β(2)-Adrenergic Receptor Signaling Acts via No Release to Mediate Ach-Induced Activation of Atp-Sensitive K(+) Current in Cat Atrial Myocytes

In atrial myocytes, an initial exposure to isoproterenol (ISO) acts via cAMP to mediate a subsequent acetylcholine (ACh)-induced activation of ATP-sensitive K(+) current (I(K,ATP)). In addition, β-adrenergic receptor (β-AR) stimulation activates nitric oxide (NO) release. The present study determined whether the conditioning effect of β-AR stimulation acts via β(1)- and/or β(2)-ARs and whether it is mediated via NO signaling. 0.1 μM ISO plus ICI 118,551 (ISO-β(1)-AR stimulation) or ISO plus atenolol (ISO-β(2)-AR stimulation) both increased L-type Ca(2+) current (I(Ca,L)) markedly, but only ISO-β(2)-AR stimulation mediated ACh-induced activation of I(K,ATP). 1 μM zinterol (β(2)-AR agonist) also increased I(Ca,L) and mediated ACh-activated I(K,ATP). Inhibition of NO synthase (10 μM L-NIO), guanylate cyclase (10 μM ODQ), or cAMP-PKA (50 μM Rp-cAMPs) attenuated zinterol-induced stimulation of I(Ca,L) and abolished ACh-activated I(K,ATP). Spermine-NO (100 μM; an NO donor) mimicked β(2)-AR stimulation, and its effects were abolished by Rp-cAMPs. Intracellular dialysis of 20 μM protein kinase inhibitory peptide (PKI) abolished zinterol-induced stimulation of I(Ca,L). Measurements of intracellular NO ([NO](i)) using the fluorescent indicator DAF-2 showed that ISO-β(2)-AR stimulation or zinterol increased [NO](i). L-NIO (10 μM) blocked ISO- and zinterol-induced increases in [NO](i). ISO-β(1)-AR stimulation failed to increase [NO](i). Inhibition of G(i)-protein by pertussis toxin significantly inhibited zinterol-mediated increases in [NO](i). Wortmannin (0.2 μM) or LY294002 (10 μM), inhibitors of phosphatidylinositol 3′-kinase (PI-3K), abolished the effects of zinterol to both mediate ACh-activated I(K,ATP) and stimulate [NO](i). We conclude that both β(1)- and β(2)-ARs stimulate cAMP. β(2)-ARs act via two signaling pathways to stimulate cAMP, one of which is mediated via G(i)-protein and PI-3K coupled to NO-cGMP signaling. Only β(2)-ARs acting exclusively via NO signaling mediate ACh-induced activation of I(K,ATP). NO signaling also contributes to β(2)-AR stimulation of I(Ca,L). The differential effects of β(1)- and β(2)-ARs can be explained by the coupling of these two β-ARs to different effector signaling pathways.