title: Calcium-dependent control of volume regulation in renal proximal tubule cells: II. Roles of dihydropyridine-sensitive and -insensitive Ca2+ entry pathways.
abstract: 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.
authors.affiliation: Department of Physiology and Cell Biology, University of Texas Medical School, Houston 77030.
authors.forename: N A
authors.initials: NA
authors.lastname: McCarty
authors.forename: R G
authors.initials: RG
authors.lastname: O'Neil
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000493:pharmacokinetics / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D002463:Cell Membrane Permeability / Q000187:drug effects / Q000502:physiology; D004095:Dihydropyridines / Q000494:pharmacology*; D007038:Hypotonic Solutions; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007687:Kidney Tubules, Proximal / Q000166:cytology* / Q000187:drug effects / Q000502:physiology; D008856:Microscopy, Fluorescence; D009543:Nifedipine / Q000494:pharmacology; D011817:Rabbits; D014700:Verapamil / Q000494:pharmacology
chemical_list: D015220:Calcium Channels; D004095:Dihydropyridines; D007038:Hypotonic Solutions; D014700:Verapamil; D009543:Nifedipine; D002118:Calcium