TUMOR-NECROSIS-FACTOR-ALPHA DECREASES INOSITOL PHOSPHATE FORMATION AND PHOSPHATIDYLINOSITOL-BISPHOSPHATE (PIP2) SYNTHESIS IN RAT CARDIOMYOCYTES

Treatment of neonatal rat cardiomyocytes for 72 h in the presence of t umor necrosis factor alpha (TNF alpha) (10 U/ml) lead to a decrease in basal and alpha(1)-adrenoceptor-induced formation of the calcium-mobi lizing second messenger inositol trisphosphate (IP3) and its metabolit es, IP2 and IP1, by 35 and 26%, respectively. The synthesis of phospha tidylinositol bisphosphate (PIP2), the substrate of PI-specific phosph olipase C, was decreased by 45% following the TNF alpha (10 U/ml) expo sure. Time courses of TNF alpha (10 U/ml)-induced alterations in rat c ardiomyocytes showed a parallel decline of basal inositol phosphate fo rmation and PIP2 synthesis suggesting that the decrease in inositol ph osphate formation was due to the reduction in PIP2 synthesis. As the T NF alpha-induced decrease of PIP2 synthesis was associated with a decr eased synthesis of the phospholipid phosphatidylinositol (PI), the pre cursor of PIP2, by 33%, the decreased availability of PIP2 is apparent ly, at least in part, the result of the decreased synthesis of PI. As an apparent functional consequence of the decrease in IP3 formation fo llowing the TNF alpha exposure, the alpha(1)-adrenoceptor-mediated ind uction of arrhythmias by 100 mu mol/l noradrenaline + 10 mu mol/l timo lol was abolished in TNF alpha-pretreated rat cardiomyocytes. To inves tigate one of the possible mechanisms of the TNF alpha-induced decreas e of PIP2 formation, the effect of TNF alpha pretreatment on glycerol- 3-phosphate dehydrogenase (GDH), a key enzyme of lipogenesis, was stud ied: Exposure of the rat cardiomyocytes for 72 h to TNF alpha induced a concentration-dependent decrease in GDH activity by maximally 55%. T he result presented are consistent with the hypothesis that the decrea sed basal and a(1)-adrenoceptor-induced formation of the second messen ger IP3 observed in chronic endotoxinemia and sepsis may be mediated b y a TNF alpha-induced decrease in the synthesis of PIP2, the substrate of PI-specific phospholipase C. This mechanism occurs following long- term exposure to low TNF alpha concentrations and is apparently distin ct from the shortterm cardiac effects induced by high concentrations o f TNF alpha