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Does suppression of responsiveness to beta-adrenoceptor activation explain the enhancement of vasoconstrictor responses by alpha 2-adrenoceptor agonists?

1. The alpha 2-adrenoceptor agonist TL99, in concentrations that had no other observable effect, enhanced constrictor responses to phenylephrine in perfused segments of the rat tail artery. 2. Vasoconstrictor responses to phenylephrine were also enhanced by propranolol (0.3 mumol/L). 3. Vasoconstrictor responses to phenylephrine in the presence of propranolol were further enhanced by TL99 (10 nmol/L). 4. The enhancement of vasoconstrictor responses by alpha 2-adrenoceptor agonists is not due to the removal of a counteracting vasodilator component produced by activation of beta-adrenoceptors.