Insulin stimulates NaCl transport in isolated perfused MTAL of Henle's loop of rabbit kidney

To elucidate the mechanism of Na⁺ retention by insulin in vivo, the direct tubular effect of insulin on NaCl transport in the in vitro microperfused medullary thick ascending limb of Henle (MTAL) was examined. Insulin at 10^-6 mol/l in the bath increased transepithelial voltage (V(te)) from 3.1 ± 0.3 to 5.7 ± 0.3 mV (n = 12, P < 0.0001). The effect of insulin on V(te) was dependent on its concentration, and the half-maximal effect of insulin was observed at 5 x 10^-9 mol/l. Insulin at 10^-6 mol/l also caused a significant decrease of luminal Cl^- concentration from 85.4 ± 5.0 to 62.8 ± 3.0 mmol/l (n = 5, P < 0.002) when the lumen was microperfused constantly at less than 1 nl/min. Insulin at 10^-6 mol/l also increased net lumen-to-bath Cl^- flux (J(Cl)) from 143 ± 15 to 292 ± 37 pmol · mm^-1 · min^-1 (n = 5, P < 0.004). When the Na⁺-K⁺-adenosinetriphosphatase (Na⁺-K⁺-ATPase) in the basolateral membrane was blocked by 10^-4 mol/l ouabain, the insulin-mediated increase in V(te) was completely suppressed. When the Na⁺-K⁺-2Cl^- cotransporter in the luminal membrane of the MTAL was blocked by 10^-4 mol/l furosemide, the insulin-mediated increase in V(te) was also abolished. To test whether adenosine 3',5'-cyclic monophosphate (cAMP) contributes to the action of insulin, we examined the effect of cAMP analogue and cAMP-dependent protein kinase inhibitor on the action of insulin. A maximal concentration (5 x 10^-4 mol/l) of dibutyryl-cAMP (DBcAMP) increased V(te) and J(Cl). Under conditions associated with a maximal response to DBcAMP, superimposed insulin induced a further increase in V(te) and J(Cl). At 5 x 10^-5 mol/l, N- [2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H-8, a cAMP-dependent protein kinase inhibitor) abolished the effect of DBcAMP completely, while it did not inhibit the action of insulin. These results indicate that insulin directly increases NaCl reabsorption in the MTAL of rabbit kidney and that this effect is independent of the adenylate cyclase-cAMP system.