The impact of phorbol ester on the regulation of amiloride-sensitive epithelial sodium channel in alveolar type ii epithelial cells.

Amiloride-sensitive sodium channel (ENaC) plays an important role in recovery from pulmonary edema. Recently, it has been shown that an activation of protein kinase C (PKC) could affect the mRNA expression of ENaC in rat parotid gland cells and A6 distal nephron epithelial cells. To determine whether an activation of PKC would regulate the mRNA expression or the function of ENaC, we stimulated rat alveolar type II epithelial cells with phorbol 12-myristate 13-acetate (PMA), a potent PKC activator, at a concentration of 100 nM. The mRNA expression of alpha-, beta-, and gamma-ENaC subunits and amiloride-sensitive current were measured. PMA inhibited the mRNA expression of all 3 ENaC subunits (alpha-ENaC: 56.0% +/- 12.1%; beta-ENaC: 62.6% +/- 15.9%; gamma-ENaC: 68.5% +/- 10.6%, respectively) and amiloride-sensitive current (control = 7.0 +/- 1.5 microA/cm(2); PMA = 1.7 +/- 0.9 microA/cm(2)) significantly at 24 hours. On the other hand, 4alpha-phorbol didecanoate 4alpha-PDD, inactive form of PMA, had no inhibitory effect on alpha- and gamma-ENaC expression or amiloride-sensitive current. However, no significant difference was seen in beta-ENaC expression between PMA and 4alpha-PDD. GF 109203X, a wide-range PKC inhibitor, blocked the inhibitory effect of PMA on all ENaC subunits mRNA expression. These results suggest that an activation of PKC may play an important role in the regulation of ENaC mRNA expression and function.