ABNORMAL CONTRACTILE FUNCTION DUE TO INDUCTION OF NITRIC-OXIDE SYNTHESIS IN RAT CARDIAC MYOCYTES FOLLOWS EXPOSURE TO ACTIVATED MACROPHAGE-CONDITIONED MEDIUM

The mechanism by which soluble mediators of immune cell origin depress myocardial contractility, either globally as in systemic sepsis, or r egionally in areas of inflammatory myocardial infiltrates, remains unc lear. When freshly isolated ventricular myocytes from adult rat hearts were preincubated for at least 24 h in medium conditioned by endotoxi n (LPS)-activated rat alveolar macrophages, their subsequent inotropic response to the beta-adrenergic agonist isoproterenol was reduced fro m 225+/-19% to 155+/-10% of the baseline amplitude of shortening (mean +/-SEM, P < 0.05). Neither baseline contractile function nor the contr actile response to high extracellular calcium were affected. To determ ine whether an endogenous nitric-oxide (NO)-signaling pathway within v entricular myocytes was responsible for their decreased responsiveness to isoproterenol, the L-arginine analogue L-NMMA was added to the pre incubation medium. While L-NMMA did not affect baseline contractile fu nction or the response of control myocytes to isoproterenol, it comple tely restored the positive inotropic response to isoproterenol in myoc ytes preincubated in LPS-activated macrophage medium. Release of NO by ventricular myocytes following exposure to activated macrophage mediu m was detected as an increase in cGMP content in a reporter-cell (RFL- 6) bioassay and also as increased nitrite content in myocyte-condition ed medium. Thus, the depressed contractile response of adult rat ventr icular myocytes to beta-adrenergic agonists by a 24-h exposure to solu ble inflammatory mediators is mediated at least in part by induction o f an autocrine NO signaling pathway.