Functional coupling of the β₂-adrenoceptor to a pertussis toxin-sensitive G protein in cardiac myocytes

Recently we demonstrated that the effects of β₂-adrenoceptor (AR) stimulation to augment Ca²⁺ current (I_Ca), cytosolic Ca²⁺ (Ca_i) transients, and contractility in rat ventricular myocytes are largely dissociated from its effect to increase cellular cAMP levels. This result suggested that β₂ARs might be coupled to signaling pathways other than the G_sα-mediated activation of adenylyl cyclase. Here we show that pertussis toxin (PTX) pretreatment specifically potentiates the responses of rat heart cells to β₂AR but not β₁AR stimulation. After PTX pretreatment, 1) the dose-response curve for the effects of the β₂AR agonist zinterol on contraction amplitude is shifted leftward and upward (EC₅₀ changed from about 1.0 μM 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 increased 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 concentration is potentiated by 2.5-fold. Similar effects of PTX are observed when β₂ARs are stimulated by isoproterenol in the presence of a selective β₁AR blocker, CGP 20712A. All effects of β₂AR agonists in both PTX-treated and control cells are abolished by a selective β₂AR blocker, ICI 118,551. In contrast, neither the base-line I_Ca, Ca transient, and contraction in the absence of βAR stimulation nor the β_iAR-mediated augmentations of these parameters are significantly altered by PTX treatment. These results demonstrate, for the first time, that the G_s-coupled β₂AR 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 β₂AR stimulation, leading to functionally opposite effects, may provide a mechanism to protect the heart from Ca²⁺ overload and arrhythmias during the response to stress.