Adaptation of rainbow trout to seawater: changes in calcitonin gene-related peptide levels are associated with an increase in hormone-receptor interaction in gill membranes.

Transfer of trout from freshwater to seawater produced changes in plasma calcitonin gene-related peptide (CGRP), calcitonin, and calcium levels. Three days after transfer, the plasma CGRP concentration increased from 517 +/- 54 (n = 6) to 1874 +/- 234 (n = 7) pg/ml. Ten hours after transfer, the plasma CT concentration in seawater-adapted trout was double the control value. Plasma calcium levels also changed: 2 days after transfer, the calcium increased 1.6-fold over the basal value. The increased plasma CGRP level was correlated with an increased specific binding of CGRP to its branchial membrane receptors. Scatchard analysis of the data revealed this increase to be mainly due to increased binding affinity, as there was no difference in binding capacity between control and transferred animals. Carbonic anhydrase activity in branchial membranes increased 2.2-fold over the control value 3 days after transfer. CGRP may thus play a role in osmoregulation and ionic balance.