Cytosolic Ca2+ and Na+ activities in perfused proximal tubules of Necturus kidney

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Abstract
To study the role of intracellular Ca in the regulation of epithelial transport of ions and H2O, cytosolic Ca ion activity .**GRAPHIC**. and cytosolic Na ion activity .**GRAPHIC**. were measured in cells of isolated perfused proximal tubules of Necturus kidney. .**GRAPHIC**. was measured with Ca2+-selective microelectrodes, .**GRAPHIC**. with Na+-selective microelectrodes. Under control conditions, i.e., Ringer solution on both sides of the epithelium, .**GRAPHIC**. averaged 71 .+-. 7 (SE) nM (n = 21) and .**GRAPHIC**. was 12.9 .+-. 0.6 mmol (n = 56). When peritubular bath Na was reduced from 100 to 10 mM by choline substitution, .**GRAPHIC**. increased from 73 .+-. 14 to 382 .+-. 69 nmol (paired t test; P < 0.001; n = 4); in different tubules, .**GRAPHIC**. decreased from 12.8 .+-. 1.9 to 8.2 .+-. 1.8 mM (P < 0.001; n = 12). Quinidine (10-4 M) increased .**GRAPHIC**. from 87 .+-. 19 to 556 .+-. 121 nM (P < 0.02; n = 5) but reduced .**GRAPHIC**. from 15.1 .+-. 1.2 to 11.8 .+-. 0.8 mM (P < 0.003; n = 8). Ouabain (10-4 M) increased both .**GRAPHIC**. and .**GRAPHIC**. .**GRAPHIC**. rose from 71 .+-. 9 to 546 .+-. 121 nmol (P < 0.005; n = 9) and .**GRAPHIC**. from 15.1 .+-. 1.8 to 70.1 .+-. 6.3 mM (P < 0.001; n = 9). The results are consistent with the existence of a Na-Ca exchange process within the contraluminal cell membrane and with the view that increased .**GRAPHIC**. inhibits the tubular transport of Na by decreasing the Na permeability of the luminal cell membrane.