THE ROLE OF PROCTOLIN IN MAINTAINING CONTRACTIONS OF THE LOCUST (LOCUSTA-MIGRATORIA) MANDIBULAR CLOSER MUSCLE

The role of proctolin in orchestrating the control of the mandibular closer muscles of the locust, Locusta migratoria, has been examined. Proctolin enhances the amplitude of the neurally evoked contractions of the closer muscle at low frequency neural stimulation (< 4 Hz). At stimulation frequencies of 4 Hz and above, proctolin, in addition to increasing amplitude, also produces a level of maintained tension between successive contractions. At 8 Hz stimulation, the amount of maintained tension produced by proctolin, is sufficient to transform the pattern of phasic contraction to one resembling a tonic contraction. Ca-45 studies indicate that the effect of proctolin on maintained tension seems to be facillitated by an influx of extracellular calcium. This influx is blocked by verapamil suggesting the existence of a calcium channel in the muscle membrane. Proctolin can also develop tonic contractions in muscles pre-exposed to depolarizing influences such as L-glutamate and 40 mM potassium. However, proctolin cannot contract a resting muscle suggesting that the calcium channel present in the muscle membrane requires both the ligand, proctolin and a sufficient membrane voltage for activation. The possibility that proctolin is working through a second messenger has been investigated.