In the present study we have examined the action of the phorbol diester tetradecanoyl phorbol acetate, an activator of protein kinase C, on the transepithelial transport of sodium, chloride and water and the production of cAMP in the isolated frog skin epithelium (Rana esculenta).
Addition of tetradecanoyl phorbol acetate to the mucosal solution resulted initially in an increase in the short-circuit current, which was followed by a progressive decrease. If the short-circuit current was first activated by addition of the antidiuretic hormone, arginine vasotocin, then the addition of tetradecanoyl phorbol acetate resulted only in a pronounced inhibition. The changes in the short-circuit current were the result of changes in the active influx of Na+.
The effect of tetradecanoyl phorbol acetate on the intracellular potential measured under short-circuited conditions (Vscc) was time-dependent. Just after addition of tetradecanoyl phorbol acetate to the mucosal solution. Vscc depolarized; this was followed by a slight hyperpolarization, after which Vscc continued to decline.
The inhibition of the Na+ transport by tetradecanoyl phorbol acetate was associated with a decline in the response to the antidiuretic hormone (arginine vasotocin), but the ability of arginine vasotocin to increase the cellular level of cAMP and to stimulate the osmotic water flow was not affected by the presence of tetradecanoyl phorbol acetate.
In skin halves in which the short-circuit current was stimulated with arginine vasotocin, addition of tetradecanoyl phorbol acetate resulted in a dose-dependent inhibition of the short-circuit current, but only minor changes in Vscc were observed.
The results presented suggest that the addition of tetradecanoyl phorbol acetate to the isolated frog skin first increases and then decreases the arginine vasotocin-sensitive sodium permeability of the apical membrane. This might be due to a stimulating effect of tetradecanoyl phorbol acetate on both the activation and deactivation (turnover) of the sodium channels.