Abstract

Cellular constituents of heart muscle contain both constitutive and inducible nitric oxide (NO) signaling pathways that modulate the contractile properties of cardiac myocytes. The identities of the inducible NO synthase (iNOS) isoform(s) expressed in cardiac muscle, and of the specific cell types expressing iNOS activity, remain poorly characterized. We amplified a 217-base pair cDNA by reverse transcriptase-polymerase chain reaction from primary cultures of inflammatory cytokine-pretreated adult rat ventricular myocytes (ARVM) that was nearly identical to other iNOS cDNA sequences. Using this 217-base pair cDNA as a probe in Northern blots, we found no evidence of iNOS mRNA in control myocytes, but both interleukin-1 beta and interferon-gamma individually increased iNOS mRNA abundance in primary cultures of ARVM, with maximal expression at 12 h. The half-life of iNOS mRNA in actinomycin C1-treated cells was 4 h. Both dexamethasone and transforming growth factor-beta attenuated the induction of iNOS mRNA abundance and enzyme activity by IL-1 beta and INF gamma. Pretreatment with dexamethasone also abolished the induction of iNOS mRNA, but not the increase in GTP cyclohydrolase mRNA in purified cardiac myocytes from lipopolysaccharide-injected rats. In order to further characterize the specific cell type producing NO, we used a NO-specific porphyrinic/Nafion-coated microsensor to record NO release from a single, isolated ARVM pretreated with IL-1 beta and IFN gamma in L-arginine-depleted medium. NO release could be detected following microinjection of L-arginine in the vicinity of the cell juxtaposed to the NO microsensor, but not following microinjection of D-arginine, and not from ARVM pretreated with L-N-monomethylarginine. Cytokine-pretreated ARVM that had been maintained in L-arginine-depleted medium also exhibited a depressed contractile response to isoproterenol after addition of L-arginine, but not D-arginine. These results indicate that altered contractile function of cardiac myocytes following exposure to specific inflammatory cytokines is due to induction of myocyte iNOS.