Na⁺ transport in isolated rat CCD: Effects of bradykinin, ANP, clonidine, and hydrochlorothiazide

We examined the effects of bradykinin (BK), atrial natriuretic peptide (ANP), hydrochlorothiazide (HCTZ), and clonidine on Na⁺ transport in isolated perfused cortical collecting ducts from rats treated with deoxycorticosterone. Arginine vasopressin was present in the bathing solution at 220 pM. Clonidine (1 μM, bathing solution) depolarized transepithelial potential difference (PDT) from -11.9 ± 2.0 (SE) to -7.4 ± 1.7 mV (P < 0.001), hyperpolarized basolateral membrane potential difference (PD_bl) from -85 ± 1 to -87 ± l mV (P < 0.01), and increased the fractional resistance of the apical membrane (FA.) from 0.81 ± 0.02 to 0.86 ± 0.02 (P < 0.03), indicating that it inhibited the Na⁺ conductance of the luminal membrane. BK (1 or 10 nM) or ANP (10 nM) in the bathing solution had no effect on PD_T, PD_bl, or FR_a. BK, ANP, or 0.1 mM luminal HCTZ also had no effect on lumen-to-bath ²²Na⁺ flux (J_l→b), whereas we showed previously that clonidine inhibits J_l→b by 30% (L. Chen, M. Paris, S. K. Williams, M. C. Reif, and J. A. Schafer. Kidney Int. 37: 366, 1990). Luminal addition of Na₊ channel blockers amiloride (10 μM) or benzamil (1 μM) reduced J_l→b, to a level not significantly different from bath-to-lumen ²²Na⁺ flux measured previously (M. Reif, S. L. Troutman, and J. A. Schafer. J. Clin. Invest. 77:1291-1298,1986), and neither BK nor HCTZ had any further effect. These results show that transcellular Na⁺ transport occurs exclusively through the apical membrane amiloride-sensitive channel, and this conductance is inhibited by clonidine but not by BK, ANP, or HCTZ.