Résumé
beta-adrenergic stimulation is a key regulator of cardiac function. The localization of major cardiac adrenergic receptors (beta(1) and beta(2)) has been investigated using biochemical and biophysical approaches and has led to contradictory results. This study investigates the functional subcellular localization of beta(1)- and beta(2)-adrenergic receptors in rat ventricular myocytes using a physiological approach. Ventricular myocytes were isolated from the hearts of rat and detubulated using formamide. Physiological cardiac function was measured as Ca2+ transient using Fura-2-AM and cell shortening. Selective activation of beta(1)- and beta(2)-adrenergic receptors was induced with isoproterenol (0.1 mu mol/L) and ICI-118,551 (0.1 mu mol/L); and with salbutamol (10 mu mol/L) and atenolol (1 mu mol/L), respectively. beta(1)- and beta(2)-adrenergic stimulations induced a significant increase in Ca2+ transient amplitude and cell shortening in intact rat ventricular myocytes (i.e., surface sarcolemma and t-tubules) and in detubulated cells (depleted from t-tubules, surface sarcolemma only). Both beta(1)- and beta(2)-adrenergic receptors stimulation caused a greater effect on Ca2+ transient and cell shortening in detubulated myocytes than in control myocytes. Quantitative analysis indicates that beta(1)- adrenergic stimulation is similar to 3 times more effective at surface sarcolemma compared to t-tubules, whereas beta(2)-adrenergic stimulation occurs almost exclusively at surface sarcolemma (similar to 100 times more effective). These physiological data demonstrate that in rat ventricular myocytes, beta(1)- adrenergic receptors are functionally present at surface sarcolemma and t-tubules, while beta(2)-adrenergic receptors stimulation occurs only at surface sarcolemma of cardiac cells.