Demonstration in Tupaia papillary muscle preparations of alpha-adrenoceptors mediating positive inotropic effects: comparison with guinea-pigs.
1. Positive inotropic responses to alpha- and beta-adrenoceptor agonists of isolated papillary muscles from the tree shrew (Tupaia), were compared with those from guinea-pigs. 2. In Tupaia, the concentration-response curve for phenylephrine, unlike that for isoprenaline, was not affected by pindolol in a concentration (10(-8) M) sufficient to block beta-adrenoceptor-mediated responses, but it was significantly shifted to the right by phentolamine (10(-6) M). In guinea-pig papillary muscles, however, the concentration-response curve for phenylephrine, like that for isoprenaline, was shifted to the right by pindolol (10(-8) M) but was unaltered by phentolamine (10(-6) M). Furthermore, when the mean concentrations of agonists inducing maximal positive inotropic responses were compared (relative to that of isoprenaline = 1.0), phenylephrine was found to be only slightly less potent (0.84 +/- 0.04; n = 5) in Tupaia and much less potent (0.33 +/- 0.06; n = 5) in the guinea-pig. 3. Although in Tupaia papillary muscles the increase in developed tension induced by a combination of phenylephrine and isoprenaline did not significantly differ from that by phenylephrine alone, it was approximately 3 times larger than that produced by phenylephrine alone in guinea-pigs. 4. These results indicate that in papillary muscles from Tupaia, unlike the guinea-pig, the positive inotropic effects of phenylephrine can be mediated by alpha-adrenoceptors.
Koga
T
T
Department of Pharmacology, Fujigotemba Research Laboratories, Chugai Pharmaceutical Co., Ltd., Shizuoka, Japan.
Shiraki
Y
Y
Sakai
K
K
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D005260:Female; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects*; D066298:In Vitro Techniques; D007537:Isometric Contraction; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects*; D010210:Papillary Muscles / Q000187:drug effects; D010656:Phenylephrine / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*; D014417:Tupaia / Q000502:physiology*; D014418:Tupaiidae / Q000502:physiology*
D000316:Adrenergic alpha-Agonists; D000318:Adrenergic beta-Agonists; D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D007545:Isoproterenol