The action of proctolin and 5-hydroxytryptamine on the oviduct of the horsefly, Tabanus proximis

Laboratory tests in the USA showed that proctolin and 5-hydroxytryptamine (5HT) caused a change in the spontaneous contractions of the oviduct of Tabanus proximus Wlk. Although both substances caused an increase in the amplitude and tonus of contractions, it was possible to distinguish between the 2 responses. 5HT (at 10-10-10-9 M) caused a pronounced increase in the frequency of phasic contractions in addition to changes in tonus. Proctolin (at 10-11 M), however, produced marked increases in both tonus and the amplitude of phasic contractions. The response profiles were remarkably similar to those previously reported for T. sulcifrons Macq. The distinctive responses obtained with the peptide and the indolamine from the 2 tabanids indicated that different receptor sites exist for each compound. Moreover, these sites may be linked to 2 different aspects of muscle function in the oviduct: the strong tonic contractions associated with egg movement and deposition mimicked by proctolin; and the small rhythmic movements that accompany the transfer of sperm simulated by 5HT. It is noted that indolamines have been implicated in the secretion from the accessory glands of several insects.