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 (VT) was not different from 0 mV, while the basolateral transmembrane voltage (VB) was -81.7±0.91 mV (n=221). The transmural resistance (RT) was 109 Ωcm2, indicating that the IMCD2 is composed of tight epithelia. The fractional apical membrane resistance (fRA) 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 (VA) or VT, and neither 2 mmol/l Ba2+ nor 10 μmol/l amiloride in the lumen affected VA and VT. Moreover, pretreatment of hamsters with deoxycorticosterone acetate (5 mg/ kg, s.c.) for 10-14 days caused only a very small change in VT in the negative direction. Amiloride in the lumen increased RT 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 VB 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 VB 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. © 1990 Springer-Verlag.