Ovarian cholesterol dynamics in teleost Channa punctatus (Bloch): relationship with reproductive cycle and response to gonadotropins

Steroidogenesis in Fundulus heteroclitus. I. Production of 17 alpha-hydroxy,20 beta-dihydroprogesterone, testosterone, and 17 beta-estradiol by prematurational follicles in vitro

In order to understand better the mechanism of gonadotropin action on steroidogenesis in prematurational follicles of Fundulus heteroclitus, follicle synthesis of 17 alpha-hydroxy, 20 beta-dihydroprogesterone (17 alpha-OH,20 beta-DHP), testosterone (T), and 17 beta-estradiol (E2) from a variety of precursors and the maturational response of oocytes were simultaneously followed in vitro. The addition of 25-hydroxycholesterol, pregnenolone, or progesterone to unstimulated follicles increased media 17 alpha-OH,20 beta-DHP, T, and E2, as well as oocyte germinal vesicle breakdown (GVBD) in a dose-dependent manner. Inhibition of cholesterol side-chain cleavage by aminoglutethimide blocked 25-hydroxycholesterol-promoted steroid accumulation and GVBD, indicating that 25-hydroxycholesterol does not directly induce GVBD, but rather is metabolized in the follicle to an active steroid (presumably 17 alpha-OH,20 beta-DHP). Likewise, trilostane, an inhibitor of delta 5-3 beta-hydroxysteroid dehydrogenase, blocked pregnenolone action. Both inhibitors also completely abolished steroid accumulation and GVBD promoted by a F. heteroclitus pituitary extract (FPE), but not GVBD induced by exogenous 17 alpha-OH,20 beta-DHP. FPE also significantly depressed T but enhanced E2 production from exogenous precursors. We have concluded from these observations that (1) cholesterol side-chain cleavage and pregnenolone conversion to progesterone are essential for gonadotropin-promoted follicle steroid production and the resulting reinitiation of meiosis by the oocyte, (2) the enzymes necessary for the conversion of cholesterol to 17 alpha-OH,20 beta-DHP, T, and E2 are present in the unstimulated, prematurational follicle, and (3) gonadotropin initiates steroidogenesis by acting at a step prior to the conversion of cholesterol to pregnenolone; it also appears to enhance aromatase activity.

Changes in tissue lipid levels in the freshwater catfishClarias batrachus associated with the reproductive cycle

The levels of free fatty acid (FFA), monoglyceride (MG), diglyceride (DG), triglyceride (TG), phospholipid (PL), free cholesterol (CF) and esterified cholesterol (CE) were measured in the liver, plasma, ovary, abdominal fat and muscle during different phases of the annual reproductive cycle in femaleClarias batrachus. During the preparatory phase, hepatic lipogenic activity predominated over mobilization and consumption. In the prespawning season, an increased hepatic lipogenic activity was maintained, but lipids were transferred from the liver to the ovary. In the spawning phase, the diminished food intake, and enhanced caloric demand for spawning behaviour and activity limited hepatic lipogenic activity, and TG lypolysis was increased as was the production of more FFA. Maximum accumulation of vitellogenin, as reflected by maximum rise in ovarian PL titre was characteristic of this phase. Marked reductions in ovarian lipid occurred during the postspawning phase. In the resting phase, there was a recovery of lipogenic activity, but PL synthesis was still inhibited. In contrast to other investigated teleosts, there were extremely high level of FFA in liver, plasma, ovary and muscle throughout the annual reproductive cycle inC. batrachus. FFA appears to be the main lipid metabolite which had a very high turnover. As evidenced by the high TG content, abdominal fat seems to be the main fat depot, not the liver and muscle.

Bioassay of fish gonadotrophin by ovarian mitochondrial cholesterol depletion

Murrel (Channa punctatus Bloch) ovarian tissues were incubated in vitro with or without piscine gonadotrophins and then subjected to subcellular fractionation followed by nonesterified cholesterol (cholesterol) assay. Gonadotrophin from salmon (SG-G100), tilapia (TL1MS), and sturgeon (S27MS) depleted cholesterol in the mitochondrial fraction, whereas cholesterol remained unchanged in other subcellular fractions. Aminoglutethimide, an inhibitor of mitochondrial cholesterol side-chain cleavage, blocked the depletion of mitochondrial cholesterol in response to SG-G100 and murrel pituitary extract. When ovarian tissue containing [4-14C]cholesterol was challenged with SG-G100 in vitro, a dose-dependent decrease of mitochondrial [4-14C]cholesterol was observed. Increasing concentrations (1, 2, 4, 6, or 8 micrograms/incubation) of SG-G100, TL1MS, and S27MS resulted in a clear linear depletion of mitochondrial cholesterol. The slope of the dose-response curve in different individual fish was found to be distinctly uniform and parallel. The slopes of the standard curves obtained with TL1MS and S27MS were greater than that with SG-G100, indicating that tilapia and sturgeon gonadotrophins are more potent. Carp pituitary gonadotrophin content, determined by using these standard curves, showed the sensitivity and precision of this bioassay.

Plasma thyroxine levels in freshwater perch: influence of season, gonadotropins, and gonadal hormones

Plasma thyroxine (T4) of a freshwater female perch Anabas testudineus (Bloch) has been measured by specific radioimmunoassay (RIA). Monitoring of plasma T4 throughout the year showed that July to early September (spawning stage) had the highest T4 and late September to January (postspawning stage) had the lowest T4. The plasma T4 cycle was coincident with the ovarian cycle. Treatment of intact perch with salmon gonadotropin (SG-G100) or ovine LH (500 ng/100 g body wt) markedly enhanced plasma T4 levels. Ovine FSH was without any effect. Ovarian steroids, estrone and 17 beta-estradiol (100 ng/100 g body wt), also significantly augmented T4 level whereas estriol or progesterone had no such effect. There was a striking fall of plasma T4 in ovariectomized fish. The drop of T4 due to ovariectomy could not be reversed by SG-G100 but administration of estrone or 17 beta-estradiol to ovariectomized fish effectively reversed the drop of plasma T4 level. These findings suggest that gonadotropin's stimulatory effect on perch thyroid is mediated via the release of ovarian steroids.

Interrelationship between plasma and ovarian cholesterol in a teleost fish

The profiles of plasma and ovarian cholesterol altered similarly as a result of seasonal influence. Fish pituitary extract and LH significantly depleted plasma and ovarian free cholesterol only, esterified cholesterol remaining unaltered. Findings indicated that plasma cholesterol was the primary source of sterol for ovarian steroidogenesis.