Bioassay of fish gonadotrophin by ovarian mitochondrial cholesterol depletion

Purification and properties of pituitary gonadotropic hormone from Indian teleosts: freshwater murrel (Channa punctatus) and carp (Catla catla)

Gonadotropic hormone (GtH) from the pituitaries of two widely different Indian teleosts, a freshwater murrel and a carp, was purified by solvent fractionation, Sephadex G-100 gel filtration, affinity chromatography on concanavalin A-Sepharose (Con A-Sepharose), and immunoaffinity chromatography. Elution profile from gel filtration showed three peaks in both cases, peak I and II were clearly separated in murrel but not in carp. Peak II demonstrated strong GtH activity in both murrel and carp but this activity could also be detected in peak I and III of carp. TSH activity was restricted to peak I in murrel and was distributed in all three peaks in carp. Chromatography on Con A-Sepharose was useful in harvesting carp glycoprotein hormones, but gave no advantage in murrel. A pure homogenous GtH from murrel and carp could be obtained by using immunoaffinity chromatography. Polyacrylamide gel electrophoresis of murrel and carp GtH showed a single discrete band. Determination of molecular weight (MW) by Sephadex G-100 gel filtration indicated that murrel and carp GtH were 42,000 and 40,000 Da, respectively. SDS-polyacrylamide gel electrophoresis of murrel and carp GtH revealed two dissimilar subunits, alpha and beta. MW of murrel and carp alpha-subunits were 18,000 and 16,000 while those of beta were 27,000 and 26,000 Da, respectively. Comparison of electrophoretic patterns of murrel and carp GtH alpha and beta with ovine LH subunits reveal distinct teleostean GtH subunits.

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.

Fundulus heteroclitus gonadotropin(s). I. Homologous bioassay using oocyte maturation and steroid production by isolated ovarian follicles

Isolated ovarian follicles from several species were cultured to develop an in vitro bioassay system for Fundulus heteroclitus gonadotropin. An extract of F. heteroclitus pituitaries, when tested in heterologous systems using follicles from Rana pipiens. Xenopus laevis, and Carassius auratus, was ineffective in provoking either germinal vesicle breakdown or steroid production. In a homologous system using F. heteroclitus follicles, F. heteroclitus pituitary extract was capable of inducing both germinal vesicle breakdown and steroid production in a dose-dependent fashion. Testosterone, estradiol-17 beta, and 17 alpha-hydroxy,20 beta-dihydroprogesterone were detected in both the culture media and the follicle extracts after F. heteroclitus pituitary extract stimulation. The steroidogenic responses resulting from the pituitary extract stimulation were dependent on the size and stage of follicular development. Only large vitellogenic follicles (1.2-1.4 mm diameter) were able to produce 17 alpha-hydroxy,20 beta-dihydroprogesterone and testosterone. Small vitellogenic follicles (less than 1.2 mm) were unresponsive to stimulation by F. heteroclitus pituitary extracts as scored by either germinal vesicle breakdown or production of 17 alpha-hydroxy, 20 beta-dihydroprogesterone and testosterone. However, estradiol-17 beta production was detected in follicles of a much wider size range: Follicles as small as 0.8 mm diameter were responsive to F. heteroclitus pituitary extract stimulation and produced a large quantity of estradiol-17 beta. There was a marked seasonal sensitivity of F. heteroclitus follicles to pituitary extract stimulation in vitro. Follicles obtained from fish outside of the breeding season (January) were less responsive to stimulation by pituitary extract or steroid. The same preparation of pituitary extract was capable of provoking germinal vesicle breakdown in follicles obtained in May. Pituitary extracts prepared during October through January were also less potent than those prepared during the breeding season (February through September). We conclude that F. heteroclitus gonadotropin(s) shows a noticeable species specificity and that F. heteroclitus follicles exhibit both a season- and a size-dependent responsiveness to gonadotropin(s). Hence, with a judicious use of the appropriate types of F. heteroclitus ovarian follicles, we have been able to demonstrate that in vitro oocyte maturation and steroid production are sensitive, homologous bioassays for F. heteroclitus gonadotropin(s).