Effects of 5-Hydroxytryptamine on the Isolated Atrium of Rabbit

Abstract

There have been numerous observations to suggest that in mammals 5-hydroxy tryptamine acts as a humoral agents (1, 2). Though 5-HT is very widespread in the body, there has been no literature to show the amount of 5-HT in the heart of the mammals. But in the cyclostome heart it has been shown that the amount of 5-HT is (6). The latter authors confirmed that 5-HT, in concentration of 10-6, had a powerful stimulant action on the isolated auricle.Stimulation was, however, preceded by a slight and transient inhibition and followed by a marked depression. Atropine could abolish the initial inhibiting effect, but not subsequent one. Studies on the isolated papillary muscle of rabbit (7) suggested that 5-HT did not increase the strength of contraction.The results obtained from the isolated auricle of rabbit suggest that 5-HT may exert triphasic or polyphasic action.The effects of 5-HT on the transmission of nerve impulses in the autonomic nervous system has been explored by several investigators. Reversible blockade of the ganglionic transmission by relatively large dose of 5-HT was demonstrated by Marrazzi et al. (8) and Douglas et al. (9). However, the intra-arterial injection of small dose of 5-HT potentiated the response of postsynaptic fibers and of the end organ to electrical stimula tion of the cervical sympathetic nerve (10) and increased the spontaneous activity of the postganglionic structures (11). The similar potentiating effect of 5-HT on the transmission of the isolated stellate ganglion of rat was demonstrated by Hertzler (12).It was, therefore, of interest to see whether 5-HT affected the transmembrane potential of the atrium or not. If it affects, it will be of value to see the mode of action of 5-HT on the transmembrane potential. METHODSSeventy albino rabbits of either sex, weighing 1.5 to 2.5 kg, were killed by bleeding through cutting the both common carotid arteries.Immediately thereafter, the thorax was para-sternally opened. The whole heart was extirpated and immersed in warmed

Differences in the Response of Three Embryologically Distinct Segments of the Isolated Canine Posterior Vena cava to Vasoactive Substances

“…6). Indirect sympathomimetic actions of 5-hydroxytryptamine similar to those ob served in this study were reported in isolated rabbit hearts [4,11].…”

Heart

“…Toda (150) showed that 5-hydroxytryptamine (5-HT) has a positive chronotropic and inotropic effect on the isolated rabbit atrium after initial brief negative effects. He suggested that 5-hydroxytryptamine acts to liberate first acetylcholine (which accounts for the initial negative chrono and inotropic responses and can be accentuated by eserine), which in turn stimulates release of norepinephrine from myocardial nerve terminals or cells.…”

Blockade With Propranolol of the Positive Inotropic Action of Serotonin in the Dog

“…In reserpinized rabbits, however, serotonin does not produce a positive inotropic action (4). Accordingly, the serotonin induced increase in myocardial contractile force is presumably mediated by noradrenaline and adrenaline, which were liberated by the administration of serotonin .…”

“…The positive inotropic action of serotonin (5-hydroxytryptamine) has extensively been demonstrated in the isolated perfused heart of cats, dogs and rabbits (1,2), in the isolated auricle of the rabbit's heart (3,4), in the cat papillary muscle (5), and in the heart of dog in situ (6). In reserpinized rabbits, however, serotonin does not produce a positive inotropic action (4).…”