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

Abstract

β1- and β2-adrenergic receptors (ARs) coexist in different regions of the heart. The β2/β1 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 β2-, but not β1-ARs, localize to caveolae, we hypothesized that modulation of f-channels and of pacemaker activity in SAN myocytes is controlled mainly by β2-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 β-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 β2 stimulation had similar effects (4.9 mV shift of the activation curve and 16.9% rate increase), but specific β1 stimulation was less effective (1.7 mV shift and 7.2% rate increase). However, after caveolar disorganization by MβCD (2%), stimulation of β1-ARs was as effective as non-specific β-AR stimulation. These data show that specific stimulation of β2-ARs 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.