ISOLATED RAT GLOMERULAR CELLS DEMONSTRATE L-TYPE CA2+-CHANNEL ACTIVITY

The presence of L-type calcium (Ca2+)-channels and the effects of Ca2+-channel antagonists on cells of rat glomeruli were investigated. Glomeruli were isolated by graded sieving and after preincubation (10 min) in zero Ca2+, the uptake of Ca-45(2+) by glomerular cells was measured. Depolarization with KCl (50 mM) or the dihydropyridine agonist Bay K 8644 (10 muM) stimulated Ca-45(2+) uptake by 13% and 24%, respectively, above control (100%), which was inhibited by nifedipine (Nif, 10 muM), P < 0.05, and by both S and R isomers of verapamil (Ver, 10 muM), P < 0.001. In a separate experimental preparation, isolated glomeruli were preloaded (45 min) with Ca-45(2+). Following a 45 min perifusion (37-degrees-C, CaCl2 1.26 mM, in the absence of Ca-45(2+)), both KCl (50 mM) and Bay K 8644 (10 muM) induced cellular Ca-45(2+) efflux with peak values above control of 11% and 15%, respectively, (P < 0.05). Exposure to Bay K 8644 preceded by depolarization with KCl resulted in enhanced Ca-45(2+) efflux identifying the presence of voltage-dependent Ca2+-channel activity. Cultured rat mesangial cells grown to confluence on coverslips were preloaded with Fura-2 and cytosolic Ca2+ was measured by microfluorometry. KCl (50 mM), gramicidin (2 muM) and/or Bay K 8644 (6 muM) stimulated Ca2+ influx which was inhibited by Ver (10 muM). Ver did not alter endothelin-stimulated Ca2+ signalling. We conclude that L-type Ca2+ channels are present on both rat glomerular (endothelial and/or mesangial) cells in vivo and on cultured mesangial cells, and their activation may be hormone specific.