Regulation of Cardiac Alternans by β-adrenergic Signaling Pathways

In cat atrial myocytes, β-adrenergic receptor (β-AR) stimulation exerts profound effects on excitation-contraction coupling and cellular Ca(2+) cycling that are mediated by β(1)- and β(2)-AR subtypes coupled to G proteins (G(s) and G(i)). In this study, we determined the effects of β-AR stimulation on pacing-induced Ca(2+) alternans. Ca(2+) alternans was recorded from single cat atrial myocytes with the fluorescent Ca(2+) indicator indo-1. Stable Ca(2+) alternans occurred at an average pacing frequency of 1.7 Hz at room temperature with a mean alternans ratio of 0.43. Nonselective β-AR stimulation as well as selective stimulation of β(1)/G(s), β(2)/G(s) + G(i), and β(2)/G(s) coupled pathways all abolished pacing-induced Ca(2+) alternans. β(1)-AR stimulation abolished alternans through stimulation of PKA and Ca(2+)/calmodulin-dependent protein kinase II, whereas β(2)-AR stimulation exclusively involved PKA and was mediated via G(s), whereas a known second pathway in cat atrial myocytes acting through G(i) and nitric oxide production was not involved in alternans regulation. Inhibition of various mitochondrial functions (dissipation of the mitochondrial membrane potential or inhibition of mitochondrial F(1)/F(0)-ATP synthase, mitochondrial Ca(2+) uptake via the mitochondrial Ca(2+) uniporter, and Ca(2+) extrusion via mitochondrial Na(+)/Ca(2+) exchange) enhanced Ca(2+) alternans; however, β-AR stimulation still abrogated alternans, provided that sufficient cellular ATP was available. Selective inhibition of mitochondrial or glycolytic ATP production did not prevent β-AR stimulation from abolishing Ca(2+) alternans. However, when both ATP sources were depleted, β-AR stimulation failed to decrease Ca(2+) alternans. These results indicate that in atrial myocytes, β-AR stimulation protects against pacing-induced alternans by acting through parallel and complementary signaling pathways.