Mechanisms for Cardiovascular Control Early in Diabetes

Augusta University, Augusta GA

Investigators

Linked publications & trials

Abstract

DESCRIPTION (provided by applicant): The variables that define diabetes are glucose and insulin, and the major complications of diabetes: hypertension and renal dysfunction, are inextricably linked to one another. Moreover, renal actions of insulin still are widely believed to contribute to the increase in blood pressure as patients progress through metabolic syndrome to overt Type II diabetes. Yet, there are zero published data that show how the changes in insulin and glucose that are measured in metabolic syndrome or Type II diabetes act within the kidney, chronically, to control kidney function and blood pressure. We developed an innovative experimental approach for clamping plasma glucose and insulin concentrations in dog kidneys, chronically, in any combination, and independent of systemic changes, while measuring renal blood flow, renal excretory function, and mean arterial pressure (MAP) 24 hours per day, to provide a unique opportunity to discover the mechanisms linking these variables. We will test the central hypothesis that the increases in plasma insulin and glucose concentrations measured in metabolic syndrome cause sodium reabsorption and increase GFR, and that hypertension ensues if the increase in GFR is attenuated. The specific aims of this project are to: 1. Test the hypothesis that combined renal tubular actions of glucose and insulin chronically stimulate tubular reabsorption and renal vasodilation, by determining whether: a. intrarenal hyperglycemia (~ 20 mg/dl increase) decreases UnaV and increases RBF and GFR;b. intrarenal hyperinsulinemia (~ 3x normal) augments those tubular effects of glucose;c&d. the stimulation of sodium reabsorption is independent of the renin-angiotensin system. 2. Test the hypothesis that glucose and insulin-induced renal vasodilation requires renal COX-2 and nNOS activity, by determining whether: a. intrarenal COX-2 inhibition in metabolic syndrome will decrease RBF and increase MAP;b&c. AngII or Tx blockade will prevent the increased RVR and MAP responses during COX-2 inhibition;d. blockade of intrarenal nNOS with SMTC will prevent the renal vasodilation and raise MAP;e. the chronic intrarenal actions of COX-2 and nNOS inhibition on RBF and MAP are additive. 3. Test the hypothesis that renal actions of insulin and glucose will cause hypertension in metabolic syndrome if GFR is not able to increase appropriately, by determining whether: a. onset of diabetes in dogs requires an increase in GFR to prevent hypertension;b. an intrarenal metabolic syndrome environment will increase MAP if GFR does not increase;c. episodes of sustained intrarenal metabolic syndrome will cause progressively lesser renal vasodilation and hyperfiltration in fat-fed dogs during the evolution of obesity;and d. angiotensin II specifically inhibits insulin signaling in the dog kidney. PUBLIC HEALTH RELEVANCE Metabolic syndrome is the earliest phase in the evolution of Type II diabetes, and patients that fall within this spectrum of deranged glucose homeostasis are at epidemic levels. Yet, despite the strong correlations between insulin and glucose with hypertension and impaired kidney function, and numerous hypotheses that insulin and glucose may have direct renal actions that can increase blood pressure, there remains no direct experimental evidence that these actions exist. Moreover, there is not an experimental model that enables these hypotheses to be tested directly. We developed such a model. We can localize metabolic syndrome levels of plasma glucose and insulin to the kidney, and maintain that for days-to-weeks while measuring renal blood flow, renal excretory function, and mean arterial pressure 24 hrs/day. Our novel hypothesis is that glucose and insulin act in concert to promote sodium retention and renal vasodilation, and, moreover, that an impaired vasodilator response enables the sodium-retaining actions to cause hypertension. Our pilot data support that.