Comparative studies on the mode of action of proctolin and phorbol-12, 13-dibutyrate in their ability to contract the locust mandibular closer muscle

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Abstract
The role of proctolin has been further investigated in the locust (Locusta migratoria) mandibular closer muscles. Radioactive calcium uptake measurements were made using protease‐dissociated muscle cells. Both the phorbol ester, phorbol‐12, 13‐dibutyrate, and proctolin produce tonic contractions which are associated with the influx of extracellular calcium. The thresholds for proctolin and the phorbol ester to contract the muscle were 1‐10 nM and 10‐100nM, respectively, while their respective thresholds for evoking measurable calcium influx into the muscle cells were 0.1‐1 nM for proctolin, and 0.1‐1 pM for phorbol‐12, 13‐dibutyrate. The effect of phorbol‐12, 13‐dibutyrate is blocked by a number of protein kinase inhibitors (at a concentration of 0.1 mM), suggesting that an activation of protein kinase can lead to calcium influx. These inhibitors, however, do not block the effect of proctolin, indicating that these two compounds work through different pathways, possibly coverging on the same final target. In light of this finding, a number of other compounds have been tested to try to ascertain how proctolin mediates an increased calcium influx.