PORCINE VENTRICULAR ENDOCARDIAL CELLS IN CULTURE EXPRESS THE INDUCIBLE FORM OF NITRIC-OXIDE SYNTHASE

1 We have investigated whether porcine endocardial cells in culture express the inducible, Ca2+-independent form of nitric oxide (NO) synthase. 2 NO synthase activity in cytosolic extracts of endocardial cells was measured by estimation of the rate of formation of L-[C-14]-citrulline from L-[C-14]-arginine. 3 Treatment of the cells in culture with lipopolysaccharide or cytokines induced a Ca2+-independent NO synthase activity in the cell cytosol. The combination of tumour necrosis factor (TNFalpha, 10 ng ml-1) and interleukin-1beta (IL-1beta, 10 ng ml-1) induced the greatest enzyme activity. 4 The increased Ca2+-independent NO synthase activity following exposure to cytokines was parallelled by an increase in guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels in the endocardial cell cytosol. 5 Simultaneous addition of dexamethasone (0.01 - 1 muM) or cycloheximide (0.03 - 3 muM) inhibited in a concentration-dependent manner TNFalpha- and IL-1beta-induced expression of Ca2+-independent NO synthase activity. Neither dexamethasone (1 muM) nor cycloheximide (3 muM) had any effect on the activity of the constitutive NO synthase. 6 The possible pathophysiological consequences of endocardial expression of the inducible NO synthase are discussed.