Phorbol Ester Activation of the Rat Vascular Myocyte Na + -H + Exchanger Isoform 1

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Abstract
Vascular myocytes from the spontaneously hypertensive rat (SHR) demonstrate elevated Na + -H + exchanger activity associated with increased cell proliferation and hyperresponsiveness to agonists such as phorbol esters. Since the Na + -H + exchanger isoform 1 (NHE-1) is stimulated by protein kinase C, we have investigated the effects of phorbol esters on NHE-1 activity and its phosphorylation in vascular myocytes of these rats. SHR cells demonstrated a larger alkalinization response to 12- O -tetradecanoylphorbol 13-acetate than Wistar-Kyoto rat (WKY) cells. Kinetic analyses indicated that whereas 12- O -tetradecanoylphorbol 13-acetate increased the maximal transport capacity of NHE-1 in both cell types, affinity for H + was increased in WKY cells and cooperativity for H + at the internal modifier site was reduced in SHR cells. In neither cell type was the subcellular distribution of NHE-1 altered by phorbol ester stimulation. NHE-1 phosphorylation was markedly reduced in WKY cells stimulated by the phorbol ester, an effect abolished by inhibition of protein kinase C. In contrast, NHE-1 phosphorylation in quiescent SHR cells was approximately double that of WKY cells and was reduced after phorbol ester treatment. Inhibition of protein kinase C in SHR cells led to a marked elevation of NHE-1 phosphorylation that was not associated with a change in exchanger activity, but WKY cells exhibited a small, insignificant rise in NHE-1 phosphorylation. Thus, the kinetic responses of NHE-1 to phorbol esters in vascular myocytes of these rat strains are different, the changes in exchanger kinetics of SHR resembling those described in human hypertension. NHE-1 phosphorylation has an inverse relationship with protein kinase C activity. However, modulation of NHE-1 phosphorylation may not be associated with concurrent alterations in activity, indicating a role for non–phosphorylation-dependent mechanisms.