[Ultrastructure of prolactin cells of Gambusia (teleost fish) in fresh water, deionized water and hypersaline medium]

Osmoregulation in the stenohaline freshwater catfish, Heteropneustes fossilis (Bloch) in deionized water

Transfer of the stenohaline catfish, Heteropneustes fossilis from tap water (TW) to deionized water (DW) resulted in an increase in the glomerular filtration rate, urine volume and osmolar and free water clearance. In a closed system, where the DW was renewed only once a day, no change in the plasma osmolality was evident for up to 14 days. When DW was renewed four times a day for 25 days, a significant reduction in the plasma osmolality was observed within 24h. When the fish were transferred back to TW, plasma osmolality increased to normal freshwater level within 24h. These observations suggest the existence of highly efficient branchial mechanisms for active uptake of salts from an exceedingly dilute ambient medium. The fact that prolactin-secreting cells as well as corticotrophs in the pituitary of the fish in DW were highly stimulated suggests the involvement of the hormones in the adaptive responses of the catfish to DW.

Control of prolactin secretion in the teleost Oreochromis mossambicus: effects of water acidification

Prolactin secretion is stimulated markedly in fish exposed to water of pH 4. This phenomenon was used to study the control of prolactin secretion. Activation occurs irrespective of changes in plasma osmolarity or plasma sodium and calcium concentrations. After acute acidification of the water, which leads to a substantial fall in plasma osmolarity and plasma electrolyte levels, the activation of the prolactin cells is less marked than after gradual acidification of the water, when plasma osmolarity, plasma sodium, and plasma total and ionic calcium levels are not noticeably affected. When fish bearing an implanted rostral pars distalis of the pituitary gland are exposed to water of pH 4, both the in situ prolactin cells and the prolactin cells of the implant become activated only when the drop in water pH is acute and followed by a reduction in plasma osmolarity and electrolyte levels. When the rate of reduction of the pH is slow and not changing plasma osmolarity or sodium and calcium levels, the in situ prolactin cells are stimulated, but not those of the implants. We conclude that the activation of the prolactin cells in situ in fish in acid water is not mediated by reductions in plasma osmolarity, plasma sodium, total calcium, or ionic calcium, but by hypothalamic mechanisms. The drop in plasma osmolarity and electrolytes probably reflects excessive osmoregulatory stress and this may hamper instead of stimulate the response of the prolactin cells to osmoregulatory disturbance. The physiological significance of the in vitro activation of prolactin cells by reduced ambient osmolarity is unclear.

Effect of osmotic pressure on prolactin release in rainbow trout: in vitro studies

To investigate a possible effect of osmotic pressure on prolactin (PRL) release in rainbow trout, we developed a technique for in vitro perifusion of trout pituitaries. Changes in osmotic pressure similar to those observed in fish plasma during transfer experiments did not induce significant modifications of PRL release. In contrast, high-amplitude variation of osmotic pressure resulted in clear modifications of PRL secretion: hyperosmotic medium caused a reduction in PRL release, while infusion of hyposmotic medium induced a transitory increase in PRL release. By using different concentrations of mannitol, we found that the modifications of prolactin secretion could not be ascribed to alterations of the ionic composition of the medium but actually resulted from variations in the osmotic pressure of the incubation medium. In further experiments osmotic pressure was decreased from 300 to 220 mOsm/kg or from 400 to 300 mOsm/kg; a similar transitory increase in PRL release was observed. Measurement of gonadotropin (GtH) in the perifusion effluent medium showed that PRL and GtH secretion followed similar patterns. Thus, our results suggest a possible mechanical effect of wide changes in osmotic pressure on pituitary cell membranes. These data indicate that the rainbow trout differs notably from nonsalmonid teleost species thus far studied in the lack of sensitivity of its PRL cells to osmotic pressure.

