Positive inotropic response to α-adrenergic stimulation in an electrically driven rat left atrium: The role of extracellular calcium

We studied the positive inotropic response induced by α-adrenergic receptor stimulation in an electrically driven rat left atrium. α-Adrenergic stimulation resulted in a prolonged positive inotropic response that reached its maximum within 5-7 min. The kinetics of the onset of the positive inotropic response were different for pure α-adrenergic, pure β-adrenergic, and mixed adrenergic stimulation. The positive inotropic responses to α- and β-adrenergic agonists were not additive. The relative inotropic response to α-adrenergic stimulation decreased when external calcium concentration was increased and disappeared when external calcium concentration was raised to 7.0 mM. The divalent cation ionophore A23187 (1 μM) produced a threefold increase of the contractility of the atrial preparation at 1.0 mM extracellular calcium, and no further α-adrenergic response was observed in its presence. Calcium channel antagonists verapamil and nifedipine markedly inhibited the response to α-adrenergic stimulation, with little effect on the β-adrenergic stimulation, at a calcium concentration of 0.5 mM. The inhibitory effect of calcium channel antagonists could be fully reversed by increasing the extracellular calcium concentration. Our data suggest that the α-adrenergic contractile response in the rat atrium involves the mobilization of extracellular calcium through verapamil-sensitive calcium channels in a mechanism different from that for the β-adrenergic response.