Functional subcellular distribution of β(1)- and β(2)-adrenergic receptors in rat ventricular cardiac myocytes

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