Ionic channel mechanisms underlying rhythmic membrane hyperpolanzation assoaated with muscle contraction in myocytes isolated from insect visceral muscle

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Myocytes isolated from the cricket lateral oviduct exhibit spontaneous rhythmic contraction Electrical recordings with the whole-cell patch damp technique under the current damp condition dearly showed that the rhythmic contraction is closely associated with the transient membrane hyperpolarization. The frequency of this rhythmic membrane hyperpolarization (RMH) increased with increasing extracellular C? whereas decreased with decreasing extracellular Ca^<2+> or with adding gadolinium, a blocker of nonselective cation channel (NSCC), into a bath solution. These results indicate that Ca^<2+> influx through Ca^<2+>-conducting channel in plasma membrane of myoccytes is responsible for the generation of RMH. By using the patch clamp technique, I have shown the presence of two types of Ca^<2+> -conducting channels in myocytes. One is a voltage-dependent L-type Ca^<2+> channel and the other is a stretch-activated Ca^<2+>-conducting nonselective cation channel. These channels are readily dist … More inguished on the basis of their voltage dependency ion selectivity membrane stretch sensitivity and pharmacological properties. The present results also indicated that Ca^<2+> influx through NSCC, may play an important role in the generation of RMH Via refilling Ca^<2+> into internal Ca^<2+> store in myocytes isolated from the cricket lateral oviduct.In order to identify the ionic channels responsible for the generation of membrane hyperpolarization, I tried to identify the Ca^<2+>-dependent K channels by using the patch damp technique. I have shown the presence of two types of Ca^<2+>-activated K channels. One is a high conductance Ca^<2+>-activated K channel (BK channel and the other is a intermediate -activated K channel (1K channel). The former channel had the single channel conductance of 140 pS and the latter channel of 50 pS wtith symmetrical high K condition. These channels are distinguished on the basis of their voltage dependency intracellular Ca^<2+> dependency and pharmacological properties. The present results have also revealed that the application of Ca^<2+> mobilizers such as caffeine and ryanodine, inhibited the generation of RMH and also the muscle contraction. This result indicates that the released Ca^<2+> from the ineternal Ca^<2+> store via Ca^<2+>-induced Ca^<2+> release (CICR) mechanism is involved in the generation of RMS.In conclusion, the present results indicate that rhythmic contraction associated with RMH in myocytes isolated from the cricket lateral oviduct is induced by released Ca^<2+> from the internal Ca^<2+> store site and a resultant increase in the intracellular Ca^<2+> may activate BK or 1K channels and thereby induce membrane hyperpolarization. Ca^<2+> influx mainly through stretch activated nonselective cation channel may play an important role for muscle contraction via supplying Ca^<2+> directly or indirectly through CICR mechanism. Less