ACTIVATORS OF PROTEIN-KINASE-A AND OF PROTEIN-KINASE-C INHIBIT MDCK CELL MYOINOSITOL AND BETAINE UPTAKE

Amino acid sequences of the myo-inositol and betaine cotransporters th at are induced in MDCK cells by hypertonicity include consensus sequen ces for phosphorylation by protein kinase A and by protein kinase C. T o test for the effect of activation of protein kinases A and C on the activity of those cotransporters, MDCK cells were exposed to activator s 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 diacyl glycerol, activators of protein kinase C, inhibited the activity of bo th 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 prolo nged incubation with a higher concentration of phorbol 12-myristate 13 -acetate, There was a doubling of activity of the myo-inositol cotrans porter and no change in the activity of the betaine cotransporter in h ypertonic and isotonic cells, Although the mechanisms of the effects o f activation of the two kinases remain to be established, it is clear that the kinases can mediate post-translational regulation of the upta ke of compatible osmolytes.