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Activators of protein kinase A and of protein kinase C inhibit MDCK cell myo-inositol and betaine uptake.

Amino acid sequences of the myo-inositol and betaine cotransporters that are induced in MDCK cells by hypertonicity include consensus sequences for phosphorylation by protein kinase A and by protein kinase C. To test for the effect of activation of protein kinases A and C on the activity of those cotransporters, MDCK cells were exposed to activators of each kinase and the activity of both cotransporters was assayed. Incubation with 8-bromoadenosine 3':5'-cyclic monophosphate (8Br-cAMP) or 3-isobutyl-1-methylxanthine (IBMX), activators of protein kinase A, and incubation with an active phorbol ester or with an active diacylglycerol, activators of protein kinase C, inhibited the activity of both cotransporters by about 30%. The relative effect of the activation of protein kinase A and of protein kinase C was similar in hypertonic and isotonic cells. The effects of activators of protein kinase A and of protein kinase C were not additive. The two cotransporters behaved differently when protein kinase C activity was down-regulated by prolonged incubation with a higher concentration of phorbol 12-myristate 13-acetate. There was a doubling of activity of the myo-inositol cotransporter and no change in the activity of the betaine cotransporter in hypertonic and isotonic cells. Although the mechanisms of the effects of activation of the two kinases remain to be established, it is clear that the kinases can mediate post-translational regulation of the uptake of compatible osmolytes.