Coronary microvascular dysfunction is an early and persistent defect in states of insulin resistance (IR) that are closely associated with inappropriate activation of the renin-angiotensin-aldosterone system (RAAS). Thus, RAAS activation is a major factor in IR-associated vascular dysfunction, acting via several mechanisms including oxidative stress, fibrosis, endothelial dysfunction and activation of serine kinases such as Rho kinase (ROK). Much existing evidence implicates angiotensin II involvement in these adverse effects;however, accumulating evidence demonstrates pronounced effects of aldosterone acting through vascular mineralocorticoid receptors (MR) as a putative mediator of vascular dysfunction in states of IR. In clinical trials, MR antagonism reduced cardiovascular mortality via unknown mechanisms. Furthermore, recent evidence demonstrates a pronounced effect of MR antagonism to improve coronary flow reserve (CFR) in diabetic patients. The exact mechanism(s) underlying MR-mediated coronary vascular dysfunction;however, remain unclear. Mounting evidence suggests that MR signaling is a primary regulator of cellular calcium homeostasis and contractility. Specifically, MR signaling appears to be a tonic regulator of L-type calcium channel (CaV1.2) function/expression in cardiomyocytes. Our recent work demonstrates that increased activation of coronary vascular MR by aldosterone leads to increased agonist- and CaV1.2-mediated coronary vasoconstrictor responsiveness, similar to that noted in states of IR. Furthermore, our data demonstrate direct activation of ROK, a primary modulator of calcium sensitivity, by MR signaling in vascular smooth muscle cells (VSMC). Therefore, this proposal seeks to investigate novel molecular mechanisms by which VSMC MR signaling promotes VSMC hypercontractility, increased coronary vasoconstriction and reductions in CFR associated with IR. We hypothesize that inappropriate activation of vascular, specifically VSMC, MRs plays a central role in the development of VSMC hypercontractility via the modulation of calcium influx and sensitivity in IR. A corollary to this hypothesis is that MR-dependent signaling in VSMC directly mediates coronary dysfunction and the impairment of coronary blood flow control in states of IR. In our proposal, we will utilize a novel, innovative VSMC-specific MR knockout (VSMC MR KO) mouse model coupled with state-of-the-art microvascular imaging and functional techniques, to evaluate the effect of MR signaling (activated by exogenous aldosterone or diet-induced IR) to modulate VSMC calcium handling, coronary arteriolar function ex vivo and CFR in vivo. To address Specific Aim 1, we will examine the relationship between MR activation and CaV1.2 channel function/expression and ROK activation in primary cultured VSMC, freshly dispersed murine coronary VSMC and intact coronary arterioles. These parameters will be correlated to functional measures including arteriolar vasoconstriction, blood pressure and CFR.
Specific Aim 2 will extend these findings and address the role of MR activation in VSMC in IR as a primary mediator of IR-associated coronary hypercontractility/dysfunction and its relationship to arteriolar function and CFR. IR will be induced in wild-type and VSMC MR KO mice with a high-fat, high-sucrose diet and will be assessed by glucose tolerance testing.
Incidence rates of type 2 diabetes and hypertension, both associated with insulin resistance (IR), are rapidly increasing in the US with an even greater increase in the Veteran population. Coronary microvascular dysfunction (i.e., reduced coronary flow reserve) is a primary finding that predicts coronary heart disease and myocardial ischemia/infarct in these patients. Furthermore, coronary vascular disease is a primary cause of mortality in this patient population. The fundamental mechanisms underlying IR-associated coronary microvascular disease;however, are not well understood and our proposed work is directed to elucidate novel mechanisms underlying the progression of these defects. A better understanding of factors involved in IR- associated coronary vascular dysfunction should help in the development of therapeutic targets for the prevention and/or treatment of these cardiovascular abnormalities in clinical states characterized by IR.