The effects of proctolin (RYLPT) on neurally evoked contractions of lo
cust oviduct muscle were studied to examine the role of proctolin as a
cotransmitter, Increasing the number of stimuli in a burst (from one
to 30 stimuli) resulted in an increase in amplitude of contraction of
locust oviduct muscle, Proctolin was capable of increasing the amplitu
de of neurally evoked contractions at lower-stimulus regimes (one- and
two-stimulus bursts) but did not do so at higher-stimulus regimes (fi
ve- and 10-stimulus bursts), The effects of proctolin were dose depend
ent within the one- and two-stimulus regimes, with thresholds at 10(-9
) M and maxima at 2.5 x 10(-8) M. Addition of proctolin increased the
basal tonus and size of a postcontraction relaxation of the oviduct mu
scle in a dose-dependent manner during all stimulus regimes, However,
the effect of proctolin on basal tonus and the postcontraction relaxat
ion was much less at the higher stimulus regimes, Previously, several
proctolin analogues have been tested for their ability to antagonize p
roctolin-induced contractions of the oviduct muscle. Since proctolin i
s proposed to be a cotransmitter rat this neuromuscular junction, one
of these analogues, cycloproctolin, was used to antagonize proctolin's
effects on neurally evoked contractions, In the presence of the antag
onist, the maximum amplitude induced by application of proctolin was d
ecreased by 22.7%, while the proctolin-induced increase in basal tonus
was decreased by 45.8%, Finally, the maximum increase in the size of
the postcontraction relaxation caused by proctolin was lowered by 32.0
%, The results of the present study show that exogenously applied proc
tolin is an excitant of the oviduct muscle at lower, rather than highe
r, stimulus regimes, and this latter inaction may be due to the corele
ase of endogenous proctolin during increased neural stimulation. (C) 1
997 John Wiley & Sons, Inc.