PATCH-CLAMP EVIDENCE FOR CALCIUM CHANNELS IN APICAL MEMBRANES OF RABBIT KIDNEY CONNECTING TUBULES

To test the hypothesis that Ca channel plays a role in renal epithelial Ca transport, we exposed and patched apical Membranes of freshly microdissected rabbit connecting tubules (CNTs). Single channel Ca currents were recorded with Ba as the charge carrier. In the cell-attached mode, 8-Br-cAMP increased the open-state probability (P(o)) to 0.6%. In excised, inside-out patches, P(o) was low spontaneously and remained low during either bath protein kinase A catalytic subunit (PKA(cs)) or Bay K 8644. Exposure to both agonists, however, unmasked Ca channels previously latent with only one, raising P(o) by 1.05% at membrane potential of -70 mV. Mean P(o) for 14 seals (2.57%) peaked at -70 mV, declining with either hyperpolarization or depolarization. The slope conductance was 25 pS. The extrapolated reversal potential (138 mV) agrees with the calculated equilibrium potential for Ca ( 1 58 mV). The Ca to Na permeability ratio exceeded 2,800. In four patches stimulated by Bay K 8644 and PK A(cs), bath nifedipine reduced P(o) from 1.03 to 0.15% at -63 mV. These patch-clamp data demonstrate a selective, 25-pS, cAMP/PKA(cs)-sensitive Ca channel in apical membranes of CNT. P(o) is stimulated by PK A(cs) and dihydropyridine (DHP) agonist, but inhibited by DHP antagonist and by depolarization. The data are consistent with the potential role of apical membrane Ca channel in epithelial Ca transport.