SUMMARY Isogenic carp Cyprinus carpio L. were acclimated to water temperatures of 15, 22 and 29°C for at least 8 weeks. The acclimations consistently resulted in slightly, but significantly, different plasma osmolality, sodium, potassium and chloride concentrations between the groups studied. Plasma total and ionic calcium levels were unaffected, indicating successful adaptation. The apparent changes in set point for plasma ion levels are explained by altered sodium pump activity and hormonal control of branchial permeability to water and ions. It appears that in 15°C-acclimated fish, a lower apparent Na+/K+-ATPase activity is compensated by strongly enhanced Na+/K+-ATPase expression (determined biochemically and immunohistochemically). In 29°C-acclimated fish, the higher ambient temperature activates the enzyme. Arrhenius plots for branchial Na+/K+-ATPase preparations of the three groups of fish suggest the occurrence of different enzyme isoforms or protein (in)stability as explanations for differences in apparent enzyme activities, rather than temperature-dependent changes in membrane fluidity. As for hormonal control over permeability, prolactin mRNA expression (and anticipated production and release) is lower in fish kept at 29°C, suggesting that control over branchial permeability to water and ions needs to be downregulated at higher temperatures. In so doing, enhanced sodium pump activity is balanced by a controlled passive ion loss to fine-tune plasma sodium levels. Basal plasma cortisol levels did not correlate positively with Na+/K+-ATPase expression, but doubling plasma cortisol levels in control fish by administering exogenous cortisol (for 7 days, using implanted minipumps and thus stress-free) enhanced Na+/K+-ATPase expression. This effect must be the result of a glucocorticoid action of the steroid: in fish, mineralocorticoid receptors have higher affinity for cortisol than glucocorticoid receptors. At a lower ambient temperature, branchial Na+/K+-ATPase expression is upregulated to counteract the temperature-inhibited activity of the sodium pump, perhaps via a mineralocorticoid receptor.
ABSTRACT In stressed tilapia, Oreochromis mossambicus , total α-melanocyte-stimulating hormone (α-MSH) levels and di-acetyl α-MSH/mono-acetyl α-MSH (di:mono) ratios are elevated. We therefore investigated the role of α-MSH in the regulation of the pituitary–interrenal axis. The corticotrophic activities of des-acetyl α-MSH, mono-acetyl α-MSH and di-acetyl α-MSH were compared. These forms of α-MSH were isolated from neurointermediate lobes and tested in a superfusion experiment with homologous interrenal tissue. The corticotrophic activity of di-acetyl α-MSH was the highest, followed by that of des-acetyl α-MSH and mono-acetyl α-MSH. Apparently, acetylation of α-MSH is of functional significance for corticotrophic action. Di-acetyl α-MSH proved to be about 100 times less potent than ACTH(1–39): the half-maximal stimulating concentrations for ACTH and di-acetyl α-MSH were 0·89 nmol/l and 110 nmol/l respectively. Surprisingly, a superfusate from neurointermediate lobes proved to be only about three times less active than a superfusate from the pituitary pars distalis, in which the corticotrophic activity is attributable to its ACTH content. When selectively stripped of all forms of α-MSH by passage through a Sepharose column coated with an antiserum against α-MSH the neurointermediate lobe superfusate was devoid of corticotrophic activity. Thus α-MSH appears to be the corticotrophic factor in the superfusate of the neurointermediate lobe. After the same treatment, the corticotrophic activity of the pars distalis superfusate was not affected. We conclude that ( in vivo ) an as yet unidentified factor is co-released with α-MSH from the neurointermediate lobe, and that this potentiates its corticotrophic activity. Journal of Endocrinology (1992) 135, 285–292
A bstract-Carp eggs were exposed immediately after fertilization to Cu concentrations of 0.3 and 0,8 (xmol/L at water pH 7.6 or pH 6.3.Mortality, the incidence of spina!cord deformation, heart rate, tail movements, hatching success, and whole-body content of K, Na, Mg, Ca, and Cu were determined over time.Light microscopical preparations of eggs (48 h after fertilization) and larvae (168 h after fertilization) were studied.At pH 7.6, Cu did not affect egg mortality, heart rate, tail movements, and whole-body K and Mg content.Hatching success increased only in the 0.3 fimol/L Cu group.Exposure to 0.8 (junol/L Cu increased larval mortality and larval deformation and decreased whole-body Na and Ca content.At pH 6.3, exposure to 0.8 |j.mol/L Cu increased egg mortality and decreased heart rate and tail movements.Furthermore, premature hatching, a concentration-dependent increase of larval mortality, and larval deformation was observed.Exposure to 0.3 and 0.8 p.mol/L Cu decreased the whole-body content of K, Na, Mg, and Ca.Uptake of Cu after hatching increased two-fold at pH 6.3 compared to the pH 7.6 groups.At pH 6.3, all Cu-exposed larvae were unable to fill their swim bladder.Also, after 168 h the yolk sac remained largely unabsorbed in the 0.3 and 0.8 ixmol/L Cu group.Exposure to 0,8 (xmol/L Cu resulted in coagulation of proteins in eggs and yolk sacs.No significant changes in any of the assessed parameters were observed in control groups of pH 6.3 and pH 7.6.
There is some consensus now that the death of many fish species, exposed to acid water, is caused by a chain of events starting with the loss of body electrolytes and eventually leading to osmoregulatory and cardiovascular failure (Muniz & Leivestad, 1980; McDonald & Wood, 1981; McDonald, 1983). Sublethal exposure to acidified water often leads to transient or chronic hypo-osmolarity of the blood plasma, mainly caused by reduced Na+ and Cl- levels (McWilliams, 1980; McDonald, 1983; Wendelaar Bonga, Van der Meij & Flik, 1984a). The severity and duration of these effects are determined by both external and internal factors. External factors, such as the calcium and aluminium concentration of the water and the presence of heavy metals, are dealt with by Wood, Potts & McWilliams, and McDonald et al. (this volume). The rate and degree of change of the environmental pH is also important. Internal factors, in particular hormones, are the subject of this chapter.
