Impact of obesity on renal structure and function in the presence and absence of hypertension: evidence from melanocortin-4 receptor-deficient mice

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The purpose of this study was to determine the long-term impact of obesity and related metabolic abnormalities in the absence and presence of hypertension on renal injury and salt-sensitivity of blood pressure. Markers of renal injury and blood pressure salt sensitivity were assessed in 52- to 55-wk-old normotensive melanocortin-4 receptor-deficient (MC4R-/-) mice and lean C57BL/6J wild-type (WT) mice and in 22-wk-old MC4R-/- and WT mice made hypertensive by N(G)-nitro-L-arginine methyl ester (L-NAME) in the drinking water for 8 wk. Old MC4R-/- mice were 60% heavier, hyperinsulinemic, and hyperleptinemic but had similar mean arterial pressure (MAP) as WT mice (115 +/- 2 and 117 +/- 2 mmHg) on normal salt diet (0.4% NaCl). A high-salt diet (4.0% NaCl) for 12 days did not raise MAP in obese or lean mice [DeltaMAP: MC4R (-/-) 4 +/- 2 mmHg; WT, 2 +/- 1 mmHg]. Obese MC4R-/- mice had 23% greater glomerular tuft area and moderately increased GFR compared with WT mice. Bowman's space, total glomerular area, mesangial matrix, urinary albumin excretion (UAE), renal TGF-beta and collagen expression were not significantly different between old MC4R-/- and WT mice. Renal lipid content was greater but renal macrophage count was markedly lower in MC4R-/- than WT mice. Mild increases in MAP during L-NAME treatment (approximately 16 mmHg) caused small, but greater, elevations in UAE, renal TGF-beta content, and macrophage infiltration in MC4R-/- compared with WT mice without significant changes in glomerular structure. Thus despite long-term obesity and multiple metabolic abnormalities, MC4R-/- mice have no evidence of renal injury or salt-sensitivity of blood pressure. These observations suggest that elevations in blood pressure may be necessary for obesity and related metabolic abnormalities to cause major renal injury or that MC4R-/- mice are protected from renal injury by mechanisms that are still unclear.