The role of environmental calcium in the extreme acid tolerance of northern banjo frog (Limnodynastes terraereginae) larvae

Many aquatically respiring animals acutely exposed to low pH waters suffer inhibition of ion uptake, and loss of branchial (gill) epithelial integrity, culminating in a fatal loss of body Na+. Environmental calcium levels ([Ca2+]e) are pivotal in maintaining branchial junction integrity, with supplemental Ca2+ reversing the negative effects of low pH in some animals. Tolerance of some naturally acidic environments by aquatic animals is further complicated by low [Ca2+]e, yet many of these environments are surprisingly biodiverse. How animals overcome the damaging actions of low pH and low environmental Ca2+ remains unknown. We examined the effects of [Ca2+]e on the response to low pH in larvae of the highly acid tolerant frog Limnodynastes terraereginae. Acute exposure to low pH water in the presence of low (5 µmol L-1) [Ca2+]e increased net Na+ efflux. Provision of additional [Ca2+]e reduced net Na+ efflux, but the effect was saturable. Acclimation to both low and high (250 µmol L-1) [Ca2+]e improved the resistance of larvae to Na+ efflux at low pH. Exposure to the Ca2+ channel inhibitor ruthenium red resulted in an abrupt loss of tolerance in low pH acclimated larvae. Acclimation to acidic water increased branchial gene expression of the intracellular Ca2+ transport protein calbindin, consistent with a role for increased transcellular Ca2+ trafficking in the tolerance of acidic water. This study supports a role for [Ca2+]e in promoting branchial integrity and highlights a potential mechanism via the maintenance of transcellular Ca2+ uptake in the acid tolerance of L. terraereginae larvae.