An L-type calcium channel in renal epithelial cells.

A voltage-activated Ca++ channel has been identified in the apical membranes of cultured rabbit proximal tubule cells using the patch-clamp technique. With 105 mm CaCl2 solution in the pipette and 180 NaAsp in the bath, the channel had a conductance of 10.4 +/- 1.0 pS (n = 8) in on-cell patches, and 9.8 +/- 1.1 pS (n = 8) in inside-out patches. In both on-cell and inside-out patches, the channel is active by membrane depolarization. For this channel, the permeation to Ba++ and Ca++ is highly selective over Na+ and K+ (PCa(Ba):PNa(K) >200:1). The sensitivity to dihydropyridines is similar to that for L-type channels where the channel was blocked by nifedipine (10 microM), and activated by Bay K 8644 (5 microM). When activated by Bay K 8644, the channel showed subconductance levels. Treatment with forskolin (12.5 microM), phorbol ester (1 microM), or stretching (40 cm water) did not activate this channel. These results indicate that this Ca++ channel is mostly regulated by membrane voltage, and appears to be an epithelial class of L-type Ca++ channel. As such, it may participate in calcium reabsorption during periods of enhanced sodium reabsorption, or calcium signaling in volume regulation, where membrane depolarization occurs for prolonged periods.