AGE-RELATED, SEX-RELATED, AND SEASONAL-CHANGES OF PLASMA-LIPOPROTEIN CONCENTRATIONS IN TROUT

Seasonal variability in physiological parameters can be attributed to seasonal variations in environmental factors and/or to the consequence of the presence of endogenous circannual rhythms. In the current stud y we have measured plasma levels of lipids and of the different lipopr otein classes in fasting trout (Oncorhynchus mykiss) between the ages of 5 and 44 months. Independent of age and sexual maturity, a circannu al variation in the low density lipoprotein concentration between 250 and 1300 mg/dl was demonstrated in both sexes. These seasonal fluctuat ions might be controlled by an endogenous biological clock synchronize d by the photoperiod. The lipoprotein profile of trout is dominated by high density lipoproteins as early as the first months of life. Their concentration increases progressively during sexual maturation from a bout 1200 mg/dl in juveniles to about 2500 mg/dl during spermiation or at the moment of ovulation. This increase is highly significantly cor related with the increased concentration of testosterone occurring in both sexes during sexual maturation. The concentration of very low den sity lipoproteins increases substantially, from about 150 mg/dl to a m aximal concentration of 800 mg/dl in females and 1100 mg/dl in males, during the deposit phase of lipid reserves which precedes the rapid in crease in the gonadosomatic ratio. In the course of rapid ovarian grow th, vitellogenin appears in the plasma of females and reaches a concen tration of 2200 mg/dl 1 month before ovulation. From these results it is concluded that season and reproductive cycle are the two main facto rs affecting basal plasma lipid and lipoprotein levels in trout. Envir onmental factors such as photoperiod or endocrine factors such as the concentration of steroid hormones can be correlated and/or involved in the regulation of these quantitative variations. These results also s uggest the presence of an endogenous biological clock able to exert an independent effect on plasma lipid and lipoprotein levels.