A postsynaptic effect of tyramine in ventricular muscle |
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Authors: | W. Brandt M. Reiter K. Seibel |
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Affiliation: | (1) Institut für Pharmakologie und Toxikologie der Technischen Universität München, Munich, Germany;(2) Abteilung für experimentelle Pharmakologie der GSF, Neuherberg |
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Abstract: | Summary Tachyphylaxis and susceptibility to pretreatment with reserpine demonstrate that tyramine increases contractility of the guinea-pig papillary muscle in concentrations up to 10–4 M by means of an indirect sympathomimetic effect.In higher concentrations (up to 3×10–3 M) tyramine causes an increase in force of contraction which is characterized by a slowing of relaxation of the isometric contraction curve. Pretreatment with reserpine reveals that this positive inotropic effect is composed of an indirect sympathomimetic and a direct post-synaptic effect; the latter is not subject to tachyphylaxis and produces by itself (at 3×10–3 M) an increase in force of contraction which amounts to about 50% of the maximum of the indirectly produced inotropic effect.The postsynaptically-induced positive inotropic effect of tyramine is not antagonized by propranolol (5×10–6 M). It is unlikely, therefore, that it is brought about by stimulation of adrenergic -receptors.The height of the isometric contraction curve in reserpine-pretreated muscles is increased by tyramine as a result of an increase in the rate of force development despite a marked concentration-dependent increase in relaxation time (t2).The direct inotropic effect of tyramine is correlated with a prolongation of the duration of the action potential (AP) which amounts to 70% at 3×10–3 M. In addition to its influence on the duration of the AP, tyramine slows the rate of depolarization and decreases the membrane resting potential. |
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Keywords: | Tyramine Postsynaptic Effect Cardiac Muscle Prolongation of Action Potential Retardation of Relaxation |
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