5-Hydroxytryptamine (5-HT) increases the sinoatrial beating rate and the force of contraction of paced left atria and papillary muscles (right ventricle) of the kitten. These effects of 5-HT are evident in tissues from noradrenaline-depleted kittens (reserpine pretreatment), and therefore elicited through myocardial 5-HT receptors. The atrial and ventricular effects of 5-HT were unaffected by concentrations of 8-OH DPAT and ketanserin saturating 5-HT1 and 5-HT2 receptors, respectively. Phenoxybenzamine blocked irreversibly sinoatrial and left atrial effects of 5-HT but not the effects of 5-HT on ventricular muscle. 5-Hydroxytryptamine causes prolongation of the ventricular action potential. Sinoatrial and left atrial 5-HT receptors are coined 5-HT3 and ventricular receptors are coined 5-HT4.
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Alberto J. Kaumann (1985) studied this question.