Abstract
Introduction: In obesity, increased glomerular capillary pressure (P GC ) may participate in renal damage.P GC is controlled in part by afferent arteriolar ( Af-Art ) resistance that in turn is regulated by two renal intrinsic feedback mechanisms, the vasoconstrictor Tubulo-Glomerular Feedback (TGF), and vasodilator Connecting Tubule Glomerular Feedback (CTGF). CTGF is initiated by an increase in NaCl transport by the epithelial sodium channel (ENaC) in the connecting tubule (CNT). Interestingly, obesity is strongly associated with hyperinsulinemia and insulin is a potent ENaC activator. Hypothesis: In obesity, hyperinsulinemia increases CTGF via activation of the ENaC, this increase, in turn, contributes to TGF attenuation leading to increased P GC and renal damage. Methods: In vivo : In zucker obese rats (ZOR) and zucker lean rats (ZLR), we measured TGF and CTGF using renal micropuncture at 9-10 weeks of age. We quantify stop-flow pressure (P SF ) as an index of P GC . We measured proteinuria as a marker of renal damage in both ZOR and ZLR. In vitro: Microdissected rabbit Af-Arts and their adherent CNTs were perfused with NaCl, insulin or ENaC inhibitor (Benzamil; BZ) to investigate the role of insulin on TGF and CTGF. Results: In-vivo : Maximal TGF response was significantly less in ZOR (6.11 ± 0.75 mmHg) in comparison to the ZLR (9.5 ± 1.1 mmHg, p<0.05). CTGF inhibition by BZ normalized the TGF response in ZOR similar to ZLR (ZOR, 14.01±1.70 mmHg vs ZLR, 11.46± 2.25 mmHg) suggesting CTGF playing a key role in TGF resetting in ZOR. Additionally, ZOR develops proteinuria (mg/24h) at 12 weeks of age (ZOR; 24.85±3.02 vs ZLR; 7.21±1.09, p<0.05 ). In-vitro microperfusion of NaCl in the CNT that elicited a half-maximal response (EC 50 , mmol/L) of Af-Art dilation was 25.0±0.8; an addition of insulin 10 -7 mol/l to the CNT lumen decreased the EC 50 to 8.1±0.8 ( P <0.05) suggesting insulin potentiates CTGF. BZ blocked the insulin-mediated CTGF (Insulin EC 50 : 7.8±0.9 vs . Insulin+BZ, EC 50 : 19.7±5.5; P <0.05). Conclusion: In-vivo : TGF is reset in ZOR due to enhanced CTGF before they develop proteinuria. In-vitro : Insulin increased CTGF during microperfusion experiments. Perspective: Insulin-induced increased in CTGF may explain higher P GC and renal damage in obesity.
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