Volume regulatory responses in frog isolated proximal cells.

Cells respond to increase in volume by activating solute efflux pathways, resulting in water loss and restoration of the original cell volume. The solute efflux pathways underlying these volume regulatory decrease (VRD) responses have been relatively well studied. However, the transduction pathways whereby the change in cell volume is converted into an intracellular signal resulting in VRD are much less well understood. We have examined VRD in isolated proximal tubule cells from the frog, with particular attention to the roles of stretch-activated channels, Ca2+ and protein kinases. Cell length was taken as an index of cell volume, and was measured continuously using a photodiode array. VRD was observed in approximately 50% of cells, and was inhibited by Ba2+, Gd3+ and 4,4'-diisothiocyanatostilbene 2,2'-disulphonic acid (DIDS), and removal of extracellular Ca2+. VRD was accelerated by the active phorbol ester, phorbol 12-myristate 13-acetate (PMA), and the phosphatase inhibitor F-; on the other hand, VRD was prolonged by 4 alpha-phorbol 12,13-didecanoate (PDC), an inactive phorbol ester), and inhibited by PMA and Gd3+, PMA and 0 Ca2+, and staurosporine. Volume regulation was unaffected by di-butyryl cAMP and 3-isobutyl-1-methyl-xanthene (IBMX). These data suggest that Ca2+ and PKC, via protein phosphorylation, play a stimulatory role in VRD.