A contribution of acid-base regulation to metabolic depression in marine ectotherms

Adaptation to permanent cold in marine ectotherms-has been discussed to be associated with mitochondrial proliferation as well as a more or less non-c ompensated decrease in metabolic rate. This process must involve the respec tive tradeoffs in energy consuming processes. Only recently it was demonstr ated that acid-base regulation is associated with a considerable cost. Meta bolic depression is accompanied by a decrease in the rate of cellular net p roton excretion lending to a decrease in the setpoint of intracellular pH r egulation. Also a shift is observed to more economic mechanisms of proton e quivalent ion exchange. In this chapter the physiological relevance of acid -base parameters in various thermal environments is reevaluated based on th ese Findings. During temperature change a wide range of tissues in ectother mal species displays pH changes in accordance with an alphastat mode of aci d-base regulation. Exeptions from this pattern described in the literature are discussed in the light of cold induced mitochondrial proliferation, of metabolic depression during (seasonal) cold and of differences between the various methods of pH(i) analysis applied. In animals adapted to various te mperature regimes it is possible to distinguish between changes in pHi elic ited by physicochemical buffering and those caused by a readjustment of the setpoints of proton equivalent ion exchange. The thermal flexibility and h igher metabolic rate of eurythermal animals is reflected in the predominant use of mure costly active mechanisms of pHi regulation during temperature induced pH(i) changes, whereas acid-base regulation appears to be less flex ible and less costly in cold stenotherms and displays largely passive tempe rature dependent pHi changes.