Calcium-dependent control of volume regulation in renal proximal tubule cells: II. Roles of dihydropyridine-sensitive and-insensitive Ca2+ entry pathways |
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Authors: | McCarty Nael A O'Neil Roger G |
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Institution: | (1) Department of Physiology and Cell Biology, University of Texas Medical School, 77030 Houston, Texas;(2) Present address: Division of Biology, California Institute of Technology, Mail Code 156-29, 91125 Pasadena, California |
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Abstract: | Summary 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 M caused intracellular free calcium concentration (Ca2+]i) to fall close to zero. Treatment with 10 M 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 M 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 Ca2+-dependent mechanism by which cell volume regulation is achieved. |
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Keywords: | intracellular calcium volume regulation regulatory volume decrease nifedipine dihydropyridine verapamil calcium fluctuation renal proximal tubule kidney tubule |
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