EFFECTS OF PROTEIN-KINASE-C MODULATORS ON NA+ K+ ADENOSINE-TRIPHOSPHATASE ACTIVITY AND PHOSPHORYLATION IN AORTAE FROM RATS WITH CIRRHOSIS/

Protein kinase C (PKC) modulates the activity and phosphorylation of t he catalytic alpha-subunit of sodium-potassium-adenosine triphosphatas e (Na+/K+ ATPase) in normal arteries. Because PKC is altered in cirrho tic aortae, Na+/K+ ATPase may also be altered in these arteries. The a im of the present study was to investigate alpha-subunit activity and phosphorylation in aortae from normal and cirrhotic rats, under baseli ne conditions and during exposure to PKC modulators. alpha-Subunit act ivity was assessed by measuring the amount of P-32 released by hydroly sis of [gamma-P-32]ATP in freshly isolated cell membranes tin the abse nce of PKC modulators only) and membrane depolarization caused by ouab ain-induced alpha-subunit inhibition in isolated aortae tin the absenc e and presence of PKC modulators). alpha-Subunit phosphorylation was a ssessed by incorporation of P-32 into alpha-subunits, Staurosporine, a PKC inhibitor, and phorbol 12,13-dibutyrate (PDBU), a PKC activator, were used. In addition, alpha-subunit expression was studied by Wester n blot analysis. In the absence of PKC modulators, the amount of P-32 released by hydrolysis of [gamma-P-32]ATP and ouabain-induced membrane depolarization were significantly lower in cirrhotic than in normal a ortae. Staurosporine suppressed ouabain-induced membrane depolarizatio n in cirrhotic and normal arteries. Ouabain-induced membrane depolariz ation was similar in cirrhotic aortae exposed to PDBU and in normal ar teries studied under baseline conditions. alpha-Subunit phosphorylatio n was significantly lower in cirrhotic than in normal aortae, in aorta e under baseline conditions, and in arteries exposed to staurosporine, Phosphorylation of the alpha-subunit was similar in cirrhotic aortae exposed to PDBU and in normal arteries under baseline conditions. West ern blot analysis showed that the amount of alpha-subunit did not sign ificantly differ between cirrhotic and normal aortae. In conclusion, a decrease in baseline Na+/K+ ATPase alpha-subunit activity occurs in a ortae from cirrhotic rats as a result of reduced basal PKC activity. T his PKC-dependent decreased alpha-subunit activity may be caused by a reduction in PKC-induced alpha-subunit phosphorylation.