (already presented for n = 24), and 1.6 pmol·kg−1·min−1 PYY1–36 infusions (n = 14).
The PYY peak value was 76 ± 23, 157 ± 22, and 182 ± 20 pmol/l after 0.2 pmol·kg−1·min−1 PYY3–36, 0.8 and 1.6 pmol·kg−1·min−1 PYY1–36 infusions, respectively, compared with a baseline value of 17 ± 1 pmol/l. One participant reported mild nausea and a feeling of coldness during the infusion of PYY3–36. Another participant reported heat flushes and abdominal pain toward the end of the PYY3–36 infusion. There was a significant difference in score for well-being between the different infusions (Fig. 4A) with lower score for well-being following the PYY3–36 infusion compared with all other infusions, and a tendency to lower scores following the higher compared with the lower PYY1–36 dose. There were no effects of PYY infusions on EI at the ad libitum lunch (placebo: 3,790 ± 328 kJ; 0.2 pmol·kg−1·min−1 PYY3–36: 4,028 ± 326 kJ; 0.8 pmol·kg−1·min−1 PYY1–36: 3,894 ± 284 kJ; 1.6 pmol·kg−1·min−1 PYY1–36: 4,216 ± 294 kJ; P = 0.40) or at the ad libitum buffet dinner (placebo: 6,980 ± 352 kJ; 0.2 pmol·kg−1·min−1 PYY3–36: 6,503 ± 528 kJ; 0.8 pmol·kg−1·min−1 PYY1–36: 7,275 ± 440 kJ; 1.6 pmol·kg−1·min−1 PYY1–36: 7,389 ± 364 kJ; P = 0.45). However, there were significant differences in appetite, with a pattern of higher premeal ratings of hunger and lower ratings of satiety following the high-dose PYY1–36 infusion, and a compensatory opposite rating in the late postprandial period (Fig. 4, B and C). Ratings of perceived ability to eat and fullness, and all appetite ratings following the 0.2 pmol·kg−1·min−1 PYY3–36 infusion, showed no clear pattern (data not shown). Infusion of PYY3–36, but not PYY1–36, produced a significant elevation of plasma FFA (Fig. 5A). Baseline glucose concentrations were higher on the placebo and low-dose PYY1–36 infusion days compared with high-dose PYY1–36 and PYY3–36 days (placebo: 5.34 ± 0.10 mmol/l; 0.2 pmol·kg−1·min−1 PYY3–36: 5.12 ± 0.12 mmol/l; 0.8 pmol·kg−1·min−1 PYY1–36: 5.41 ± 0.13 mmol/l; 1.6 pmol·kg−1·min−1 PYY1–36: 5.14 ± 0.12 mmol/l; P = 0.0009), but no significant difference was found in the repeated-measurement analysis of glucose concentration (Fig. 5B). No significant difference was found in the repeated-measurement analysis of insulin concentrations (Fig. 5C), but the postprandial iAUC values were significantly higher following PYY3–36 and high-dose PYY1–36 infusions compared with low-dose PYY1–36 and placebo (placebo: 27,179 ± 3,369 pmol·min−1·l−1; 0.2 pmol·kg−1·min−1 PYY3–36: 39,526 ± 6,476 pmol·min−1·l−1; 0.8 pmol·kg−1·min−1 PYY1–36: 27,465 ± 3,665 pmol·min−1·l−1; 1.6 pmol·kg−1·min−1 PYY1–36: 36,960 ± 5,717 pmol·min−1·l−1; P = 0.004). No differences were observed in TG concentrations (Pinfusion = 0.97; data not shown). PYY3–36 and high-dose PYY1–36 produced significantly higher increases in heart rate compared with placebo and low-dose PYY1–36 (Fig. 5D). Baseline values of heart rate were not significantly different (placebo: 55.7 ± 2.5 beats/min; 0.2 pmol·kg−1·min−1 PYY3–36: 54.2 ± 2.5 beats/min; 0.8 pmol·kg−1·min−1 PYY1–36: 55.7 ± 2.3 beats/min; 1.6 pmol·kg−1·min−1 PYY1–36: 56.1 ± 2.4 beats/min; P = 0.73). No differences between infusions were observed for EE (Pinfusion = 0.28, data not shown), but a tendency to lower RQ following PYY3–36 and high-dose PYY1–36 infusions compared with placebo was observed (Fig. 6).
