pH effect on osmotic response of collecting tubules to vasopressin and 8-CPT-cAMP

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.