Calmodulin mediates contraction of the oviducts of Locusta migratoria

Abstract

Calmodulin (CaM) is a multifunctional calcium binding protein which is an essential mediator in the contraction of mammalian smooth muscle. In the present study, evidence is provided that CaM is important in mediating insect visceral muscle contraction. Calmodulin is present in the oviducts of the locust, Locusta migratoria, at c. 55 ng/mg protein, 2.7 times that found in the external ventral protractor of the VIIth abdominal segment, a skeletal muscle. The calmodulin inhibitors trifluoperazine (TFP) and N-(6-aminohexyl)-5-chloro-l-naphthalenesulfonamide (W7) inhibited proctolin-induced contractions in a dose-dependent manner. The effective dose for 50% inhibition (ED 50) of contraction for TFP was 1.6×10 −4 M and for W7 was 3.0×10 −4 M. High potassium-induced contractions were also inhibited dose-dependently by TFP and W7 with ED 50s of 1.8×10 −4 M and 5.1×10 −4 M, respectively. The protein kinase C (PKC) inhibitor chelerythrine had no effect on either proctolin- or high potassium-induced contractions of the locust oviduct indicating that TFP and W7 were not simply antagonizing PKC in producing their inhibitory effects. At least 15 calmodulin binding proteins (CaMBPs) were visualized in the locust oviduct muscle after probing SDS polyacrylamide gels with 35S-VU-1 CaM. Furthermore the binding of CaM to proteins in the gel was inhibited in a dose-dependent manner in the presence of either TFP or W7 paralleling the physiological inhibition of contraction. These results indicate that the contraction of the locust oviducal muscle is, at least in part, mediated by calmodulin, possibly through a CaMBP such as myosin light chain kinase, similar to the process of contraction in smooth muscles of vertebrates.