Environmental control of prolactin synthesis in the teleost fish Oreochromis (formerly Sarotherodon) mossambicus

In the cichlid fish Oreochromis mossambicus prolactin cell activity is inversely related to the osmolarity and the Ca2+ concentration of the ambient water. Prolactin cell activity was estimated, at the end of a 3-week experimental period, by determination of the rate of prolactin synthesis during incubation of the rostral parts of the pituitary gland, in the presence of [3H]lysine. Since the secretory activity of isolated prolactin cells is known to be inversely related to the osmolarity of the incubation medium, the possibility was investigated that the effects of changes in ionic composition of the ambient water on prolactin secretion in vivo are mediated by changes in osmolarity of the blood plasma. No support was found for this hypothesis. In fish exposed to high water osmolarities prolactin cell activity was reduced, while plasma osmolarity increased. In contrast, at high Ca2+ concentrations of the water, when prolactin secretion was inhibited to a similar extent, plasma osmolarity was significantly reduced. Although direct effects of plasma osmolarity on prolactin cells cannot be excluded completely, it is unlikely that plasma osmolarity is the predominant factor in the control of prolactin cell activity in situ. The physiological significance of the capacity of isolated prolactin cells to respond to changes in osmolarity of the ambient medium--a capacity shared with some other endocrine cell types--is therefore unclear.

Response of prolactin cells in the goldfish adapted to diluted seawater with or without calcium

The activity of prolactin (PRL) cells was investigated in goldfish gradually adapted to diluted (30%) artificial seawater (ASW) or directly immersed in 23% ASW and in similar saline solutions free of calcium. When Ca2+ is omitted, PRL cells appear slightly more active than those in saline solutions with calcium, nuclear areas having intermediate values between those in freshwater (FW) and those in diluted SW. Nuclear areas are also slightly larger in Mg-free SW. Other signs of stimulation are not apparent. These data are compared to those obtained in other teleost species. The lack of stimulation in PRL cells of goldfish adapted to deionized water (DW) suggests that calcium plays a minor role in controlling prolactin secretion in this teleost.

[Ultrastructure of prolactin cells and survival of Gambusia sp. (teleost fish) in deionized water enriched in calcium or sodium]

The ultrastructure of the prolactin (PRL) cells of Gambusia was studied in animals kept in deionized water (EDes) and in EDes supplemented either with Ca2+ (10,2 and 15,3 mM) or with Na+ (10,2 and 15,3 mM). In environments supplemented with CaCl2 the maximal survival was 40 days. The stimulation of PRL cells was similar to that described in EDes. On the contrary, in environments supplemented with NaCl, 50% of the animals are still alive after 45 days. PRL cells are slightly stimulated and their ultrastructural aspect is similar to that observed in controls kept in freshwater. The significance of these results is discussed in relation with our present knowledge of the pituitary control of osmoregulation in teleost fish.

Specific effect of calcium ions on the calcium-sensitive cells of the pars intermedia in the goldfish

Cytological changes in the calcium-sensitive (Ca-s) cells (formerly termed PAS-positive cells) of the pars intermedia were investigated in the goldfish after adaptation to deionized water (DW), with or without addition of sodium, potassium and magnesium. These ions were added as chloride salts at concentrations similar to those present in fresh water (FW). The marked stimulation of the Ca-s cells is not inhibited in DW supplemented with Na+ (0.35 mM/l), K+ (0.05 mM/l), and Mg2+ (0.2 mM/l) for a period of 24 days. The inhibition of the response to DW with calcium chloride (2 mM/l). These data show that chloride ions are not responsible for the regression of the Ca-s cells observed in goldfish kept in DW supplemented with calcium chloride. The effect of calcium ions on the Ca-s cells appears to be specific. These results support the hypothesis that the Ca-s cells synthesize a factor (hypercalcin?) involved in calcium regulation, and that its release is influenced by the calcium content of the environment. The role of the pars intermedia in calcium metabolism is strengthened by the present results. Biochemical data suggest the presence of a hypercalcemic factor in the pituitary of fish (Parsons et al. 1978) and are in agreement with the present cytological findings.

PAS-positive cells of the pars intermedia are calcium-sensitive in the goldfish maintained in a hyposmotic milieu

Cytological changes in the pars intermedia of the goldfish were investigated after adding calcium to deionized water (DW). In fish maintained in DW, the PAS-positive cells are highly stimulated in comparison to cells of fish kept in fresh water (FW). In DW supplemented with calcium at the same concentration as in FW (2 mM/l), the hyperactivity of the PAS-positive cells is prevented. When calcium ions are added 60 h before the animals are sacrificed, the PAS positive cells start to show signs of regression and their granules are stored: the release of the granular material appears to be suppressed by calcium. In the goldfish, the PAS-positive cells, homologous to a similar cell type in the eel, react only very weakly with the PAS technique. The name "calcium-sensitive cells" appears to be more appropriate in the goldfish for this particular cell type, secreting an unknown factor. This factor, different from the prolactin produced in the rostral pars distalis of the hypophysis, might be an equivalent of a "hypercalcin".