Excitation-contraction Coupling in Skeletal Muscle Fibers from Adult Domestic Honeybee

Excitation-contraction coupling was characterized in enzymatically isolated adult honeybee skeletal muscle fibers. The voltage-dependent Ca(2+) current (I(Ca)) underlies action potential (AP) depolarization phase in honeybee muscle. A single AP leads to rapid and transient cytoplasmic Ca(2+) increase ("Ca(2+) transient"), which afterwards returns toward baseline following an exponential time course. Trains of APs elicit a staircase increase of Ca(2+), as a result of multiple Ca(2+) transient summation. Surprisingly, the nifedipine-sensitive I(Ca) is blocked by allethrin, a pyrethroid insecticide, revealing myotoxic effects of this neurotoxic insecticide for honeybees. Ca(2+) transients are under the control of Ca(2+) entry through voltage-activated Ca(2+) channels. Indeed, Ca(2+) transient amplitude depends on extracellular Ca(2+) concentration, and bell-shaped relationships are obtained for both I(Ca) integral and the Ca(2+) transient peak in response to depolarizations of increasing amplitude. The slow inactivation kinetics of I(Ca) induces long-lasting Ca(2+) transients that tend to reach a plateau and to return toward a resting level after the end of the stimulation. A Ca(2+)-induced Ca(2+) release mechanism is suggested by two results. First, caffeine (>or=5 mM) and 4-cmc (>0.4 mM), two activators of the sarcoplasmic reticulum Ca(2+) release channels (CRCs), induce Ca(2+) elevations in the absence of extracellular Ca(2+). Second, ryanodine (5 microM) a plant alkaloid that binds specifically to CRCs, depresses voltage-induced Ca(2+) transients. Honeybee muscle fibers represent a valuable model to study invertebrate excitation-contraction coupling and insecticide myotoxicity toward useful insects.