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

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 beta-phorbol-12-myristate-13-acetate (PMA) on the rates of bicarbonate absorption (JHCO3) and fluid absorption (Jv) 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) approximately 10(-5) M) within 10 min caused a significant increase of JHCO3 and Jv. 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(3) M) 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 inhibit JHCO3 and Jv. Amiloride (10(-3) M) or EIPA (10(-5) M) significantly inhibits JHCO3 and Jv, while there is no additive effect of PMA and amiloride or EIPA on PCT transport. An inactive phorbol-ester, 4 alpha-phorbol, that does not activate protein kinase C, had no effects on JHCO3 and Jv. Capillary perfusion of PMA (10(-8) M) significantly stimulate both JHCO3 and Jv; 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.