title: Involvement of plasma membrane Ca2+ channels, IP3 receptors, and ryanodine receptors in the generation of spontaneous rhythmic contractions of the cricket lateral oviduct.
abstract: In the present study, the isolated cricket (Gryllus bimaculatus) lateral oviduct exhibited spontaneous rhythmic contractions (SRCs) with a frequency of 0.29±0.009 Hz (n=43) and an amplitude of 14.6±1.25 mg (n=29). SRCs completely disappeared following removal of extracellular Ca2+ using a solution containing 5mM EGTA. Application of the non-specific Ca2+ channel blockers Co2+, Ni2+, and Cd2+ also decreased both the frequency and amplitude of SRCs in dose-dependent manners, suggesting that Ca2+ entry through plasma membrane Ca2+ channels is essential for the generation of SRCs. Application of ryanodine (30 μM), which depletes intracellular Ca2+ by locking ryanodine receptor (RyR)-Ca2+ channels in an open state, gradually reduced the frequency and amplitude of SRCs. A RyR antagonist, tetracaine, reduced both the frequency and amplitude of SRCs, whereas a RyR activator, caffeine, increased the frequency of SRCs with a subsequent increase in basal tonus, indicating that RyRs are essential for generating SRCs. To further investigate the involvement of phospholipase C (PLC) and inositol 1,4,5-trisphosphate receptors (IP3Rs) in SRCs, we examined the effect of a PLC inhibitor, U73122, and an IP3R antagonist, 2-aminoethoxydiphenyl borate (2-APB), on SRCs. Separately, U73122 (10 μM) and 2-APB (30-50 μM) both significantly reduced the amplitude of SRCs with little effect on their frequency, further indicating that the PLC/IP3R signaling pathway is fundamental to the modulation of the amplitude of SRCs. A hypotonic-induced increase in the frequency and amplitude of SRCs and a hypertonic-induced decrease in the frequency and amplitude of SRCs indicated that mechanical stretch of the lateral oviduct is involved in the generation of SRCs. The sarcoplasmic reticulum Ca2+-pump ATPase inhibitors thapsigargin and cyclopiazonic acid impaired or suppressed the relaxation phase of SRCs. Taken together, the present results indicate that Ca2+ influx through plasma membrane Ca2+ channels and Ca2+ release from RyRs play an essential role in pacing SRCs and that Ca2+ release from IP3Rs may play a role in modulating the amplitude of SRCs, probably via activation of PLC.
authors.affiliation: Department of Biology, Tokyo Gakugei University, Tokyo 184-8501, Japan.
authors.forename: Hirotake
authors.initials: H
authors.lastname: Tamashiro
authors.affiliation: Department of Biology, Tokyo Gakugei University, Tokyo 184-8501, Japan. Electronic address: myoshi@u-gakugei.ac.jp.
authors.forename: Masami
authors.initials: M
authors.lastname: Yoshino
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D005260:Female; D006135:Gryllidae / Q000502:physiology*; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000378:metabolism*; D009119:Muscle Contraction; D010057:Oviducts / Q000502:physiology; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism*
chemical_list: D053496:Inositol 1,4,5-Trisphosphate Receptors; D019837:Ryanodine Receptor Calcium Release Channel; D002118:Calcium
keywords: Cricket; IP(3) receptor; Lateral oviduct; Myogenic contraction; Ryanodine receptor