Temperature adaptation in fish

Altering the body temperature of fish causes immediate changes in both the functional and biochemical properties of secretory epithelial tissues. The present account describes how the intestinal tract of the goldfish adapts to regulate its function after a sudden exposure to a change in environmental temperature. Adaptation can be seen to take place in three distinct steps. The first stage, occurring 15-20h after the change in environmental temperature, alters the ability of actively transported non-electrolytes to increase the microvillar membrane permeability to sodium. The passive permeability of the cells to sodium is also regulated at this stage. The second adaptive change involves changes in membrane phospholipids and amino acid transport. This occurs after about 32-48h. Regulation is still not complete, however, the intracellular sodium remaining low and intracellular potassium remaining high for 2-3 weeks. There is then a final regulation of sodium-pump activity. This change, which is concerned with pump turnover rather than with number of pump sites, is probably connected with the synthesis of new cells. These changes are discussed in detail, and an attempt is made to relate fine changes in phospholipid fatty acyl composition to known changes in membrane function.