Studies on the relationship between osmotic or ionic regulation and thyroid gland activity in two salmonid fishes, Salmo gairdneri Richardson and Oncorhynchus kisutch Walbaum

Abstract

In freshwater-acclimated rainbow trout a single intraperitoneal injection of ovine TSH significantly elevated plasma thyroxine (T4) levels within I h after the injection. In seawateradapted trout the increase in T, after TSH-treatment was not evident until 6 h after the injection. TSH caused a transient fall in plasma Na+ and C1-between 3 h and 9 h after the injection in seawater-adapted fish and plasma Na+ was lowered in freshwater-adapted trout 24 h after the injection. Although there were clear histological changes in the thyroid gland of freshwater-adapted trout after TSH-injection, no such changes were evident in seawateradapted fish.Plasma thyroid hormone levels and thyroid histology in freshwater-adapted rainbow trout and coho salmon transferred to sea water, and seawater-adapted trout transferred to fresh water showed no consistent changes compared with controls.The data are interpreted to indicate that although ambient salinity may have indirect effects on thyroid activity there is no direct involvement in ionic or osmotic regulation in the two species.

Measurement and regulation of thyroidal status in teleost fish

“…The response of thyroid tissue to a standa~'xtized TSH dose is not fixed for a given species, but varies with season and physiologic state (Milne and Leatherland, 1980;Grau et al, 1985;Specker and Richman, 1984;Specker and Schreck, 1984;Specker and Kobuke, 1987;Swanson and Dickhoff, 1987). For the killifish (Fundulus heteroclitus, Cyprinodontidae), the seasonal differences in the TSH-induced plasma T4 level were not explained by any alterations in plasma T4 clearance, supporting the view that the seasonal changes in TSH action probably occurred at the level of the thyroid tissue itself (Grau et al, 1985).…”

“…3). In the hyperactive state, the epithelial cell nucleus is more basal and spherical and possesses a more prominent nucleolus (Ortman and Billig, 1966;Eales, 1979a;Milne and Leatherland, 1980;Nishioka et aL, 1987). Furthermore, electron microscopy indicates that TSH increases the amount of rough endoplasmic reticulum, the proportion of cell area occupied by lysosomes and the number of microvilli or pseudopods at the luminal surface.…”

“…The T 4 release can be assessed by immunoassay of plasma T 4 or by chromatographic analysis of endogenously labelled iodothyronines released from thyroids of fish previously injected with *I-. T 4 increases in a dose-dependent manner with no change in T 3 Eales, 1975, 1976;Brown et aL, 1978;Milne and Leatherland, 1980;Ng et al, 1982;Swanson et al, 1988, Inui et al, 1989. The T4 (but not T3) concentration is also raised in the perfusion medium by treating thyroid tissue extirpated from parrotfish with TSH .…”

“…While this broad array of investigational approaches provides fish researchers with valuable tools, it also creates confusion concerning choice of method and data interpretation, especially because simultaneously employed methods do not always agree (e.g. Eales, 1964;Drury and Eales, 1968;Chan and Eales, 1976;Milne and Leatherland, 1980;Vijayan et al, 1988). What guidance can be provided regarding methodology to potential investigators of thyroid function in fish?…”

Changes in plasma prolactin and growth hormone concentrations during freshwater adaptation of juvenile chum salmon (Oncorhynchus keta) reared in seawater for a prolonged period

“…However, they do not appear to control the development of hypoosmoregulatory capacity, although they probably affect behaviour, growth and morphological development such as body silvering (Milne and Leatherland 1980;Miwa and Inui 1985;Ikuta et al 1985;Iwata et al 1987). Kinetic analyses of T 4 secretion rate and other parameters of T 4 metabolism were made in juvenile coho salmon by Specker et al (1984).…”