Xiao R P, Ji X, Lakatta E G
Gerontology Research Center, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224.
Mol Pharmacol. 1995 Feb;47(2):322-9.
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.
最近我们证明，在大鼠心室肌细胞中，β2 -肾上腺素能受体（AR）刺激增强钙电流（ICa）、胞质钙（Cai）瞬变和收缩力的效应，在很大程度上与其增加细胞内cAMP水平的效应无关。这一结果表明，β2ARs可能与Gsα介导的腺苷酸环化酶激活以外的信号通路偶联。在此我们表明，百日咳毒素（PTX）预处理特异性增强大鼠心脏细胞对β2AR而非β1AR刺激的反应。PTX预处理后，1）β2AR激动剂齐帕特罗对收缩幅度影响的剂量 - 反应曲线向左上方移动（EC50从约1.0微摩尔变为70纳摩尔），2）在负载indo - 1的细胞中，齐帕特罗（10^(-5) M）对Cai瞬变和收缩幅度的最大效应分别比对照细胞增加1.7倍和2.0倍，3）相同齐帕特罗浓度诱导的ICa幅度增加增强了2.5倍。当在选择性β1AR阻滞剂CGP 20712A存在的情况下用异丙肾上腺素刺激β2ARs时，观察到PTX有类似的效应。选择性β2AR阻滞剂ICI 118,551消除了PTX处理细胞和对照细胞中β2AR激动剂的所有效应。相比之下，PTX处理既不显著改变无βAR刺激时的基线ICa、Cai瞬变和收缩，也不改变β1AR介导的这些参数的增强。这些结果首次证明，与Gs偶联的β2AR可同时激活一条通过PTX敏感G蛋白导致心脏细胞功能抑制的通路。β2AR刺激期间一种以上G蛋白的激活导致功能上相反的效应，这可能为心脏在应激反应期间免受钙超载和心律失常提供一种机制。
