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Department of Physiology, University of Mississippi Medical Center, Jackson, Mississippi 39216
Recently we reported that hindquarter blood flow, measured 24 h/day, decreased progressively over the first 6 days of type 1 diabetes in rats. That response, coupled with the tendency of mean arterial pressure to increase, suggested a vasoconstrictor response. The purpose of this study was to measure the changes in cardiac output together with the renal hemodynamic and excretory responses to allow integrative determination of whether vasoconstriction likely accompanies the onset of type 1 diabetes. Rats were instrumented with a Transonic flow probe on the ascending aorta and with artery and vein catheters, and cardiac output and mean arterial pressure were measured continuously, 24 h/day, throughout the study. The induction of diabetes, by withdrawing intravenous insulin-replacement therapy in streptozotocin-treated rats, caused a progressive decrease in cardiac output that was 85 ± 5% of control levels by day 7. This was associated with significant increases in glomerular filtration rate, renal blood flow, and microalbuminuria as well as urinary fluid and sodium losses, with a negative cumulative sodium balance averaging 15.7 ± 1.6 meq by day 7. Restoring insulin-replacement therapy reversed the renal excretory responses but did not correct the negative sodium balance, yet cardiac output returned rapidly to control values. Increasing sodium intake during the diabetic and recovery periods also did not significantly affect the cardiac output response during any period. These results indicate that cardiac output decreases significantly at the onset of type 1 diabetes without glycemic control, and although volume loss may contribute to this response, there also is a component that is not volume or sodium dependent. We suggest this may be due to vasoconstriction, but to what extent local blood flow autoregulation or active vasoconstriction may have mediated that response is not known.
peripheral vascular resistance; sodium excretion; thromboxane; angiotensin II
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