Cold tolerance is linked to osmoregulatory function of the hindgut in Locusta migratoria

There is growing evidence that maintenance of ion and water balance determines cold tolerance in many insects. The hindgut of terrestrial insects is critical for maintaining organismal homeostasis as it regulates solute and water balance of the haemolymph. Here, we used ex vivo everted gut sacs of Locustamigratoria to examine the effects of temperature (0-30°C), thermal acclimation, hypoxia, and ionic and osmotic forces on bulk water and ion (Na⁺, K⁺ and Cl^-) movement across the rectal epithelium. These findings were related to simultaneous in vivo measurements of water and ion balance in locusts exposed to similar temperatures. As predicted, we observed a critical inhibition of net water and ion reabsorption at low temperature that was proportional to the in vivo loss of water and ion homeostasis. Further, cold-acclimated locusts, which are known to defend ion and water balance at low temperature, were characterised by improved reabsorptive capacity at low temperature. These findings strongly support the hypothesis that transport mechanisms in the hindgut at low temperature are essential for cold tolerance. The loss of osmoregulatory capacity at low temperature was primarily caused by reduced active transport, while rectal paracellular permeability to fluorescein isothiocyanate dextran was unchanged at 0 and 30°C. During cold exposure, water reabsorption was independent of major cation gradients across the epithelia, while a reduction in mucosal Cl^- availability and an increase in mucosal osmolality markedly depressed water reabsorption. These findings are discussed in the context of existing knowledge and with suggestions for future physiological studies on cold acclimation and adaptation in insects.