Daily rhythms of liver cAMP, total liver lipids, prolactin-like hormone and growth hormone cell activities in Sarotherodon mossambicus acclimated to different photoperiod regimes

Circadian pattern of hepatosomatic index, liver glycogen and lipid content, plasma non-esterified fatty acid, glucose, T3, T 4, growth hormone and cortisol concentrations in Oncorhynchus mykiss held under different photoperiod regimes and fed using demand-feeders

The circadian patterns of several tissue and plasma metabolites, and several plasma hormone concentrations are described in rainbow trout (Oncorhynchus mykiss) that were held in groups under three different photoperiod regimes, and given free access to a demand-feeder. Regardless of photoperiod regime, all the measured parameters showed significant diel rhythms that appeared to be synchronized by dawn; dawn was represented by the concomitant onset of both light and feeding. The diel increases in hepatic glycogen content, and plasma T4 and cortisol concentrations were in phase with the main period of feeding activity, whereas the peaks in plasma T3 and glucose concentrations that may also be triggered by feeding activity, were delayed by several hours. The peaks in hepatosomatic index, plasma non-esterified fatty acids and plasma growth hormone concentrations were 180° out of phase with the main period of feeding activity, and associated with periods of hypophagia and low activity.

Norepinephrine: a potent activator of glycogenolysis and gluconeogenesis in rockfish hepatocytes

Norepinephrine (NOR) is a potent activator of carbohydrate metabolism in isolated hepatocytes from copper rockfish (Sebastes caurinus), increasing rates of glycogenolysis fourfold with an EC50 of 6.3 nM. Nanomolar concentrations of NOR also enhance gluconeogenesis. Epinephrine (EPI) activates both pathways to a smaller extent; the corresponding EC50 for glycogenolysis is 320 nM. There is no significant difference between the magnitude of glucose production in response to comparable doses of NOR, bovine glucagon, and catfish glucagon-like peptide. Experiments with an adrenergic agonist (isoproterenol) and antagonists (propranolol, prazosin, atenolol) indicate that NOR effects are mediated through beta-adrenoceptors. Catecholamine-activated glycogenolysis measured at 100 nM EPI or NOR is poorly correlated with a 30-50% rise in intracellular cAMP. Glucose production following catecholamine administration is not linear: 50% of the hourly glucose output is released within the first 17 min (NOR) and 5 min (EPI), respectively. During hepatocyte incubation (60 min at 15 degrees), added NOR and EPI (100 nM) were not degraded to any significant extent. In the absence of added hormones, rockfish hepatocytes produce 7.41 +/- 0.89 mumol glucose x g-1 packed cells x hr-1 at 15 degrees, with gluconeogenesis accounting for 35.0% of the total production. The rate of glucose output, which is linear for at least 60 min, is not correlated with the initial hepatocyte glycogen level.

Effects of diet and feeding time on daily variations in plasma insulin, hepatic c-AMP and other metabolites in a teleost fish,Dicentrarchus labrax L

Juvenile sea bass, 1.5 years old, of mixed sex, held on long photoperiods were fed early on the photoperiod and late on the photoperiod, using different diets. Fish fed natural diets showed a daily rhythmicity of plasma insulin, liver c-AMP, plasma glucose, liver glycogen and muscle glycogen content, however, fish fed a commercial diet did not show this daily rhythmicity except for plasma insulin levels. In addition, these fish had significantly lower levels of plasma insulin, liver c-AMP and plasma glycerol than the group fed on the natural diets at similar feeding times. The time of feeding also induced different rhythmicity patterns in hormones and metabolites as well as a significant change in their mean levels. These facts are discussed in relation with the pre-feeding activity and increased appetite exhibited by the fish fed late on the photoperiod and with their implications on fish culture.

