Up-Regulating the Heme Oxygenase System with Hemin Improves Insulin Sensitivity and Glucose Metabolism in Adult Spontaneously Hypertensive Rats

Accumulating clinical evidence indicates that impaired glucose tolerance is a common phenomenon in essential hypertension. Although recent evidence underscores the role of heme-oxygenase (HO) in diabetes, its effects on insulin sensitivity and glucose metabolism in spontaneously hypertensive rat (SHR), a model of essential hypertension with characteristics of metabolic syndrome including insulin resistance/impaired glucose metabolism remains largely unclear. Here we report the effects of the HO inducer, hemin, and the HO blocker, chromium-mesoporphyrin on insulin sensitivity and glucose metabolism in SHRs. Adult SHRs were severely hypertensive but normoglycemic. Hemin therapy lowered blood pressure, increased plasma insulin, decreased glycemia, and enhanced insulin sensitivity by improving glucose tolerance (ip glucose tolerance test) and insulin tolerance (ip insulin tolerance test) but reduced insulin resistance (homeostasis model assessment index). These effects were accompanied by increased gastrocnemius muscle HO-1, HO activity, cGMP, cAMP alongside antioxidants including bilirubin, ferritin, superoxide dismutase, catalase, and the total antioxidant capacity, whereas oxidative/inflammatory mediators like 8-isoprostance, nuclear-factor-kappa B, activating-protein-1, activating-protein-2, c-Jun-NH2-terminal-kinase, and heme were abated. Furthermore, hemin reduced proteinuria/albuminuria and enhanced the depressed levels of adiponectin, AMP-activated protein-kinase, and glucose transporter-4 in SHRs, suggesting that although SHRs are normoglycemic, insulin signaling and renal function may be impaired. Contrarily, the HO inhibitor chromium-mesoporphyrin exacerbated oxidative stress, aggravated insulin resistance, glucose tolerance, insulin tolerance and nephropathy. Hemin also enhanced HO signaling in Wistar Kyoto and Sprague Dawley rats and increased insulin sensitivity albeit less intensely than in SHRs, suggesting greater selectivity of HO in SHRs with dysfunctional insulin signaling. These results suggest that perturbations of insulin signaling may be a forerunner to hyperglycemia in essential hypertension. By concomitantly potentiating insulin-sensitizing agents, suppressing insulin/glucose intolerance, and abating oxidative stress, HO inducers may prevent metabolic and cardiovascular complications in essential hypertension. (Endocrinology 151: 549-560, 2010)