Involvement of Protein Tyrosine Kinase in Osmoregulation of Na⁺ Transport and Membrane Capacitance in Renal A6 Cells

Abstract.

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.