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

Most insects have the ability to alter their cold tolerance in response to temporal temperature fluctuations, and recent studies have shown that insect cold tolerance is closely tied to the ability to maintain transmembrane ion-gradients that are important for the maintenance of cell membrane potential (V_m). Accordingly, several studies have suggested a link between preservation of V_m and cellular survival after cold stress, but none have measured V_m in this context. We tested this hypothesis by acclimating locusts (Locusta migratoria) to high (31°C) and low temperature (11°C) for four days before exposing them to cold stress (0°C) for up to 48 hours and subsequently measuring ion balance, cell survival, muscle V_m, and whole animal performance. Cold stress caused gradual muscle cell death which coincided with a loss of ion balance and depolarisation of muscle V_m. The loss of ion-balance and cell polarisation were, however, dampened markedly in cold-acclimated locusts such that the development of chill injury was reduced. To further examine the association between cellular injury and V_m we exposed in vitro muscle preparations to cold buffers with low, intermediate, or high [K⁺]. These experiments revealed that cellular injury during cold exposure occurs when V_m becomes severely depolarised. Interestingly we found that cellular sensitivity to hypothermic hyperkalaemia was lower in cold-acclimated locusts that were better able to defend V_m whilst exposed to high extracellular [K⁺]. Together these results demonstrate a mechanism of cold-acclimation in locusts that improves survival after cold stress: Increased cold tolerance is accomplished by preservation of V_m through maintenance of ion homeostasis and decreased K⁺-sensitivity.