Stimulatory Effect of 5-Hydroxytryptamine (Serotonin) on Secretion by Malpighian Tubules of Insects

Abstract

IT is well known that the Malpighian tubules of insects are under hormonal control; in several cases a diuretic hormone has been found (refs. 1–5 and unpublished results of D. E. M. P.) and others suggest the involvement of an antidiuretic hormone^6–9. Such substances as acetylcholine, ATP and pitressin do not stimulate the tubules to secrete¹⁰. 5-Hydroxytryptamine (5-HT), however, has been found to be a potent stimulator of fluid secretion by Malpighian tubules. In Rhodnius prolixus and Carausius morosus the threshold for the effect on isolated tubules is at a concentration of about 5 × 10⁻⁸ moles/l. In both these insects, the increased rate of secretion is very like that which can be induced by treatment with their diuretic hormones (Fig. 1). Furthermore, the behaviour of the trans-wall potential difference in tubules from Rhodnius after treatment with 5-HT (Fig. 2) is similar to the changes which occur when diuretic hormone is added (unpublished results of S. H. P. M.). The question is whether or not the hormones and 5-HT are identical.