In vitro effects of E coli endotoxin on K+-activated para-nitrophenylphosphatase activity and ouabain binding in dog hearts.

The in vitro effect of E coli endotoxin on the activity of K+-activated paranitrophenylphosphatase (K+-PNPPase), an enzyme which represents the partial activity of (Na+ + K+)-ATPase enzyme system, was studied in isolated adult dog heart myocytes. The results were correlated with ouabain-binding studies. Endotoxin had an inhibitory effect on the Vmax for K+ activation as well as the Vmax for Mg++ and para-nitrophenylphosphate (PNPP) saturation. The inhibition was concentration-dependent and noncompetitive with K+, Mg++, and PNPP and furthermore, reversible. Endotoxin did not displace the bound 3H-ouabain from receptor sites nor did it affect the capacity of ouabain binding, indicating that the total enzyme concentration was not altered. From these findings, it is concluded that endotoxin in vitro affects the myocardial (Na+ + K+)-ATPase enzyme system by decreasing the turnover number of the enzyme molecule. The ability of endotoxin to modify myocyte membrane-associated enzyme activity may be responsible for altered heart metabolism and function in endotoxemia.