Differential effects of phorbol ester (PMA) on blocker-sensitive ENaCs of frog skin and A6 epithelia

Activation of protein kinase C with phorbol 12-myristate 13-acetate (PMA) caused complex transient perturbations of amiloride-sensitive short-circuit Na+ currents (I-Na) in A6 epithelia and frog skins that were tissue and concentration dependent. A noninvasive channel blocker pulse method of noise analysis (18) was used to investigate how PMA caused time-dependent changes of apical membrane epithelial Na+ channel (ENaC) single-channel currents, channel open probabilities (P-o), and channel densities (N-T) In A6 epithelia, 5 and 50 nM PMA caused within 7 min concentration-dependent sustained decreases of P-o (similar to 55% below control, 50 nM) and rapid compensatory transient increases of N-T within 7 min (similar to 220% above control, 50 nM), resulting in either small transient increases of I-Na at 5 nM PMA or small biphasic decreases of I-Na at 50 nM PMA. In contrast to A6 epithelia, 50 and 500 nM: PMA in frog skin caused after a delay of at least 10 min transient increases of N-T to similar to 60-70% above control at 30-60 min. Unlike A6 epithelia, P-o was increased similar to 15% above control within 7 min and remained within +/-10-15% of control for the duration of the 2-h experiments. Despite differences in the time courses of secondary inhibition of transport in A6 epithelia and frog skin, the delayed downregulation of transport was due to time-dependent decreases of NT from their preelevated levels in both tissues. Whereas P-o is decreased within minutes in A6 epithelia as measured by noise analysis or by patch clamp (8), the discrepancy in regulation of N-T in A6 epithelia as measured by noise analysis and patch clamp is most likely explained by the inability of on-cell patches formed before treatment of tissues with PMA to respond to regulation of their channel densities.