β2-Adrenergic receptor agonists stimulate L-type calcium current independent of PKA in newborn rabbit ventricular myocytes

Selective stimulation of beta(2)-adrenergic receptors (ARs) in newborn rabbit ventricular myocardium invokes a positive inotropic effect that is lost during postnatal maturation. The underlying mechanisms for this age-related stimulatory response remain unresolved. We examined the effects of beta(2)-AR stimulation on L-type Ca2+ current (I-Ca,I-L) during postnatal development. I-Ca,I-L was measured (37 degrees C; either Ca2+ or Ba2+ as the charge carrier) using the whole-cell patch-clamp technique in newborn (1 to 5 days old) and adult rabbit ventricular myocytes. Ca2+ transients were measured concomitantly by dialyzing the cell with indo-1. Activation of beta(2)-ARs (with either 100 nM zinterol or 1 mu M isoproterenol in the presence of the beta(1)-AR antagonist, CGP20712A) stimulated I-Ca,I-L twofold in newborns but not in adults. The beta(2)-AR-mediated increase in Ca2+ transient amplitude in newborns was due exclusively to the augmentation of I-Ca,I-L. Zinterol increased the rate of inactivation of I-Ca,I-L and increased the Ca2+ flux integral. The beta(2)-AR inverse agonist, ICI-118551 (500 nM), but not the beta(1)-AR antagonist, CGP20712A (500 nM), blocked the response to zinterol. Unexpectedly, the PKA blockers, H-89 (10 mu M), PKI 6-22 amide (10 mu M), and Rp-cAMP (100 mu M), all failed to prevent the response to zinterol but completely blocked responses to selective beta(1)-AR stimulation of I-Ca,I-L in newborns. Our results demonstrate that in addition to the conventional beta(1)-AR/cAMP/PKA pathway, newborn rabbit myocardium exhibits a novel beta(2)-AR-mediated, PKA-insensitive pathway that stimulates I-Ca,I-L. This striking developmental difference plays a major role in the age-related differences in inotropic responses to beta(2)-AR agonists.