HETEROGENEITY OF BETA-ADRENOCEPTORS IN RIGHT ATRIA ISOLATED FROM COLD-EXPOSED RATS

  • 1 January 1984
    • journal article
    • research article
    • Vol. 230  (2) , 450-454
Abstract
The chronotropic response of right atria isolated from 5-day-cold-exposed rats to isoproterenol and norepinephrine (NE) was studied. A large increase in the sensitivity of the pacemaker to isoproterenol and a decrease in the sensitivity to NE occurred. Determination of pA2 [competitive agonistic activity] values of propranolol and metoprolol using isoproterenol and NE as agonists and analysis of the slopes of Schild plots suggested that in atria isolated from control rats the chronotropic effect of isoproterenol and NE resulted from the preferential interaction of the catecholamines with a homogenous .beta.-1 adrenoceptor population. After cold exposure the affinity of atrial adrenoceptors for propranolol increased when the agonist was isoproterenol and decreased when norepinephrine was used. The slopes of the Schild plots of metoprolol when the agonists were isoproterenol or NE were not unitary unless the experiments were performed in the presence of butoxamine. However, butoxamine prevented the demonstration of cold-induced supersensitivity to isoproterenol, leaving the subsensitivity to NE unaffected. It is concluded that cold-induced heterogeneity of the atrial .beta. adrenoceptors is responsible for the increased sensitivity to isoproterenol. Probably, subsensitivity to NE resulted from conformational alterations of the atrial .beta.-1 adrenoceptors.