Our study confirms previous findings (4, 5, 8, 18) of a suppression of food intake, i.e., lower ad libitum EI, following PYY3–36 infusions at a rate of 0.8 pmol·kg−1·min−1, but the picture is less clear for subjective appetite sensations (VAS ratings), probably due to the statistical power being too low, with only four subjects completing the high-dose PYY3–36 infusions. We found that five of nine subjects experienced adverse effects, mainly in the form of nausea, but also in some instances accompanied by abdominal discomfort and heat flushes. These effects were of a severity that precluded continuation of the infusions. Subjective ratings of well-being were lower during the infusion period in the four completing subjects who did not report nausea. Although these ratings were back to normal before the ad libitum lunch meal, it cannot be ruled out that discomfort, at least partly, can account for the lower ad libitum EI. The infusions of 0.2 pmol·kg−1·min−1 PYY3–36, on the other hand, were generally well tolerated with fewer and milder reported side effects, but in accordance with other studies (8, 18) this infusion rate did not result in significant lowering of EI or in differences in appetite ratings.
If we compare our plasma concentrations of PYY, we find that they were somewhat higher than in the three previous studies reporting infusion of 0.8 pmol·kg−1·min−1 (5, 8, 18), but also among these studies great differences are seen in plasma peak levels of PYY. Since the differences among studies in baseline values are generally lower than differences in peak levels, it is likely due to different preparation of the infusion. In the present study, the subjects' own plasma was used as carrier in the preparation of the infusion compared with Hemaccel (4, 5, 18, 19) and human serum albumin (8) in other studies, probably resulting in different adsorption of the peptide to infusion bag and tubing. Since actual concentration of the hormone in the infusion liquid after passage through the tubing is generally not reported, it is difficult to compare doses across studies, and differences in assays used to analyze plasma PYY concentrations may add to these discrepancies. Plasma levels closest to the ones in the present study were found in the study by Degen et al. (8), who also reported side effects in the form of nausea, vomiting, abdominal discomfort, fullness and sweating, with plasma levels in the order of >74 pmol/l leading to nausea. Our observations with regard to side effects are also in accord with a previous study with nasal administration of PYY3–36, in which doses resulting in a fourfold increase in plasma PYY levels from baseline to peak were associated with dizziness and nausea (20). Furthermore, PYY has been shown to have emetic effects in dogs, both with endogenous PYY induced by cisplatin treatment (21) and with exogenous PYY infused at doses below 100 pmol/kg (14). This corresponds well with our findings, where a 90-min infusion amounting to a total dose of 72 pmol/kg caused nausea in five of nine subjects. Both Degen et al. (8) and Le Roux et al. (18) demonstrated a dose-dependent decrease in EI, and their data suggest that plasma levels should reach at least 60 pmol/l to have significant effects on EI. Neary et al. (19) found that doses of 0.4 pmol·kg−1·min−1, giving rise to plasma concentrations around 60 pmol/l, resulted in a borderline significant reduction of food intake, whereas our infusion with 0.2 pmol·kg−1·min−1 resulting in mean plasma PYY concentrations of 76 pmol/l had no effect on food intake. Consequently, the therapeutic window for reduction of food intake without nausea is narrow, and PYY therapy will require methods of administration that produce a stable plasma level within this therapeutic range.
PYY3–36 infusions produced progressive increases in FFA until a peak was reached 30 min postprandially with 182 and 95% increases from baseline following high- and low-dose PYY3–36, respectively. The PYY3–36 infusions moreover resulted in increased postprandial insulin responses and for the high-dose infusion increased postprandial glucose response. The only other PYY3–36 infusion study where insulin and glucose were measured is that of Neary et al. (19), who, in accord with our results, found no differences in fasting glucose and insulin, but they did not measure postprandial values. The increased FFA and postprandial insulin and glucose concentrations could be caused by a PYY-induced impairment in insulin sensitivity, resulting in higher release of FFA from adipose tissue but without any obvious effects on glucose and insulin levels before a challenge is presented in the form of a meal. However, we find it more likely that increased lipolysis was the primary event, with the higher levels of FFA producing insulin resistance with resulting higher postprandial insulin and glucose concentrations. An activation of the sympathetic nervous system could be the cause of an increased lipolysis, which is supported by the data indicating higher heart rate for both low- and high-dose PYY3–36 infusions, and higher EE, during the high-dose PYY3–36 infusions. Measurements of catecholamine concentrations could help clarify this picture, but such samples were not taken in the present study. The decrease in RQ during the PYY3–36 infusions is as we expected, as the higher FFA concentrations result in higher fat oxidation, but the increased EE also indicates sympathoadrenal activation. It remains unclear whether the reduced ratings of well-being associated with the PYY3–36 infusions could be the reason for the hypothesized increased sympathoadrenal activity.
