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 (∼55% below control, 50 nM) and rapid compensatory transient increases of N T within 7 min (∼220% above control, 50 nM), resulting in either small transient increases of I Naat 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 ∼60-70% above control at 30-60 min. Unlike A6 epithelia, P o was increased ∼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 N T 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.