Ins 1,4,5-P3 and Ca2+ signaling in quiescent neonatal cardiac myocytes

Activation of alpha 1-adrenergic receptors in neonatal cardiac myocytes results in changes in contractile activity and the induction of hypertrophic growth. The biochemical mechanisms responsible for these diverse effects are not yet established, but presumably involve the associated alpha 1-adrenergic stimulation of phosphatidylinositol (PI) hydrolysis, with concomitant generation of Ins 1,4,5-P3 and diacylglycerol. This study examined whether alpha 1-adrenergic generation of Ins 1,4,5-P3 in intact, quiescent, neonatal cardiac myocytes resulted in a Ca2+ signal. Stimulation of myocytes with norepinephrine in the presence of propranolol caused accumulation of inositol mono-, bis and trisphosphates. However, alpha 1-adrenergic stimulation did not alter cytosolic free Ca2+ levels in 85% of the myocytes examined. Direct generation of Ins 1,4,5-P3, by photolysis of microinjected caged Ins 1,4,5-P3, was also unable to alter cytosolic free Ca2+ levels, despite the presence of Ins 1,4,5-P3 receptors. Taken together, these data indicated that alpha 1-adrenergic stimulation did not initiate Ca2+ signaling because Ins 1,4,5-P3-induced Ca2+ mobilization was not operative in quiescent neonatal cardiac myocytes. Normal excitation-contraction Ca2+ handling mechanisms were present in these cells, as illustrated by depolarization- and caffeine-induced Ca2+ transients. Analysis of these same myocytes following 48 h in the presence of norepinephrine and propranolol showed a 40% increase in the ratio of protein to DNA and a 350% increase in release of atrial naturietic factor, compared to control cells, indicating the normal operation of alpha 1-adrenergic-induced hypertrophic growth. Therefore, the assumption that Ca(2+)-dependent processes will be activated by receptor signaling pathways coupled to enhanced phosphatidylinositol turnover in cardiac cells must be avoided. In addition, the data presented in this study clearly indicated that an increase in cytosolic free Ca2+ was not necessary for the induction of alpha 1-adrenergic-mediated cardiac hypertrophy.