Alpha-adrenoceptor-mediated increase in cytosolic free calcium in isolated cardiac myocytes

The effect of alpha-adrenoceptor stimulation on the concentration of cytosolic free calcium (Cai2+) was determined by measuring indo-l fluorescence in isolated ventricular cardiomyocytes from normal and streptozotocin-diabetic rat; 1.3 x 10(5) alpha 1-adrenoceptors per normal myocyte and an unaltered number of these receptors in cells from diabetic rats were detected using the alpha 1-selective ligand WB-4101. Under basal conditions, Cai2+ was found to be 154 +/- 4 nM (n = 34) reaching a value of 192 +/- 10 nM (n = 15) after stimulation of myocytes with a maximal dose of methoxamine for 5 min. Under the same conditions the leakage of dye produced a significantly smaller increase of basal values of 169 +/- 5 nM (n = 17). Indo-l loaded cells did not respond to beta-stimulation unless in the presence of KCl (50 mM), demonstrating the specificity of methoxamine action. Treatment of cells with nifedipine or chelation of extracellular calcium by EGTA did not modify the alpha-adrenergic response. Experiments with cardiomyocytes from streptozotocin-diabetic rats showed an unaltered modulation of Cai2+ by both alpha- and beta-receptor stimulation. It is concluded that signalling by alpha 1-adrenoceptors in ventricular cardiomyocytes results in mobilization of intracellular calcium stores.