Somatostatin and altered medium osmotic pressure elicit rapid changes in prolactin release from the rostral pars distalis of the tilapia, Oreochromis mossambicus, in vitro.

Prolactin (PRL) cells in the rostral pars distalis of the tilapia Oreochromis mossambicus respond to somatostatin (SRIF) and reduced medium osmotic pressure within 10-20 min of exposure during perifusion incubation. Pieces of rostral pars distalis tissue were removed from freshwater-adapted tilapia and were preincubated in [3H]leucine in static culture (355 m phi smolal) for 48 hr. Following preincubation, they were placed in the perifusion apparatus and baseline release was established for 3 hr in hyperosmotic medium (355 m phi smolal). Exposure to hyposmotic medium (280 m phi smolal) resulted in a rapid and steep rise in the release of [3H]PRL, which remained elevated for more than 2 hr. When SRIF was added simultaneously with hyposmotic medium, the rise in PRL release normally initiated by reduced osmotic pressure was prevented. Somatostatin also quickly reduced release that had been previously elevated by exposure to hyposmotic medium. The time course of these changes suggests that SRIF and altered osmotic pressure act on PRL secretion in at least partial independence of effects which they may have on PRL synthesis in the tilapia pituitary.

Studies on the regulation of growth hormone release from the proximal pars distalis of male tilapia, Oreochromis mossambicus, in vitro.

The in vitro effects of several factors, including cortisol, somatostatin (SRIF), and medium osmotic pressure, on growth hormone (GH) release from the tilapia pituitary were examined in relation to fish size. Spontaneous GH release from the proximal pars distalis (PPD) of approximately 60-g fish was significantly less than that from tissue of fish weighing either approximately 120 or approximately 280 g when incubated in 340 m phi smolal medium. While GH content of the PPD cultures (tissue + medium measured by densitometry) increased consistently with fish size, GH concentration (per microgram of tissue protein) was variable, being highest in 120-g fish and lowest in 280-g fish. Moreover, GH concentration was not related to GH release. Fish size also appeared to be important in the responsiveness of GH cells to stimulation by cortisol (Nishioka et al., 1985) and by increased osmotic pressure. In cultures of PPD from approximately 60-g fish, in which spontaneous release was relatively low, cortisol and increased medium osmotic pressure significantly enhanced release. Cortisol and hyperosmotic medium were without significant effect, however, on GH release from PPD of approximately 120-g fish, which showed high spontaneous release. In contrast, SRIF, a potent inhibitor of GH secretion, was effective in lowering GH release regardless of fish size. Nevertheless, SRIF was apparently more effective in inhibiting GH release from tissue of 60-g fish than from tissue of 120-g fish. Our data suggest that GH secretion may be augmented when smaller tilapia (approximately 60 g) are transferred to seawater, a situation in which blood cortisol and osmotic pressure would presumably be elevated.

The role of calcium in prolactin release from the pituitary of a teleost fish in vitro.

The release of PRL from the pituitary of a teleost fish, the tilapia (Oreochromis mossambicus), has been previously shown to be dependent on calcium. However, the source(s) and specific action(s) of calcium in the secretory process have not been identified. Also undefined are the mechanisms by which regulators of PRL cell function may alter calcium distribution. In the present investigation, the elevation of medium K+ concentration during static incubations to a depolarizing concentration (56 mM) produced no change in cumulative PRL release over control levels during the 18-20 h of incubation. During perifusion incubation, exposure to high K+ concentrations briefly stimulated (less than or equal to 10 min) and then depressed PRL release. In contrast, reduced medium osmotic pressure elicited a rapid elevation in PRL release that was sustained for 2 h or more. D600, a calcium entry blocker, at 10(-5) M diminished the K+-induced pulses of PRL release. The same concentration, however, did not alter the release of PRL evoked by reduced osmotic pressure. In contrast, CoCl2, which blocks a range of calcium-mediated processes in addition to calcium influx, suppressed PRL release during perifusion and static incubations in hyposmotic medium. These findings suggest that while PRL secretion from the tilapia pituitary is calcium dependent, calcium entry through voltage-regulated plasmalemma channels may not be a prerequisite to the actions of reduced osmotic pressure.

The thyroid gland of the Hawaiian parrotfish and its use as an in vitro model system.

Thyroid tissue in the Hawaiian parrotfish, Scarus dubius, is organized into two discrete lobes, one anterior to the first pair of afferent branchial arteries and the other between the first and second pairs. Lobes or pieces of thyroid lobes from S. dubius were incubated in static or perifusion culture using a simple defined medium. In static culture, this thyroid tissue releases thyroxine (T4) responding to bovine thyrotropin (bTSH) in a dose-related manner during 4- and 24-hr incubations. At 24 hr, however, substantially lower concentrations of bTSH are required to evoke a significant elevation of T4 than at 4 hr. Exposure to bTSH also elicits morphological changes within 4 hr, which include colloid resorption and an increase in the height of the follicular epithelium. During perifusion culture, T4 release rises rapidly within 20 to 30 min following the initiation of exposure to bTSH and remains elevated for between 4 and 8 hr thereafter. In spite of high plasma triiodothyronine (T3) concentrations, the parrotfish thyroid releases no detectable T3 during in vitro culture. This is the first direct evidence in support of the notion that plasma T3 in a teleost fish may be derived principally, perhaps exclusively, from the peripheral monodeiodination of T4.