G(i)-dependent suppression of β1-adrenoceptor effects in ventricular myocytes from NE-treated guinea pigs

Abstract

It has been suggested that there is a preferential coupling in heart muscle between the inhibitory G protein (G(i)) and the β2-subtype of the β-adrenergic receptor (β-AR), since pertussis toxin (which inactivates G(i)) reveals latent β2-ARs in rat and mouse myocytes. We have previously shown that guinea pigs treated with norepinephrine (NE) for 7 days have myocytes that are desensitized to β-AR-agonist stimulation, and that pertussis toxin restores these responses. The purpose of the present investigation was to determine whether pertussis toxin specifically upregulated β2-ARs in myocytes from NE-treated guinea pigs. The sole β-AR subtype in control guinea pig myocytes was confirmed as β1-AR by radioligand binding, single-cell autoradiography, and concentration-response curves to isoproterenol in contracting myocytes. In contrast, a minor pool of β2-ARs was observed in rat myocytes by use of the same methods. NE treatment decreased the maximum isoproterenol response (relative to high Ca2+) from 0.89 ± 0.06 to 0.58 ± 0.08 (n = 7, P < 0.01) and the pD2 (- log EC50) from 8.8 ± 0.2 to 7.5 ± 0.2 (n = 7, P < 0.01). Pertussis toxin treatment increased the isoproterenol-to-Ca2+ ratio to 0.88 ± 0.04 (n = 6, P < 0.05) and the pD2 to 8.6 ± 0.3 (P < 0.01). This was not mediated by increases in either number or function of β2-ARs. G(i) is therefore able to modulate β1-AR responses in guinea pig myocytes.