Excitation-contraction coupling in skeletal muscle fibers from adult domestic honeybee

Abstract

Excitation–contraction coupling was characterized in enzymatically isolated adult honeybee skeletal muscle fibers. The voltage-dependent Ca²⁺ current (I _Ca) underlies action potential (AP) depolarization phase in honeybee muscle. A single AP leads to rapid and transient cytoplasmic Ca²⁺ increase (“Ca²⁺ transient”), which afterwards returns toward baseline following an exponential time course. Trains of APs elicit a staircase increase of Ca²⁺, as a result of multiple Ca²⁺ transient summation. Surprisingly, the nifedipine-sensitive I _Ca is blocked by allethrin, a pyrethroid insecticide, revealing myotoxic effects of this neurotoxic insecticide for honeybees. Ca²⁺ transients are under the control of Ca²⁺ entry through voltage-activated Ca²⁺ channels. Indeed, Ca²⁺ transient amplitude depends on extracellular Ca²⁺ concentration, and bell-shaped relationships are obtained for both I _Ca integral and the Ca²⁺ transient peak in response to depolarizations of increasing amplitude. The slow inactivation kinetics of I _Ca induces long-lasting Ca²⁺ transients that tend to reach a plateau and to return toward a resting level after the end of the stimulation. A Ca²⁺-induced Ca²⁺ release mechanism is suggested by two results. First, caffeine (≥5 mM) and 4-cmc (>0.4 mM), two activators of the sarcoplasmic reticulum Ca²⁺ release channels (CRCs), induce Ca²⁺ elevations in the absence of extracellular Ca²⁺. Second, ryanodine (5 µM) a plant alkaloid that binds specifically to CRCs, depresses voltage-induced Ca²⁺ transients. Honeybee muscle fibers represent a valuable model to study invertebrate excitation–contraction coupling and insecticide myotoxicity toward useful insects.