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Journal of Experimental Biology, Vol 200, Issue 4 785-792, Copyright © 1997 by Company of Biologists

JOURNAL ARTICLES

Increased concentrations of haemolymph Mg2+ protect intracellular pH and ATP levels during temperature stress and anoxia in the common shrimp

F. J. Sartoris and H. O. Portner

The effects of temperature (0, 5, 10, 15 and 20 °C) and of anoxia (at 5 °C) on extracellular Mg2+ concentration ([Mg2+]e), intracellular pH (pHi) and ATP and lactate levels were investigated in intermoult adults of the common shrimp Crangon crangon. All animals caught in summer (summer animals) showed a slight but significant increase in [Mg2+]e at low temperatures. In contrast, at every temperature tested, a few of the animals caught in winter (winter animals) showed elevated [Mg2+]e during short- term (4 h) but not during long-term (6 days) incubations. The reasons for the overshoot in Mg2+ concentrations in individual animals remain unexplained, but a protective effect of extracellular Mg2+ on intracellular pH and on ATP concentrations was visible at haemolymph Mg2+ concentrations above 15 mmol l-1. The influence of high extracellular [Mg2+] on pHi and intracellular ATP and lactate levels under normoxic and anoxic conditions was tested using an incubation medium containing 150-250 mmol l-1 Mg2+. When haemolymph Mg2+ levels were manipulated by exposure of the animal to high levels of Mg2+ in the external medium, animals with a haemolymph [Mg2+] below the threshold concentration of 15 mmol l-1 had significantly lower values of intracellular pH than animals with haemolymph [Mg2+] above 15 mmol l-1. In addition, the elevation of haemolymph [Mg2+] by incubation in high-[Mg2+] water prevented the drop in pHi and the rise in lactate levels induced by anoxia. The protective effect of high levels of extracellular Mg2+ did not depend upon the [Ca2+]/[Mg2+] ratio but only on [Mg2+]e. However, experiments with isolated muscle tissues showed no dependence of muscle intracellular pH on [Mg2+]e under both normoxic and anoxic conditions, leading to the conclusion that the protective effect is evoked via a central, possibly anaesthetising, effect of high [Mg2+]e. The dependence of pHi and muscle [ATP] on extracellular [Mg2+] resembles the protective effect of high Mg2+ levels on the post- ischaemic mammalian heart.

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