Electrophysiological study of inner medullary collecting duct of hamsters

The electrophysiological properties of the hamster mid-inner medullary collecting duct (IMCD₂) cells were examined in isolated and perfused preparations by intracellular impalement with conventional 1 mol/l KCl microelectrodes and cable analysis. The transmural voltage (V _T) was not different from 0 mV, while the basolateral transmembrane voltage (V _B) was –81.7±0.91 mV (n=221). The transmural resistance (R _T) was 109 cm², indicating that the IMCD₂ is composed of tight epithelia. The fractional apical membrane resistance (fR _A) was 0.98±0.003 (n=10). Abrupt changes in the luminal concentration of Na⁺, K⁺ or Cl^– did not alter the apical membrane voltage (V _A) or V _T, and neither 2 mmol/l Ba²⁺ nor 10 mol/l amiloride in the lumen affected V _A and V _T. Moreover, pretreatment of hamsters with deoxycorticosterone acetate (5 mg/ kg, s.c.) for 10–14 days caused only a very small change in V _T in the negative direction. Amiloride in the lumen increased R _T and increased the voltage divider ratio very slightly. However, an abrupt increase in K⁺ concentration in the bath from 5 mmol/l to 50 mmol/l or addition of 2 mmol/l Ba²⁺ to the bath depolarized the basolateral membrane by 39 mV and 29 mV, respectively. In the presence of 2 mmol/l Ba²⁺ in the bath, a reduction of HCO₃ ^– concentration from 25 mmol/l to 2.5 mmol/l depolarized V _B by 20.4 mV. No Cl^– conductance was demonstrated in the basolateral membrane. Addition of ouabain to the bath or elimination of K⁺ from the bath caused only very small changes in V _B of the IMCD₂ as compared to the marked responses ovserved in the medullary thick ascending limb of the loop of Henle and in the upper portion of the descending limb of the long-looped nephron. These findings are compatible with the view that either a weak Na⁺-K⁺ pump or an ouabain-resistant pump in combination with high K⁺ conductance in the basolateral membrane mainly accounts for the maintenance of the intracellular concentration of Na⁺ and K⁺. The IMCD₂ may contribute little to the transmural transport of Na⁺ and K⁺. The physiological significance of HCO₃ ^– conductance in the basolateral membrane remains to be established.