Effect of changing the light/dark schedule, the time of onset of the light or dark period, or the daylength, on rhythms of epidermal cell proliferation

Rhythms of labeling and mitotic indices were studied in the hindlimb epidermis of the anuran tadpole Rana pipiens under different light/dark (LD) cycles and daylengths in order to examine the role of the various parameters of the lighting regimen in setting the periods of the rhythms and the timing of the cell proliferation peaks. Altering the time of, or inverting, the 12 h light period on a 24 h day resulted in phase shifting of basically bimodal circadian rhythms with peaks in the light and dark. Thus the cell proliferation rhythms were entrained to the LD cycle. These rhythms also entrained to noncircadian schedules since they lengthened on a 15L:15D cycle and shortened on a 9L:9D cycle, although the bimodal characteristic of a peak in the light and a peak in the dark remained. Studies of 18L:6D and 6L:18D cycles in which either the time of onset of light or dark was changed relative to the 12L:12D control indicated that the onset of dark may regulate the timing of the labeling index peaks while the onset of light may determine the time of occurrence of mitotic index peaks. Control of the timing of labeling and mitotic index peaks by different parameters of the LD cycle suggests a mechanism for cell cycle regulation by the environmental lighting schedule. Analysis of the rhythms on all the cycles studied suggested that labeling index rhythms equal the length of, or twice the length of, the dark period. Mitotic index rhythms equal the daylength or a multiple of the length of the dark period.

Catecholamine effect on cyclic adenosine 3':5'-monophosphate level in isolated catfish hepatocytes

The effect of catecholamines (epinephrine, norepinephrine, isoproterenol, and phenylephrine) on cyclic adenosine 3':5'-monophosphate (cAMP) level in isolated catfish (Ictalurus melas) liver cells was studied in the presence or absence of alpha (phentolamine) and beta (propranolol)-receptor antagonists. All catecholamines increased the hepatocyte cAMP level: the rank of their potency was epinephrine = isoproterenol greater than norepinephrine greater than phenylephrine. Propranolol completely blocked the catecholamine effect; phentolamine was ineffective. Results confirm previous findings (L. Brighenti, A. C. Puviani, M. E. Gavioli, and C. Ottolenghi, 1987, Gen. Comp. Endocrinol. 66, 306-313) that epinephrine and norepinephrine act via beta-receptor activation. However, the comparison of the effects of isoproterenol and phenylephrine on cAMP with those on phosphorylase alpha and on glycogen breakdown suggests that a more complex mechanism is possibly involved in the catecholamine effect on catfish glycogenolysis.

Seasonal variations in body growth rates and circulating levels of growth hormone in the goldfish, Carassius auratus

Several times throughout the year, changes in serum growth hormone (GH) levels over a 24-h period were determined in goldfish maintained under photoperiods and temperatures simulating natural (Edmonton) environmental conditions. In the goldfish a reproducible daily rhythm in circulating GH levels was not present at any time of the year. The average serum GH level over the daily sampling period and the instantaneous relative growth rate in goldfish sampled at the various times of the year were also determined. The highest mean daily serum GH levels were found in March and June, whereas the lowest level was found in goldfish sampled in November. Changes in mean daily serum GH levels were closely correlated to seasonal changes in daylength. The highest growth rate was found in goldfish sampled in July, whereas the lowest growth rates were found in February and March. Female goldfish exhibited a faster growth rate than male goldfish at certain times of the year, but sexual differences in growth rate were correlated with sexual differences in serum GH levels only in November when female goldfish had a higher serum GH level than male goldfish.

Daily rhythms of insulin and glucose levels in the plasma of sea bass Dicentrarchus labrax after experimental feeding

Significant and inverse circadian rhythms are demonstrated in glucose and plasma insulin in fish fed a natural diet. The highest glucose levels are found during the light period, around feeding time, and the insulin level peaks during the dark period. As a possible cause for the insulin rhythmicity, the daily variations in several plasma amino acids are considered. The feeding times could be a training factor for metabolic rhythms, which are maintained even during a fast of 7 days. The differences in the compositions of the diets could be responsible for the lack of circadian rhythms in fish fed on a commercial diet.