A REGULATED CALCIUM-CHANNEL IN APICAL MEMBRANES OF RENAL PROXIMAL TUBULE CELLS

Using the single-channel patch-clamp technique, we identified a Ca2+ c hannel in the apical membranes of rabbit cultured proximal tubule cell s. The channel is permeable to both Ca2+ and Ba2+ but not to monovalen t cations. In on-cell patches, the channel opened infrequently and had a conductance of 4.6 +/- 0.9 pS (n = 5) with 105 mM CaCl2 in the pipe tte. Although addition of forskolin (12.5 mu M) was without effect, ad dition of phorbol 12-myristate 13-acetate (PMA, 1 mu M) activated the channel. At 0 mV pipette voltage (resting state) in the on-cell patche s, PMA increased open probability (P-o) from 0 to 6.9 +/- 2.3% (n = 5) within 1-3 min of PMA application. Likewise, stretching the membrane patch (-10 to -30 mmHg) activated this channel (P-o increased to 5.3 /- 2.1%, n = 3, at 0 mV applied pipette potential), with results consi stent with a mechanosensitive channel. The channel displayed only mode st voltage sensitivity, with mild activation on membrane hyperpolariza tion but with inactivation on strong depolarization. The addition of t he L-type Ca2+ channel blocker (antagonist), nifedipine (10 mu M), com pletely blocked this channel in both on-cell and inside-out patches, w hereas the agonist, BAY K 8644 (5 mu M) was without effect. It is conc luded that this channel is a nifedipine-sensitive, protein kinase C-re gulated Ca2+ channel and that it may play a role in Ca2+ signaling in the proximal tubule cells, particularly during periods of mechanical s tress.