Muscle membrane potential and insect chill coma.

Chill-susceptible insects enter a reversible paralytic state, termed chill coma, at mild low temperatures. Chill coma is caused by neuromuscular impairment, allegedly triggered by cold-induced depolarization of muscle resting membrane potential (Vm). We used five Drosophila species that vary in cold tolerance (chill coma temperature spanning ∼11°C) and repeatedly measured muscle Vm during a downward temperature ramp (20 to -3°C). Cold-tolerant species were able to defend their Vm down to lower temperatures, which is not explained by species-specific differences in initial Vm at 20°C, but by cold-tolerant drosophilids defending Vm across a broad range of temperatures. We found support for a previously suggested 'critical threshold' of Vm, related to chill coma, in three of the five species. Interestingly, the cold-tolerant Drosophila species may enter coma as a result of processes unrelated to muscle depolarization as their Vm was not significantly depolarized at their chill coma temperatures.