β₂-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 β₁- and/or β₂-ARs and whether it is mediated via NO signaling. 0.1 μM ISO plus ICI 118,551 (ISO-β₁-AR stimulation) or ISO plus atenolol (ISO-β₂-AR stimulation) both increased L-type Ca²⁺ current (I_Ca,L) markedly, but only ISO-β₂-AR stimulation mediated ACh-induced activation of I_K,ATP. 1 μM zinterol (β₂-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 β₂-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-β₂-AR stimulation or zinterol increased [NO]_i. L-NIO (10 μM) blocked ISO- and zinterol-induced increases in [NO]_i. ISO-β₁-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 β₁- and β₂-ARs stimulate cAMP. β₂-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 β₂-ARs acting exclusively via NO signaling mediate ACh-induced activation of I_K,ATP. NO signaling also contributes to β₂-AR stimulation of I_Ca,L. The differential effects of β₁- and β₂-ARs can be explained by the coupling of these two β-ARs to different effector signaling pathways.