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Induction by endogenous noradrenaline of an α 1 ‐adrenoceptor‐mediated positive inotropic effect in rabbit papillary muscles
Author(s) -
Hattori Yuichi,
Takeda Youji,
Nakaya Haruaki,
Kanno Morio
Publication year - 1993
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1993.tb13754.x
Subject(s) - prazosin , tyramine , inotrope , propranolol , endocrinology , medicine , chemistry , phenylephrine , isoprenaline , electrophysiology , reserpine , stimulation , antagonist , receptor , biology , blood pressure
1 The possible involvement of α 1 ‐adrenoceptors in the inotropic and electrophysiological responses to endogenous noradrenaline released by tyramine was examined in rabbit papillary muscles. 2 A concentration‐dependent positive inotropic effect was produced by tyramine. This effect of tyramine was not observed in muscles from rabbits pretreated with reserpine. 3 The positive inotropic effect of tyramine was greatly inhibited by propranolol, but not altered by prazosin. However, when β‐adrenoceptors were blocked by pretreatment with propranolol, tyramine still produced a positive inotropic effect, an effect which was antagonized by prazosin. 4 Tyramine caused a decrease in action potential duration (APD) and an increase in action potential amplitude in a concentration‐dependent manner. Isoprenaline also produced the same electrophysiological effects. These electrophysiological effects of both agents were inhibited by propranolol. 5 When β‐adrenoceptors were blocked by propranolol, the observed prazosin‐sensitive positive inotropic effect of tyramine was not accompanied by any change in APD. In contrast, APD was markedly prolonged by α 1 ‐adrenoceptor stimulation with phenylephrine in the presence of propranolol, in association wh the positive inotropic effect. 6 It is concluded that in rabbit papillary muscles, endogenous noradrenaline causes a positive inotropic effect predominantly mediated by β‐adrenoceptors, but can still evoke a positive inotropic effect through α 1 ‐adrenoceptors when β‐adrenoceptor stimulation is eliminated. This suggests that the α 1 ‐adrenoceptor‐mediated positive intropic mechanism(s) may be masked by simultaneous activation of β‐adrenoceptors. In addition, this study indicates that APD prolongation is not involved in the α 1 ‐adrenoceptor‐mediated inotropic responses to endogenous noradrenaline.