Seasonal changes in plasma thyroxine kinetics in coho salmon Oncorhynchus kisutch during smoltification

Abstract

Growth-accelerated yearling coho salmon smolts, weighing approximately 200 g, were collected in spring and cannulated via the dorsal aorta; they were held individually in containers during the process. A cannula was connected outside the container with a syringe, and the fish received an intra-arterial injection of 13.3 ng/g body weight thyroxine (T4). A group of growth-accelerated smolts weighing more than 190 g were used as control fish. Another group of normal body-sized fish was monitored smoltification parameters through the parr–smolt transformation to evaluate the validity of the growth-accelerated smolts. Although condition factor (CF) of the growth-accelerated smolts decreased from February to March, it did not change afterward until a significant increase in July. The plasma T4 levels increased from February to a peak in late April and decreased thereafter. Gill Na+, K+–ATPase activity showed a peak in late June. According to the smolt parameters, the smoltification developed from March and desmoltfied in July. Peak levels of plasma T4 1 min after the T4 injection were lower during mid smolt stage than those at the beginning and end of smoltification. The T4 metabolic clearance rate (MCR) increased rapidly and attained a peak in late March, and decreased thereafter when the plasma T4 level was at the peak. The T4 secretion rate exhibited also a peak in late March with a tendency to decrease afterward, similar to the MCR. These results indicate that the highest MCR of circulating T4 appears in the early stage of smoltification when plasma T4 level is still low, and MCR lowers when the fish reach the full smolt stage. In the full smolt stage, the increased circulating T4 at the period of T4 surge is the result of the lower T4 metabolism.