Effects of specific alpha-adrenoceptive agents on extraneuronal uptake (uptake2) of isoproterenol in perfused rat heart

Effects of specific alpha-adrenoceptive agents (alpha 1-agonist, alpha 1-antagonist, alpha 2-agonist and alpha 2-antagonist) on the extraneuronal accumulation of 3H-isoproterenol in the perfused rat heart were examined. The extraneuronal accumulation of 3H-isoproterenol in the hearts perfused with 3H-isoproterenol (10(-6)M) under COMT inhibition by tropolone (10(-4)M) was about 6 times higher than that of intact COMT. The increase in the accumulation by COMT inhibition was regarded as 100% and the effects of specific alpha-adrenoceptive agents on the accumulation was evaluated. alpha 1-agonists, methoxamine and phenylephrine, did not affect the accumulation. alpha 1-antagonists, prazosin, bunazosin and YM-12617, significantly decreased the accumulation of 3H-isoproterenol and these IC50 values were 2 x 10(-6)M, 3.5 x 10(-6)M and 2.3 x 10(-5)M, respectively. alpha 2-agonists, clonidine and guanabenz, significantly reduced the accumulation and these IC50 values were 3.4 x 10(-5)M and 2.9 x 10(-7)M, respectively. The alpha 2-antagonist, yohimbine, did not affect the accumulation. The present experiments clearly demonstrated that the tested alpha 1-antagonists and alpha 2-agonists inhibited uptake2 in rat heart but the tested alpha 1-agonists and an alpha 2-antagonist did not inhibit it.