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 β-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 (10⁻³ 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 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, 4α-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.