TEMPORAL EFFECTS OF CYTOKINES ON NEONATAL CARDIAC MYOCYTE CA2-CYCLASEACTIVITY( TRANSIENTS AND ADENYLATE)

This study investigates the hypothesis that inflammatory cytokines, in terleukin (IL)-1 alpha IL-1 beta, and tumor necrosis factor (TNF), inf luence cardiac function by affecting calcium homeostasis and that this effect is mediated by the beta-adrenergic-adenylate cyclase system. A fter 4 days in culture, neonatal rat ventricular myocytes were treated with cytokines (10 ng/ml) for short (2 h) or longer (18 h) times. Myo cyte calcium, contractility, and adenylate cyclase were measured under each condition. Anticipated stepwise increases in adenylate cyclase a nd intracellular calcium were found in controls (non-cytokine-treated) with 10(-7) M isoproterenol, 10(-7) M isoproterenol + 0.1 mM guanosin e triphosphate, and 10(-9) M forskolin. Cells in the presence of cytok ine for 2 h show increased basal calcium levels but no changes in aden ylate cyclase activities, and isoproterenol fails to elevate adenylate cyclase levels or affect contractile shortening. After long-term trea tment with IL-1 beta or TNF, but not IL-1 alpha, the significantly ele vated levels of basal systolic calcium remain, and isoproterenol incre ases adenylate cyclase activity, unlike after short exposure. Forskoli n maximally activates adenylate cyclase following both short- and long -term incubation, but the stepwise increase in activity is blunted fol lowing prolonged exposure. Thus short-term cytokine treatment blocks t he adrenergic receptor-mediated increases in adenosine 3',5'-cyclic mo nophosphate, dissociating adenylate cyclase activation from cytokine-m ediated increases in cell calcium, whereas longer treatment apparently produces direct affects on adenylate cyclase. Time-dependent differen ces in contractile response were found with IL-1 alpha at 2 h and TNF at 18 h, implying that myofibrillar responsiveness to increased cytopl asmic calcium is dependent on both cytokine species and exposure time.