Involvement of protein tyrosine kinase in osmoregulation of Na+ transport and membrane capacitance in renal A6 cells

Renal A6 cells have been reported in which hyposmolality stimulates Na+ tra nsport by increasing the number of conducting amiloride-sensitive 4-pS Nachannels at the apical membrane. To study a possible role of protein tyrosi ne kinase (PTK) in the hyposmolality-induced signaling, we investigated eff ects of PTK inhibitors on the hyposmolality-induced Na+ transport in A6 cel ls. Tyrphostin A23 (a PTK inhibitor) blocked the stimulatory action of hypo smolality on a number of the conducting Na+ channels. Tyrphostin A23 also a bolished macroscopic Na+ currents (amiloride-sensitive short-circuit curren t, I-Na) by decreasing the elevating rate of the hyposmolality-increased I- Na. Genistein (another type of PTK inhibitor) also showed an effect similar to tyrphostin A23, Brefeldin A (BFA), which is an inhibitor of intracellul ar translocation of protein, blocked the action of hyposmolality on I-Na by diminishing the elevating rate of the hyposmolality-increased I-Na, mimick ing the inhibitory action of PTK inhibitor. Further, hyposmolality increase d the activity of PTK. These observations suggest that hyposmolality would stimulate Na+ transport by translocating the Na+ channel protein (or regula tory protein) to the apical membrane via a PTK-dependent pathway. Further, hyposmolality also caused an increase in the plasma (apical) membrane capac itance, which was remarkably blocked by treatment with tyrphostin A23 or BF A. These observations also suggest that a PTK-dependent pathway would be in volved in the hyposmolality-stimulated membrane fusion in A6 cells.