ALPHA(1A)-ADRENOCEPTOR-MEDIATED AND ALPHA(1B)-ADRENOCEPTOR-MEDIATED POSITIVE CHRONOTROPIC EFFECTS ON ISOLATED RAT ATRIUM

The positive chronotropic effect of alpha1-adrenoceptor subtypes was s tudied on the isolated right atrium of adult male Wistar rats. In the presence of beta-blocker propranolol, chloroethylclonidine (Chl) (an i rreversible alpha1B inactivator) reduced the positive chronotropic eff ect of phenylephrine (Phe). When the basal rate was lowered by choline rgic stimulation. Chl showed less effect on the increase of spontaneou s beat rate. H-7, a protein kinase C (PKC) inhibitor. lowered the chro notropic effect to Phe under physiological condition, but did not affe ct the Phe-induced positive chronotropic effect when basal rate was le ssened by carbachol. These suggested that the chronotropic effect was mainly mediated by alpha1B receptors, and partially related to the act ivation of PKC by second messengers under physiological conditions. Th e compensatory effect of the alpha1-adrenoceptors was mainly mediated by alpha1A receptors, which is scarcely involved in the activation of PKC.