The hindgut muscularis of the stable fly, Stomoxys calcitrans: Some of its structural, motile and pharmacological properties

Abstract

The muscles of the hindgut of the stable fly, Stomoxys calcitrans L., are arranged in a fine regular lattice of longitudinal and circular fibres with diameters that range between 7 and 16 μm. The hindgut is innervated by a proctodeal nerve that branches and projects along the rectum and colon. Four distinct patterns of motile activity were evident in the hindgut; compression, peristalsis, reverse peristalsis and segmentation. Compressions that were recorded myographically showed distinctive activity patterns. When the proctodeal nerve was stimulated by a single electrical pulse, the longitudinal muscles of the hindgut responded with a single monophasic contraction that lasted 800 ms. If the frequency of the stimulating pulse was increased, individual responses could be distinguished until the stimulus reached 10 pulses/s. At this point, a complete fusion of contractile events occurred and the amplitude doubled. The hindgut showed a sensitivity to l-glutamate in micromolar amounts with a threshold for increasing the frequency of phasic contractions at a concentration of 10 −5 M. At 10 −4 M, glutamate and above tonic changes were evident. The threshold concentration for hindgut excitation with proctolin fell between 10 −15 and 10 −10 M. At these levels, increases in the amplitude and frequency of phasic contractions were generally evident. At a proctolin concentration of 10 −9 M and above, tonic changes were observed. Evidence for proctolin desensitization of the stable fly hindgut is discussed. Although 80% of the stable fly preparations tested were unresponsive to leucomyosuppressin, 20% did whow a weak inhibitory reaction.