Functional coupling of the beta 2-adrenoceptor to a pertussis toxin-sensitive G protein in cardiac myocytes.

Recently we demonstrated that the effects of beta 2-adrenoceptor (AR) stimulation to augment Ca2+ current (ICa), cytosolic Ca2+ (Cai) transients, and contractility in rat ventricular myocytes are largely dissociated from its effect to increase cellular cAMP levels. This result suggested that beta 2ARs might be coupled to signaling pathways other than the Gs alpha-mediated activation of adenylyl cyclase. Here we show that pertussis toxin (PTX) pretreatment specifically potentiates the responses of rat heart cells to beta 2AR but not beta 1AR stimulation. After PTX pretreatment, 1) the dose-response curve for the effects of the beta 2AR agonist zinterol on contraction amplitude is shifted leftward and upward (EC50 changed from about 1.0 microM to 70 nM), 2) in indo-1-loaded cells, the maximal effects of zinterol (10(-5) M) on Cai transient and contraction amplitudes are additionally increased 1.7- and 2.0-fold, respectively, over those in control cells, and 3) the increase in ICa amplitude induced by the same zinterol concentration is potentiated by 2.5-fold. Similar effects of PTX are observed when beta 2ARs are stimulated by isoproterenol in the presence of a selective beta 1AR blocker, CGP 20712A. All effects of beta 2AR agonists in both PTX-treated and control cells are abolished by a selective beta 2AR blocker, ICI 118,551. In contrast, neither the base-line ICa, Cai transient, and contraction in the absence of beta AR stimulation nor the beta 1AR-mediated augmentations of these parameters are significantly altered by PTX treatment. These results demonstrate, for the first time, that the Gs-coupled beta 2AR can simultaneously activate a pathway that leads to functional inhibition in cardiac cells via a PTX-sensitive G protein. The activation of more than one G protein during beta 2AR stimulation, leading to functionally opposite effects, may provide a mechanism to protect the heart from Ca2+ overload and arrhythmias during the response to stress.