Characterization of stretch-activated calcium permeable cation channels in freshly isolated myocytes of the cricket (Gryllus bimaculatus) lateral oviduct

Intermediate conductance Ca2+-activated potassium channels are activated by functional coupling with stretch-activated nonselective cation channels in cricket myocytes

Cooperative gating of localized ion channels ranges from fine-tuning excitation-contraction coupling in muscle cells to controlling pace-making activity in the heart. Membrane deformation resulting from muscle contraction activates stretch-activated (SA) cation channels. The subsequent Ca2+ influx activates spatially localized Ca2+-sensitive K+ channels to fine-tune spontaneous muscle contraction. To characterize endogenously expressed intermediate conductance Ca2+-activated potassium (IK) channels and assess the functional relevance of the extracellular Ca2+ source leading to IK channel activity, we performed patch-clamp techniques on cricket oviduct myocytes and recorded single-channel data. In this study, we first investigated the identification of IK channels that could be distinguished from endogenously expressed large-conductance Ca2+-activated potassium (BK) channels by adding extracellular Ba2+. The single-channel conductance of the IK channel was 62 pS, and its activity increased with increasing intracellular Ca2+ concentration but was not voltage-dependent. These results indicated that IK channels are endogenously expressed in cricket oviduct myocytes. Second, the Ca2+ influx pathway that activates the IK channel was investigated. The absence of extracellular Ca2+ or the presence of Gd3+ abolished the activity of IK channels. Finally, we investigated the proximity between SA and IK channels. The removal of extracellular Ca2+, administration of Ca2+ to the microscopic region in a pipette, and application of membrane stretching stimulation increased SA channel activity, followed by IK channel activity. Membrane stretch-induced SA and IK channel activity were positively correlated. However, the emergence of IK channel activity and its increase in response to membrane mechanical stretch was not observed without Ca2+ in the pipette. These results strongly suggest that IK channels are endogenously expressed in cricket oviduct myocytes and that IK channel activity is regulated by neighboring SA channel activity. In conclusion, functional coupling between SA and IK channels may underlie the molecular basis of spontaneous rhythmic contractions.

BK Channels Are Activated by Functional Coupling With L-Type Ca2+ Channels in Cricket Myocytes

Large-conductance calcium (Ca2+)-activated potassium (K+) (BK) channel activation is important for feedback control of Ca2+ influx and cell excitability during spontaneous muscle contraction. To characterize endogenously expressed BK channels and evaluate the functional relevance of Ca2+ sources leading to BK activity, patch-clamp electrophysiology was performed on cricket oviduct myocytes to obtain single-channel recordings. The single-channel conductance of BK channels was 120 pS, with increased activity resulting from membrane depolarization or increased intracellular Ca2+ concentration. Extracellular application of tetraethylammonium (TEA) and iberiotoxin (IbTX) suppressed single-channel current amplitude. These results indicate that BK channels are endogenously expressed in cricket oviduct myocytes. Ca2+ release from internal Ca2+ stores and Ca2+ influx via the plasma membrane, which affect BK activity, were investigated. Extracellular Ca2+ removal nullified BK activity. Administration of ryanodine and caffeine reduced BK activity. Administration of L-type Ca2+ channel activity regulators (Bay K 8644 and nifedipine) increased and decreased BK activity, respectively. Finally, the proximity between the L-type Ca2+ channel and BK was investigated. Administration of Bay K 8644 to the microscopic area within the pipette increased BK activity. However, this increase was not observed at a sustained depolarizing potential. These results show that BK channels are endogenously expressed in cricket oviduct myocytes and that BK activity is regulated by L-type Ca2+ channel activity and Ca2+ release from Ca2+ stores. Together, these results show that functional coupling between L-type Ca2+ and BK channels may underlie the molecular basis of spontaneous rhythmic contraction.

Involvement of plasma membrane Ca2+ channels, IP3 receptors, and ryanodine receptors in the generation of spontaneous rhythmic contractions of the cricket lateral oviduct

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.

Signaling pathway underlying the octopaminergic modulation of myogenic contraction in the cricket lateral oviduct

Octopamine (OA), a biogenic monoamine, is a neurotransmitter and neuromodulator in invertebrates. Here, we report the effect of OA on the spontaneous rhythmic contractions (SRCs) of the lateral oviduct of the cricket Gryllus bimaculatus and the possible signaling pathway involved. Application of OA increased both the frequency and amplitude of SRCs in a dose-dependent manner. The effect of OA was inhibited by subsequent application of the OA receptor antagonist epinastine, indicating that the action of OA is mediated by OA receptor. To investigate the predominant signaling pathway underlying the action of OA, we first examined a possible involvement of the cAMP/cAMP-dependent protein kinase A (PKA) signaling pathway. Application of the membrane-permeable cAMP analog 8-Br-cAMP had little effect on SRCs and the effect of OA was not influenced by subsequent application of the PKA inhibitor H89, indicating that the cAMP/PKA signaling pathway is not the predominant pathway in the action of OA. Next, we examined a possible involvement of the second messenger inositol 1,4,5-trisphosphate in the action of OA. The effect of OA on SRCs was inhibited by subsequent application of the phosphoinositide-specific phospholipase C (PLC) inhibitor U73122, indicating that the PLC pathway is involved in the action of OA. The OA-induced increase in the frequency of SRCs was inhibited by pretreatment of the cell with the ryanodine receptor antagonist tetracaine but was not significantly affected by the IP3 receptor antagonist 2-aminoethoxydiphenyl borate (2-APB). On the other hand, the OA-induced increase in the amplitude of SRCs was inhibited by pretreatment of the cells with 2-APB but was not significantly affected by tetracaine. Taken together, these results suggest that the OA-induced excitatory effect on SRCs is mediated by the PLC signaling pathway: Ca2+ release from IP3 receptors may contribute to the modulation of the amplitude of SRCs, whereas Ca2+ release from ryanodine receptors may contribute to the modulation of the frequency of SRCs.