In the present study, PYY1–36 infusions had no effect on subsequent EI. However, subjective appetite ratings suggested an effect of low-dose PYY1–36 in lean individuals with lower ratings of hunger and perceived ability to eat, whereas this effect was not present in the obese participants. In contrast with this, high-dose PYY1–36 resulted in increased hunger and decreased satiety in the period before the ad libitum meal, whereafter a compensatory opposite rating was observed. The difference between low- and high-dose PYY1–36 in VAS ratings of hunger could be speculated to be due to different study groups with more obese subjects in the high-dose PYY1–36 part of the study. Besides effects on appetite, the only other significant effects seen with PYY1–36 were a higher postprandial insulin response and increases in heart rate following high-dose PYY1–36. Since PYY1–36 is thought to be cleaved by DPP-IV, the infusion of PYY1–36 should give rise to higher circulating concentrations of PYY3–36, and thereby similar effects should be seen with administration of the two different peptide fractions. Our data therefore suggest that this cleavage process is not a complete and rapid process such as is seen with the cleavage of GLP-1, which has a half-life of 1–2 min (24).
The difference between lean and obese subjects in appetite ratings could be speculated to be due to different expression of the Y1, Y2, and Y5 receptors, which have opposite effects on appetite, with the Y2 receptor signaling anorexigenic effects (5), and the Y1 and Y5 orexigenic effects (11, 16, 17, 23). Another possibility is lower activity of DPP-IV in obese compared with lean humans, thereby giving rise to higher circulating levels of PYY3–36 in the lean compared with obese participants, but this theory is not supported by previous data showing an equal ratio between PYY1–36 and PYY3–36 in obese compared with lean individuals (18). Furthermore, the appetite regulation in obese may be speculated to be affected more by other factors, such as psychological factors, which could tend to blur the assessment of the effects of a physiological factor like PYY. Finally, a previous infusion study in rats comparing effects on EI of PYY1–36 and PYY3–36 found that PYY1–36 seemed to be around 10-fold less potent than PYY3–36. This suggests that much higher doses than those used in the present study could be needed to find significant effects on EI, and on fat and glucose metabolism, in humans after PYY1–36 administration (6) and the present study might not have reached highest tolerable dose of PYY1–36.
In summary, PYY1–36 infusions had no effects on EI, and no clear dose-dependent effect was observed in relation to appetite following PYY1–36 infusions, but data indicate that effects might differ between lean and obese individuals. Low-dose PYY1–36 had no effects on EE, FFA, glucose, insulin or TG concentrations, whereas high-dose PYY1–36 resulted in a somewhat increased heart rate and postprandial insulin response. The anorectic effects of PYY3–36 were confirmed in this study but may be due to discomfort and it seems that the therapeutic window, resulting in decreased EI without side effects, is narrow. PYY3–36 caused increased thermogenesis, lipolysis, and increased postprandial insulin and glucose responses. The mechanisms behind these findings are unknown and call for further studies.
This study was supported by EC-FP6 (contract number: LHM-CT-2003-503041) and an institutional grant from AdiTech Pharma AB.
None of the authors has any conflict of interest. A. Astrup was in 2004–2005 consultant for Aditech Pharma AB, who has patents pending for PYY. J. J. Holst's work is supported by the Novo Nordisk Foundation and by the Danish Medical Research Council. The study was supported by an institutional grant from AdiTech Pharma AB.
Present address of A. Flint: Novo Nordisk, Copenhagen, Denmark.
This publication reflects the authors' views and not necessarily those of the EC. The authors acknowledge the valuable technical and clinical work performed by I. Timmermann, M. Gudmundsson, L. Albæk, S. Spilgaard, L. B. Thielsen, I. Krog-Mikkelsen, O. Hels, and B. K. Vistisen. We also thank C. Kostecki, K. G. Rossen, Y. Rasmussen, B. Hoielt, and K. B. H. Larsen for providing all the meals from the metabolic kitchen and C. Cuthbertson for proofreading the manuscript.
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 © 2007 by American Physiological Society