BK Channels Are Activated by Functional Coupling With L-Type Ca Channels in Cricket Myocytes.

Large-conductance calcium (Ca)-activated potassium (K) (BK) channel activation is important for feedback control of Ca influx and cell excitability during spontaneous muscle contraction. To characterize endogenously expressed BK channels and evaluate the functional relevance of Ca 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 Ca 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. Ca release from internal Ca stores and Ca influx via the plasma membrane, which affect BK activity, were investigated. Extracellular Ca removal nullified BK activity. Administration of ryanodine and caffeine reduced BK activity. Administration of L-type Ca channel activity regulators (Bay K 8644 and nifedipine) increased and decreased BK activity, respectively. Finally, the proximity between the L-type Ca 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 Ca channel activity and Ca release from Ca stores. Together, these results show that functional coupling between L-type Ca and BK channels may underlie the molecular basis of spontaneous rhythmic contraction.