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 mem-branes 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 in-creased the open-state probability (P.) to 0.6%. In excised, inside-out patches, P. was low spontaneously and remained low during either bath protein kinase A catalytic subunit (PKA3) or Bay K 8644. Exposure to both agonists, however, unmasked Ca channels previously latent with only one, raising P. by 1.05 % at membrane potential of-70 mV. Mean P. for 14 seals (2.57%) peaked at-70 mV, declining with either hyperpolarization or depolarization. The slope con-ductance was 25 pS. The extrapolated reversal potential (138 mV) agrees with the calculated equilibrium potential for Ca ( 158 mV). The Ca to Na permeability ratio exceeded 2,800. In four patches stimulated by Bay K 8644 and PKA,, bath nifedi-pine reduced P. from 1.03 to 0.15 % at-63 mV. These patch-clamp data demonstrate a selective, 25-pS, cAMP/PKA,.-sen-sitive Ca channel in apical membranes ofCNT. P. is stimulated by PKA, and dihydropyridine (DHP) agonist, but inhibited by DHP antagonist and by depolarization. The data are consistent with the potential role ofapical membrane Ca channel in epithe