To study the role of intracellular calcium in the regulation of epithelial transport of ions and water, cytosolic calcium ion activity (aiCa) and cytosolic sodium ion activity (aiNa) were measured in cells of isolated perfused proximal tubules of Necturus kidney. aiCa was measured with Ca2+-selective microelectrodes, aiNa with Na+-selective microelectrodes. Under control conditions, i.e., Ringer solution on both sides of the epithelium, aiCa averaged 71 +/- 7 (SE) nM (n = 21) and aiNa was 12.9 +/- 0.6 mmol (n = 56). When peritubular bath sodium was reduced from 100 to 10 mM by choline substitution, aiCa increased from 73 +/- 14 to 382 +/- 69 nmol (paired t test; P less than 0.001; n = 4); in different tubules, aiNa decreased from 12.8 +/- 1.9 to 8.2 +/- 1.8 mM (P less than 0.001; n = 12). Quinidine (10(-4) M) increased aiCa from 87 +/- 19 to 556 +/- 121 nM (P less than 0.02; n = 5) but reduced aiNa from 15.1 +/- 1.2 to 11.8 +/- 0.8 mM (P less than 0.003; n = 8). In contrast, 10(-4) M ouabain increased both aiCa and aiNa; aiCa rose from 71 +/- 9 to 546 +/- 121 nmol (P less than 0.005; n = 9) and aiNa from 15.1 +/- 1.8 to 70.1 +/- 6.3 mM (P less than 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 aiCa inhibits the tubular transport of sodium by decreasing the sodium permeability of the luminal cell membrane.

Antidiuretic hormone (ADH) increased the electrical capacitance of apical membrane of the toad bladder; this effect was modulated by the osmotic gradient across the tissue. Capacitance was measured from the transepithelial voltage response to constant-current pulses using bladders depolarized with KCl-sucrose serosal solution to reduce basolateral resistance and with Na-free mucosal solution to increase apical membrane resistance. Addition of ADH (20 mU/ml) increased capacitance by 28 +/- 9% (mean +/- SD) in the absence and by 8 +/- 3% in the presence of an osmotic gradient (200 mosM, mucosal side hypotonic). With bladders stimulated in the absence of an osmotic gradient, rapidly imposing a gradient resulted in a peak rate of water flow that declined to 40% of the peak value after 15-20 min. ADH-dependent capacitance also decreased with a similar time course. Removal of ADH reversed the capacitance change (t1/2 = 10-15 min), but the reversal was slower than the decline in water flow to basal levels (t1/2 less than 5 min). Colchicine and cytochalasin B also inhibited the ADH-induced capacitance increase. The capacitance change was also inhibited when the mucosal solution was made hypertonic with raffinose. The results are interpreted within the framework of a previously proposed model of ADH-stimulated water transport in which cytoplasmic vesicular structures fuse with the apical plasma membrane.

Pancreatic polypeptide (PP) has been considered to be predominantly under cholinergic-vagal control and may therefore serve as an indicator of vagal tone. We found similar basal plasma concentrations of PP in athletes with bradycardia and in untrained subjects. We further observed that physical exercise--an adrenergic situation--induces a several-fold rise in PP plasma concentration. Maximal plasma PP levels correlated positively with maximal blood pressure. The exercise-induced rise of PP was completely abolished by propranolol. These findings suggest that stimulation of the adrenergic system releases PP and that therefore plasma PP concentrations cannot be used to determine cholinergic-vagal activity.

The effect of peritubular and luminal pH changes on hydraulic conductance, (Lp, 10(-7) cm X s-1 X atm-1) in the isolated perfused rabbit cortical collecting tubule (CCT) was tested at 37 degrees C before and after administration of 20 microU/ml vasopressin or 10(-4) M 8-[p-chlorophenylthio]-adenosine cyclic monophosphate (8-CPT-cAMP). In vasopressin experiments when bath pH was changed from 7.58 to 7.16 or from 7.58 to 6.70, mean Lp decreased from 249 +/- 32 to 199 +/- 23 (n = 5; P less than 0.01) and from 231 +/- 38 to 201 +/- 36 (n = 5; NS), respectively. In contrast, in 8-CPT-cAMP experiments when bath [HCO3] was kept constant while CO2 was elevated the hydroosmotic response was increased. Using 2.5 mM HCO3, Lp at 0.4% CO2 was 275 +/- 15 and at 6% CO2 it was 352 +/- 50 (n = 4; paired t test; P less than 0.05). At 8.5 and 21.5 mM HCO3 raising CO2 from 2 to 13% and from 4 to 32% increased Lp from 237 +/- 71 to 410 +/- 32 (n = 4; paired t test; P less than 0.05) and from 282 +/- 45 to 449 +/- 63 (n = 6; paired t test; P less than 0.001), respectively. Reducing luminal pH from 7.40 to 5.40 had no effect on either vasopressin- or cAMP-induced changes in Lp. Accordingly, lowering the bath pH by increasing the PCO2 at constant [HCO3] markedly stimulates the response to 8-CPT-cAMP, whereas lowering the bath pH by reducing [HCO3] inhibits the vasopressin response.

Quinidine, a compound thought to increase cytosolic calcium ion activity, has been found to inhibit the hydrosmotic response to vasopressin (VP) and adenosine 3',5'-cyclic monophosphate (cAMP) in the toad urinary bladder. To test whether this drug has a similar action in the mammalian nephron, the effect of quinidine on the hydraulic conductivity of the isolated perfused rabbit cortical collecting tubule (CCT) exposed to either 20 microU/ml VP or 10(-4) M 8-(p-chlorophenylthio) - adenosine 3',5' - cyclic monophosphate (8-CPT-cAMP) was studied. Quinidine had no effect on the basal water permeability of the CCT. Quinidine sulfate (10(-4) M) reduced the VP-stimulated water permeability from 280 +/- 50 X 10(-7) to 115 +/- 41 X 10(-7) cm X s-1 X atm-1 (P less than 0.05). The hydrosmotic response to 8-CPTcAMP was likewise reduced following exposure to quinidine. This effect was shown to be dose dependent. In paired experiments, inhibition of the response to 10(-4) M 8-CPTcAMP averaged 11% at 10(-6) M, 27% at 5 X 10(-6) M, 53% at 5 X 10(-5) M, and 50% at 10(-4) M quinidine. Inhibition of the response to 8-CPTcAMP was estimated to be half maximal at approximately 5 X 10(-6) M quinidine. Tubules were protected against the quinidine-induced inhibition by the addition of 6.5 X 10(-5) M quin 2-acetoxymethylester in the presence of low peritubular Ca concentration. These results are consistent with the view that elevated cytosolic Ca ion levels inhibit the increase in water permeability elicited by VP or exogenous cAMP in the mammalian CCT.