Localization of f-channels to caveolae mediates specific β2-adrenergic receptor modulation of rate in sinoatrial myocytes

beta(1)- and beta(2)-adrenergic receptors (ARs) coexist in different regions of the heart. The beta(2)/beta(1) expression ratio is higher in the sinoatrial node (SAN) than in atria and ventricles, but the specific contribution of either type of receptor to rate modulation is still not well established. We have recently demonstrated that pacemaker ("funny") f-channels are located in lipid rafts of the rabbit SAN. Since in ventricular myocytes beta(2)-, but not beta(1)-ARs, localize to caveolae, we hypothesized that modulation of f-channels and of pacemaker activity in SAN myocytes is controlled mainly by beta(2)-AR activation. To address this point, we investigated the caveolar localization of proteins by co-immunoprecipitation and immunocytochemistry, and found that f-channels interact with caveolin 3. We also recorded If current and spontaneous activity from SAN myocytes, and found that beta-AR activation by the non-selective agonists isoproterenol and fenoterol shifted the If activation curve similarly (by 6.3 and 5.3 mV) and increased similarly spontaneous rate (by 23.1% and 21.6%, respectively). Specific 2 stimulation had similar effects (4.9 mV shift of the activation curve and 16.9% rate increase), but specific 0 1 stimulation was less effective (1.7 mV shift and 7.2% rate increase). However, after caveolar disorganization by M beta CD (2%), stimulation of beta(1)-ARs was as effective as non-specific -AR stimulation. These data show that specific stimulation of beta(2)-ARZs is the main mechanism by which heart rate is modulated through a positive shift of the If activation curve and that this mechanism requires specific membrane compartmentation. (c) 2006 Elsevier Inc. All rights reserved.