Role of extracellular calcium in contractions produced by activation of postsynaptic alpha-2 adrenoceptors in the canine saphenous vein.
Canine saphenous vein (CSV) has been shown to contain both postsynaptic alpha-1 and alpha-2 adrenoceptors. Our previous studies have shown that activation of postsynaptic alpha-1 adrenoceptors in this tissue utilizes both extracellular and intracellular Ca++ to produce contractions. In the present study, the source of calcium mobilized by activation of postsynaptic alpha-2 adrenoceptors in CSV was elucidated. Contractions of tissue rings to the supramaximal concentrations of three selective alpha-2 agonists, B-HT 920, M-7 and clonidine, were determined in the absence and presence of 5 mM La . In the presence of La , clonidine and M-7 produced small but statistically significant contractions (8-14% of control) which were abolished when the alpha-1 adrenoceptors were inactivated by phenoxybenzamine (10(-7) M, 30 min). In contrast, contractions to B-HT 920 were abolished completely in the presence of La . All the three alpha-2 agonists stimulated 45Ca++ uptake into CSV (0.3-0.4 mmol/kg wet weight, 10 min). 45Ca++ efflux studies demonstrated that the selective alpha-2 agonist, B-HT 920 (10(-5) M plus 10(-7) M phenoxybenzamine), did not induce an increase in the rate of 45Ca++ efflux. In contrast, an augmented 45Ca++ efflux rate was observed with the alpha-1 agonist, phenylephrine (10(-4) M plus 10(-7) M rauwolscine). These results suggest that activation of postsynaptic alpha-2 adrenoceptors in CSV utilizes primarily extracellular Ca++ to produce contractions.
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D000818:Animals; D001381:Azepines / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D008729:Methoxamine / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects*; D009131:Muscle, Smooth, Vascular / Q000502:physiology*; D009284:Naphthols / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D012501:Saphenous Vein / Q000502:physiology
D001381:Azepines; D009284:Naphthols; D011942:Receptors, Adrenergic, alpha; C009957:5,6-dihydroxy-2-dimethylaminotetralin; C024763:talipexole; D008729:Methoxamine; D002118:Calcium