Calcium-dependent control of volume regulation in renal proximal tubule cells: II. Roles of dihydropyridine-sensitive and -insensitive Ca2+ entry pathways.

The Ca2+ entry pathways in the basolateral plasma membrane of the isolated, nonperfused proximal straight tubule (PST) of rabbit kidney were investigated using fura-2 fluorescence microscopy. Under isotonic conditions, reduction of bath [Ca2+] from 1 mM to 1 microM caused intracellular free calcium concentration ([Ca2+]i) to fall close to zero. Treatment with 10 microM verapamil, a calcium channel blocker, had a similar effect. Treatment with verapamil or low Ca2+ also induced fluctuations in cell volume. However, isotonic treatment with 10 microM nifedipine, a dihydropyridine (DHP)-type calcium channel blocker, did not affect [Ca2+]i or cell volume, indicating that the endogenous Ca2+ entry pathway is verapamil-sensitive but DHP-insensitive. When cells were exposed to hypotonic solutions in the presence of 1 mM Ca2+, they swelled and underwent normal RVD while [Ca2+]i increased transiently to a peak before decreasing to a late phase plateau level above the baseline level (see McCarty, N.A., O'Neil, R.G. 1991. J. Membrane Biol. 123:149-160). When cells were swollen in the presence of verapamil or low bath [Ca2+], RVD was abolished and [Ca2+]i fell well below the baseline during the late phase response. In contrast, when cells were swollen in the presence of nifedipine, RVD and the late phase rise in [Ca2+]i were abolished, but [Ca2+]i did not fall below the baseline level in the late phase, indicating that nifedipine inhibited the swelling-induced Ca2+ entry but that Ca2+ entry by another pathway was undisturbed. It was concluded that PST cells are characterized by two Ca2+ permeability pathways in the basolateral membrane. Under both isotonic and hypotonic conditions, Ca2+ entry occurs at a slow rate via a verapamil-sensitive, DHP-insensitive "baseline" Ca2+ entry pathway. Cell swelling activates a separate DHP-sensitive, verapamil-sensitive Ca2+ entry pathway, which is responsible for the supply of Ca ions to the Ca(2+)-dependent mechanism by which cell volume regulation is achieved.