Cold-induced depolarization of insect muscle: Differing roles of extracellular K+ during acute and chronic chilling

Abstract

Insects enter chill coma, a reversible state of paralysis, at temperatures below their critical thermal minimum (CT min ), and the time required for an insect to recover after a cold exposure is termed chill coma recovery time (CCRT). The CT min and CCRT are both important metrics of insect cold tolerance that are used interchangeably, although chill coma recovery is not necessarily permitted by a direct reversal of the mechanism causing chill coma onset. Nevertheless, onset and recovery of coma have been attributed to loss of neuromuscular function due to depolarization of muscle fibre membrane potential (V m does depolarize muscle resting potential and slow CCRT following prolonged cold exposure. However, onset of chill coma at the CT min relates to an as-yet-unknown effect of temperature on neuromuscular function.

Cold-acclimation improves chill tolerance in the migratory locust through preservation of ion balance and membrane potential

“…Numerous studies have shown how chronic cold stress changes hemolymph [K + ] and [Na + ] in locusts Findsen et al, 2013) and other insects (Koštál et al, 2004(Koštál et al, , 2006MacMillan and Sinclair, 2011b;MacMillan et al, 2014MacMillan et al, , 2015bCoello Alvarado et al, 2015;Des Marteaux and Sinclair, 2016) and consistent with these earlier observations we found that ion balance of locusts was disrupted during prolonged cold stress (≥4 h). Specifically we found that muscle ion concentrations generally remained constant (Fig.…”

“…As previously observed in cold-stressed locusts, we found that E K depolarized considerably, while E Na remained fairly constant during the prolonged cold stress (Fig. 2C,F) (MacMillan et al, 2014). Cold-acclimated locusts retained a more polarized E K than their warm-acclimated conspecifics because they were able to better maintain transmembrane K + balance, a pattern also observed in crickets (Coello Alvarado et al, 2015), fire bugs (Koštál et al, 2004), and cockroaches (Koštál et al, 2006).…”

“…Muscle ion concentrations were calculated using the muscle water content. Muscle ion concentrations were corrected on the assumption of a 4% residual hemolymph volume in muscle tissue (MacMillan et al 2014):…”

“…For measurement, locusts were placed on a pre-cooled plate to maintain a body temperature of 0°C. Extensor tibialis fibres were accessed by gently cutting open the cuticle as described by MacMillan et al (2014). The exposed muscle fibres were penetrated using the glass electrode, and when a sudden drop in potential was recorded this was interpreted as having entered a cell, and the drop in potential was noted as the membrane potential of the given cell if this could be repeated for a minimum of three times.…”

“…When a chill-susceptible insect is gradually cooled, reduced ion pump activity causes the V m of excitable tissues to depolarize, and this depolarization often coincides with the onset of chill coma (Pichon and Treherne, 1974;Wareham et al, 1974;Hosler et al, 2000;Andersen et al, 2015a;Overgaard and MacMillan, 2016). Following chill coma onset, hemolymph ion balance is progressively lost, which further depolarizes V m (MacMillan et al, 2014;Overgaard and MacMillan, 2016). Thus the development of chill injury in insects is almost always associated with increased hemolymph [K + ], which has a marked depolarizing effect on cells, including muscle cells (Hoyle, 1953;Wood, 1963).…”