Protein kinase C regulation of rat jejunal transport systems: mechanisms involved in bicarbonate absorption

We examined whether PKC modulates the transport systems involved in bicarbonate movements across the plasma membranes of rat jejunum. Results of enzymatic assays give evidence that under basal conditions cPKC is present in both basolateral (BLM) and apical (BB) membranes of the enterocyte. In BLM the basal expression of the kinase is significantly lower than in BB; however treatment with Ca2+ and phorbol 12-myristate 13-acetate (PMA) causes a significant increase, thus suggesting an asymmetrical kinase translocation. To explore the effect of PKC activation on membrane-bound transport mechanisms, “in vitro” phosphorylated membrane vesicles were used to perform uptake studies. Results suggest that PKC activation exerts an inhibitory effect on basolateral Cl-/HCO3- antiport, whereas basolateral HCO3- conductive pathway seems to be stimulated and Cl- conductance unaffected. Apical, but not basolateral, Na+/H+ exchanger is inhibited by PKC activation. The specificity of the response to PKC was confirmed by using staurosporine or 4-a-PMA. The inhibition of both apical Na+/H+ and basolateral Cl-/HCO3- exchange activities suggests that the overall action of PKC determines a reduction of transepithelial bicarbonate transport.