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+ transport by increasing the number of conducting amiloride-sensitive 4-pS Na+ channels at the apical membrane. To study a possible role of protein tyrosine kinase (PTK) in the hyposmolality-induced signaling, we investigated effects of PTK inhibitors on the hyposmolality-induced Na+ transport in A6 cells. Tyrphostin A23 (a PTK inhibitor) blocked the stimulatory action of hyposmolality on a number of the conducting Na+ channels. Tyrphostin A23 also abolished macroscopic Na+ currents (amiloride-sensitive short-circuit current, 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 intracellular translocation of protein, blocked the action of hyposmolality on I-Na by diminishing the elevating rate of the hyposmolality-increased I-Na, mimicking the inhibitory action of PTK inhibitor. Further, hyposmolality increased the activity of PTK. These observations suggest that hyposmolality would stimulate Na+ transport by translocating the Na+ channel protein (or regulatory protein) to the apical membrane via a PTK-dependent pathway. Further, hyposmolality also caused an increase in the plasma (apical) membrane capacitance, which was remarkably blocked by treatment with tyrphostin A23 or BFA. These observations also suggest that a PTK-dependent pathway would be involved in the hyposmolality-stimulated membrane fusion in A6 cells.