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 (I-Ca), cytosolic Ca2+ (Ca-i) tra nsients, and contractility in rat ventricular myocytes are largely dis sociated from its effect to increase cellular cAMP levels. This result suggested that beta(2)ARs might be coupled to signaling pathways othe r than the G(s alpha)-mediated activation of adenylyl cyclase. Here we show that pertussis toxin (PTX) pretreatment specifically potentiates the responses of rat heart cells to beta(2)AR but not beta(1)AR stimu lation. After PTX pretreatment, 1)the dose-response curve for the effe cts of the beta(2)AR agonist zinterol on contraction amplitude is shif ted leftward and upward (EC(50) changed from about 1.0 mu M to 70 nM), 2) in indo-1-loaded cells, the maximal effects of zinterol (10(-5) M) on Ca-i transient and contraction amplitudes are additionally increas ed 1.7- and 2.0-fold, respectively, over those in control cells, and 3 ) the increase in I-Ca amplitude induced by the same zinterol concentr ation is potentiated by 2.5-fold. Similar effects of PTX are observed when beta(2)ARs are stimulated by isoproterenol in the presence of a s elective beta(1)AR blocker, CGP 20712A. All effects of beta(2)AR agoni sts in both PTX-treated and control cells are abolished by a selective beta(2)AR blocker, ICI 118,551. In contrast, neither the base-line I- Ca, Ca-i transient, and contraction in the absence of beta AR stimulat ion nor the beta(1)AR-mediated augmentations of these parameters are s ignificantly altered by PTX treatment. These results demonstrate, for the first time, that the G(s)-coupled beta(2)AR can simultaneously act ivate a pathway that leads to functional inhibition in cardiac cells v ia a PTX-sensitive G protein. The activation of more than one G protei n during beta(2)AR stimulation, leading to functionally opposite effec ts, may provide a mechanism to protect the heart from Ca2+ overload an d arrthythmias during the response to stress.