Acute hormonal modulation of NHE3 activity is partly mediated by kinases, including protein kinase C (PKC). We examined the role of NHE3 phosphorylation in regulating its activity in response to PKC activation by phorbol 12-myristate 13-acetate (PMA). In pooled NHE-deficient fibroblasts transfected with NHE3, PMA increased NHE3 activity and phosphorylation. When six potential PKC target serines were mutated, NHE3 phosphorylation was drastically reduced and PMA failed to regulate NHE3 phosphorylation or function. To examine whether NHE3 phosphorylation is sufficient for functional regulation by PKC, we exploited the heterogeneous response of NHE3 activity to PMA in individual clones of transfectants. Clones with stimulatory, inhibitory, or null responses to PMA were observed. Despite the diverse functional response, changes in NHE3 phosphorylation as revealed by tryptic phosphopeptide maps were similar in all clones. We conclude that although phosphorylation appears to be necessary, it is insufficient to mediate PKC regulation of NHE3 function and factors extrinsic to the NHE3 protein must be involved.

proximal tubule; hormones; sodium chloride transport; sodium bicarbonate transport

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