NO EVIDENCE FOR HOMEOVISCOUS ADAPTATION IN A HETEROTHERMIC TISSUE - TUNA HEAT-EXCHANGERS

Many poikilotherms are known to adjust the membrane composition of the ir cells in response to a temperature change so that membrane fluidity , and therefore function, is conserved. Such compensatory changes in m embrane composition are considered ''homeovvscous adaptations.'' In th is study, we examined a heterothermic tissue, the visceral rete mirabi le of the bluefin tuna, for evidence of homeoviscous adaptation. We me asured the proportions of phospholipid fatty acids and phospholipid he ad groups as a function of position along the rete thermal gradient, w hich has been estimated to be similar to 10 degrees C. We found no eff ect of position along the rete on the composition of either phospholip id fatty acids or head groups. Our results were unexpected in light of our previous demonstration of compensation of metabolic enzyme activi ty in the same tissue. The lack of evidence for a homeoviscous respons e may be due to the fluctuating nature of the thermal gradient along t he visceral retia; i.e., membranes may be adapted to a eurythermal exi stence rather than being fine-tuned to a particular temperature.