Differential Actions of 5-Hydroxytryptamine on The Isolated Rat Right and Left Atria

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Parimongkol Wongchuengam
Rachanee Naiwatanakul
Prakorn Chudapongse
Prasan Dhumma-Upakorn


The positive chronotropic and inotropic effects of 5-hydroxytryptamine(5-HT) on the isolated rat right and left atria respectively have been investigated in the presence of 5-HT antagonists, i.e. ,methysergide and cyproheptadine, and beta-adrenergic blocking agent,i.e. , propranolol. Methysergide (0.47 μg/ml) or cyproheptadine(0.02 μg /ml) reduced the positive chronotropic effect of 5-HT (2.0 μg /ml) on the right atria to about 50% of controls, but completely abolished the positive inotropic effect of 5-HT on the left atria. Propranolol (0.15 μg /ml) attenuated the positive chronotropic effect of 5-HT similar to those of methysergide and cyproheptadine. However, propranolol did not reduce the positive inotropic effect induced by 5-HT on the left atria. Combinations of 5-HT antagonists with beta blocker totally inhibited the positive chronotropic effect. The positive chronotropic response of the isolated right atria from reserpinized rats was also abolished by either methysergide or cyproheptadine alone.

It is concluded that in the rats the effect of 5-HT on the right atrial rates is mediated by the combination of the direct effect on 5-HT receptor and indirect effect through catecholamine release from the intra-cardiac stores. In contrast, the positive inotropic effect on the left atria is due to a direct action of 5-HT and does not involve endogenous catecholamine release.


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