Abstract
There have been numerous observations to suggest that in mammals 5-hydroxytryptamine 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 little (3). The effects of 5-HT on the blood pressure are complex and vary according to the species (2, 4). The positive inotropic and chronotropic responses of the isolated auricle of rabbit to 5-HT were demonstrated by McCawley et al. (5) and Sinha et al. (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 stimulation 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.
