alpha 1- and alpha 2-adrenoceptor stimulation in the isolated perfused hindquarters of the rat: an in vitro model.
In the isolated perfused hindquarters of rats pretreated with reserpine, vasoconstrictor responses were induced by activation of postjunctional alpha 1- and alpha 2-adrenoceptors. B-HT 920 and methoxamine were used as selective stimulants of alpha 2- and alpha 1-adrenoceptors, respectively. Prazosin (pA2 8.83) exhibited a competitive antagonism toward methoxamine-induced vasoconstrictor responses, whereas yohimbine (pA2 8.95) effectively antagonized B-HT 920-induced vasoconstriction in a competitive manner. When calcium was omitted from the perfusion fluid, no vasoconstriction via alpha 2-adrenoceptors could be evoked. On the other hand, raising the extracellular calcium concentration in the perfusion fluid enhanced the alpha 2-adrenoceptor-mediated vasoconstrictor effects. Vasoconstriction via postjunctional alpha 1-adrenoceptors seemed hardly, if at all, dependent on extracellular calcium. The calcium entry blockers Co2+, La3+, and nisoldipine selectively inhibited the alpha 2-adrenoceptor-induced vasoconstriction in a noncompetitive manner. The results further support the concept that extracellular calcium is a prerequisite for the process of vasoconstriction induced by activation of postjunctional alpha 2-adrenoceptors.
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D001381:Azepines / Q000494:pharmacology; D002118:Calcium / Q000502:physiology; D006614:Hindlimb / Q000098:blood supply; D066298:In Vitro Techniques; D008297:Male; D008729:Methoxamine / Q000494:pharmacology; D009543:Nifedipine / Q000031:analogs & derivatives / Q000494:pharmacology; D015737:Nisoldipine; D011224:Prazosin / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012039:Regional Blood Flow / Q000187:drug effects; D012110:Reserpine / Q000494:pharmacology; D014661:Vasoconstriction / Q000187:drug effects*; D015016:Yohimbine / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D001381:Azepines; D015016:Yohimbine; D015737:Nisoldipine; C024763:talipexole; D012110:Reserpine; D008729:Methoxamine; D009543:Nifedipine; D002118:Calcium; D011224:Prazosin