CHARACTERIZATION OF POSTJUNCTIONAL ALPHA-1 AND ALPHA-2 ADRENOCEPTORS ACTIVATED BY EXOGENOUS OR NERVE-RELEASED NOREPINEPHRINE IN THE CANINE SAPHENOUS-VEIN

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  • 1 January 1984
    • journal article
    • research article
    • Vol. 230  (3) , 699-705
Abstract
Experiments were designed to characterize .alpha.-1 and .alpha.-2 adrenoceptor-mediated effects in the canine saphenous vein. Rings of saphenous vein were mounted for isometric tension recording in physiological saline solution. Contractile responses evoked by .alpha.-1 adrenoceptor agonists, cirazoline or St 587 were inhibited by .alpha.-1 antagonists, prazosin (pA2 [effective concentration of agonist to antagonist] = 7.9) or phenoxybenzamine, but were relatively resistant to the .alpha.-2 adrenoceptor antagonist rauwolscine. The responses to .alpha.-2 adrenoceptor agonists, xylazine or B-HT 920, were relatively resistant to prazosin or phenoxybenzamine but were antagonized by rauwolscine (pA2 = 8.7). After phenoxybenzamine, the .alpha.-2 agonists, M-7, guanfacine, UK 14304, B-HT 920 and xylazine evoked similar maximal increases in tension which were considerably smaller (approximately 50%) than that attained by .alpha.-1 adrenoceptor stimulation. The different concentration-effect characteristics of these responses were also revealed using norepinephrine. Prazosin produced a biphasic effect on the concentration-response curve of norepinephrine, being more potent against responses above 50% of the maximum (pA2 = 7.9) compared to lower increases in tension (pA2 = 6.2). After .alpha.-1 adrenoceptor blockade with prazosin, rauwolscine was more effective against responses below 50% of the maximum, compared to higher increases in tension. The .alpha.-1 and .alpha.-2 adrenoceptor-mediated concentration-effect curves to norepinephrine are almost coincident. .alpha.-2 adrenergic stimulation produces only partial activation of the vascular smooth muscle. Contractile responses produced by sympathetic nerve stimulation or by tyramine were antagonized more effectively by the combination of prazosin plus rauwolscine than by either blocker given alone, suggesting that .alpha.-1 and .alpha.-2 adrenoceptors are innervated by sympathetic nerves in the canine saphenous vein.

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