Time- and dose-dependent effects of protein kinase C on proximal bicarbonate transport

Summary Activation of protein kinase C has been shown to cause both stimulation and inhibition of transport processes in the brush-border membrane and renal tubule. This study was designed to examine the dose-response nature and time-dependent effect of 4 -phorbol-12-myristate-13-acetate (PMA) on the rates of bicarbonate absorption (J_HCO3) and fluid absorption (J_v) in the proximal convoluted tubule (PCT) of rat kidney. Bicarbonate flux was determined by total CO₂ changes between the collected fluid and the original perfusate as analyzed by microcalorimetry. Luminal perfusion of PMA (10^–10 10^–5 M) within 10 min caused a significant increase ofJ_HCO3 andJ_v. A peaked curve of the dose response was observed with maximal effect at 10^–8 M PMA on both bicarbonate and fluid reabsorption, which could be blocked completely by amiloride (10^–3m) and EIPA (10^–5 M). On the other hand, with an increase of perfusion time beyond 15 min, PMA (10^–8 and 10^–6 M) could inhibitJ_HCO3 andJ_v. Amiloride (10^–3 M) or EIPA (10^–5 M) significantly inhibitsJ_HCO3 andJ_v, while there is no additive effect of PMA and amiloride or EIPA on PCT transport. An inactive phorbol-ester, 4-phorbol, that does not activate protein kinase C, had no effects onJ_HCO3 andJ_v. Capillary perfusion of PMA (10^–8 M) significantly stimulate bothJ_HCO3 andJ_v; however, PMA did not affect glucose transport from either the luminal side or basolateral side of the PCT. These results indicate that activation of endogenous protein kinase C by PMA could either stimulate or inhibit both bicarbonate and fluid reabsorption in the PCT dependent on time and dose, and these effects are through the modulation of Na⁺/H^– exchange mechanism.