title: Vascular smooth muscle: availability of calcium through alpha-adrenoceptor stimulation.
abstract: In strip preparations from the rabbit main pulmonary artery, it was not possible to differentiate convincingly between alpha 1- and alpha 2-adrenoceptors, when selective agonists such as methoxamine, St 587, B-HT 920, and clonidine, as well as selective antagonists such as prazosin and yohimbine were used for receptor characterization. Measurements of the membrane potential of the vascular smooth-muscle cells with intracellular glass micropipettes showed similar depolarizations in response to the alpha 1-selective methoxamine and the alpha 2-selective B-HT 920. These observations suggest the occurrence of a uniform alpha-adrenoceptor population in the rabbit main pulmonary artery. However, in contrast to alpha 1-agonists, contractile responses to alpha 2-agonists differed considerably in their dependence on external Ca, as revealed in Ca withdrawal experiments, as well as by the use of Ca antagonists. Furthermore, inactivation of the guanine nucleotide-binding inhibitory protein (Ni protein) by pertussis toxin impaired vasoconstriction, in response to alpha 2-agonists, but left that to alpha 1-agonists unaffected. It is suggested that alpha 1- and alpha 2-agonists induce two distinct conformational changes in an otherwise uniform alpha-adrenoceptor of the rabbit main pulmonary artery. By the mediation of Ni protein, alpha 2-agonists appear to allow influx of Ca into the vascular muscle cells through a type of Ca channel that is unlikely to be controlled by voltage.
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D001381:Azepines / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D005260:Female; D008297:Male; D008729:Methoxamine / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D037342:Pertussis Toxin; D011224:Prazosin / Q000494:pharmacology; D011817:Rabbits; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D014661:Vasoconstriction / Q000187:drug effects; D010566:Virulence Factors, Bordetella / Q000494:pharmacology; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D000316:Adrenergic alpha-Agonists; D001381:Azepines; D011942:Receptors, Adrenergic, alpha; D010566:Virulence Factors, Bordetella; D015016:Yohimbine; C024763:talipexole; D037342:Pertussis Toxin; D008729:Methoxamine; D002118:Calcium; D011224:Prazosin