Cortisol was previously shown to rapidly (10-20 min) reduce the release of prolactin (PRL) from pituitary glands of tilapia ( Oreochromis mossambicus). This inhibition of PRL release by cortisol is accompanied by rapid reductions in 45 Ca 2+ and cAMP accumulation. Cortisol's early actions occur through a protein synthesis-independent pathway and are mimicked by a membrane-impermeable analog. The signaling pathway that mediates rapid, nongenomic membrane effects of glucocorticoids is poorly understood. Using the advantageous characteristics of the teleost pituitary gland from which a nearly pure population of PRL cells can be isolated and incubated in defined medium, we examined whether cortisol rapidly reduces intracellular free calcium ([Formula: see text]) and suppresses L-type voltage-gated ion channel activity in events that lead to reduced PRL release. Microspectrofluorometry, used in combination with the Ca 2+ -sensitive dye fura 2 revealed that cortisol reversibly reduces basal and hyposmotically induced [Formula: see text] within seconds ( P < 0.001) in dispersed pituitary cells. Somatostatin, a peptide known to inhibit PRL release through a membrane receptor-coupled mechanism, similarly reduces [Formula: see text]. Under depolarizing [K + ], the L-type calcium channel agonist BAY K 8644, a factor known to delay the closing of L-type Ca 2+ channels, stimulates PRL release in a concentration-dependent fashion ( P < 0.01). Cortisol (and somatostatin) blocks BAY K 8644-induced PRL release ( P < 0.01; 30 min), well within the time course over which its actions occur, independent of protein synthesis and at the level of the plasma membrane. Results indicate that cortisol inhibits tilapia PRL release through rapid reductions in [Formula: see text] that likely involve an attenuation of Ca 2+ entry through L-type voltage-gated Ca 2+ channels. These results provide further evidence that glucocorticoids rapidly modulate hormone secretion via a membrane-associated mechanism similar to that observed with the fast effects of peptides and neurotransmitters.
Varying and contending models have been proposed to explain how teleost ionocytes facilitate ion uptake from freshwater environments. A recently characterized member of the clc Cl − channel family ( clc2c ) has been proposed as a conduit for basolateral Cl − transport by Na + /Cl − cotransporter ( ncc )‐expressing ionocytes of stenohaline zebrafish. It is unknown to what extent clc2c is expressed in gill of euryhaline species, such as Mozambique tilapia, and whether expression is modulated in response to extrinsic (salinity) and intrinsic (hormones) factors. Here, we investigated prolactin (Prl) and local osmotic control of clc2c mRNA levels. First, branchial clc2c expression was markedly enhanced in freshwater‐acclimated tilapia compared with seawater‐acclimated tilapia. To investigate endocrine control of this salinity‐dependent expression, we injected hypophysectomized tilapia with ovine Prl. Ovine Prl induced a >8000‐fold increase in clc2c expression from saline‐injected controls. To then test whether Prl regulates clc2c expression in a gill‐autonomous fashion, we incubated gill filaments for 24 hours in the presence of homologous Prls, Prl 177 and Prl 188 . By 24 hours, Prl 188 stimulated clc2c expression ~5‐fold from controls. We then incubated filaments in media ranging from 280 to 450 mOsm/kg for 3 hours to investigate the possibility that extracellular osmolality exerts a local effect on clc2c expression. clc2c showed higher expression with decreasing media osmolalities. Collectively, our results suggest that both hormonal and extracellular osmotic conditions direct clc2c expression in branchial ionocytes. We propose that multifactorial modulation of clc2c expression contributes to the excellent adaptability of Mozambique tilapia to variations in environmental salinity. Support or Funding Information Supported by Start‐Up Funds (Skidmore College) to J.P.B.
Smoltification is a transformation that occurs in some species of salmon, during which solitary fish in fresh water become schooling fish and migrate to the sea. This process is accompanied by large increases in plasma T4. T4 secretion rate and other parameters of T4 metabolism in juvenile coho salmon were estimated by applying kinetic analyses to measurements of the disappearance of injected T4 radiotracer from plasma. Studies were performed at the beginning (March) and end (May) of the increase in T4 concentration in fresh water and seawater. Early and intensive sampling permitted characterization of a very fast initial component of the T4 disappearance curve when analyses included a zero time datum derived from an independent estimate of plasma volume. The plasma volume, equal to 1.77% of body weight, was obtained by measuring the disappearance of radiolabeled albumin from the plasma in two other groups of animals in fresh water and seawater. There were 3- to 7-fold changes in T4 production, distribution, and metabolism between March and May, whereas environment (fresh water vs. seawater) had relatively minor effects on T4 kinetics. In fresh water, the T4 secretion rate was 4.48 ng/h in March and 1.50 ng/h in May. The total T4 pool size was 37.8 ng in March and 12.2 ng in May. Plasma-tissue T4 fluxes were 3- to 7-fold greater in May. Relatively less T4 was distributed in tissue in May (63% vs. 83%), and T4 spent much less time in tissue in May than in March during each pass through the tissue space (11 min vs. 3.1 h). We propose that the difference in secretion rate and a redistribution of T4 between blood and tissues contribute to both the rise and fall in the plasma T4 concentration between March and May. Changes in T4 kinetics during salmonid smoltification resemble those occurring during amphibian metamorphosis and mammalian gestation and neonatal life, and may reflect an increased requirement and an important role for thyroid hormones during periods of rapid development in vertebrates in general. (Endocrinology 115: 399–406, 1984)
