17α,20β,21-Trihydroxy-4-pregnen-3-one: the probable maturation-inducing steroid of the Lusitanian toadfish

17,20β,21-Trihydroxy-4-pregnen-3-one (17,20β,21-P) was identified as the major metabolite of incubations of Lusitanian toadfish Halobatrachus didactylus ovarian follicles with [³H]-17hydroxyprogesterone. The potency of several steroids in inducing germinal vesicle breakdown of follicle-enclosed oocytes of Lusitanian toadfish was systematically examined by using an in vitro germinal vesicle breakdown (GVBD) bioassay. 17,20β-Dihydroxy-4-pregnen-3-one (17,20β-P) and 17,20β,21-P, two confirmed maturation-inducing steroids (MIS) in teleosts, were the most potent in inducing GVBD with ED₅₀s ranging between 9 and 271 nM. Structure-activity relationships followed similar patterns to what has been observed in similar bioassays, i.e. a vital requirement for 17- and 20β-hydroxyl groups in C21 steroids and a reduction in activity of 14 and 5–6%, respectively, for 5-pregnene and 5β-pregnanes compared to 4-pregnenes. Corticosteroids, testosterone and 17β-oestradiol were ineffective. Folliculated oocytes stimulated by pituitary homogenate produced 17,20β,21-P from endogenous substrates in amounts one order of magnitude higher than 17,20β-P. These results strongly support the hypothesis that 17,20β,21-P is the likely MIS in this species.     

Reproductive biology and endocrine regulation of final oocyte maturation of captive white bass

The ovarian development, and plasma levels of gonadotropin II (GtH II) and sex-steroid hormones at the end of vitellogenesis were examined in captive white bass Morone chrysops. The changes in plasma hormone levels and oocyte morphology associated with gonadotropinreleasing hormone agonist (GnRHa)-induced final oocyte maturation (FOM) were studied. Although plasma 17β-oestradiol (E₂) and oocyte diameter increased, there were no changes in GtH II, testosterone (T), 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) or 17,20β,21-dihydroxy-4-pregnen-3-one (17,20β,21-P) in non-hormone-treated females, and no FOM was observed. Treatment with a sustained-release GnRHa delivery system (GnRHa implant) induced two FOM cycles separated by about 24 h, with the release of approximately equal numbers of eggs in each spawn. Plasma GtH II levels were elevated significantly throughout FOM, reaching a maximum of 9·07 ± 1·55 ng ml^?1 in ovulated fish. Both plasma E₂ and T increased soon after the GnRHa treatment, but E₂ declined in fish undergoing germinal vesicle (GV) migration. Plasma T increased further during FOM (7·55 ± 2·87 ng ml^?1), but declined precipitously at ovulation. A surge in plasma 17,20β-P and 17,20β,21-P (4·11 ± 0·97 ng ml^?1 and 3·10 ± 0·77 ng ml^?1, respectively) was observed in females undergoing GV breakdown (GVBD). Based on the involvement of different sex-steroid hormones, FOM was separated into two stages. Early FOM included lipid-droplet coalescence and GV migration, and was associated with elevations in plasma GtH II and T. Late FOM included GVBD and yolk-globule coalescence, and was associated with elevations in plasma GtH II, 17,20β-P and 17,20β,21-P. The results of this study point to the absence of a surge in plasma GtH II as the missing link in the reproductive axis responsible for the failure of captive white bass to undergo FOM at the end of vitellogenesis. Sustained elevation of plasma GtH II via treatment with a GnRHa implant induced two consecutive spawns with an overall egg production two- to eightfold higher than previously obtained from captive broodstocks, and similar to annual egg production Values reported for wild fish.     

Steroids involved with final oocyte maturation in the winter flounder.

A number of androgens and progestogens including 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20-P) were examined in female winter flounder as possible maturation inducing steroids (MIS). During final oocyte maturation serum levels of testosterone (T) and 17 beta-hydroxy-5 beta-androsten-3-one (5 beta-T) peaking at over 200 ng/ml and pregnenolone (PE) at 40 ng/ml were the predominant steroids found from each major group. High levels of T and 5 beta-T were correlated with oocyte stages characterized by germinal vesicle migration. Of the PEs measured, maximum serum levels of PE, 3 beta,17 alpha-hydroxy-5-pregnen-20-one (17-PE) and 3 beta,17 alpha, 20 beta-dihydroxy-5-pregnene (17,20-PE) were found during later oocytes stages associated with germinal vesicle breakdown. Levels of 17,20-P, an established MIS in most fish, were almost non-detectable (less than 0.1 ng/ml serum) in females throughout all stages of final oocyte maturation. Incubations of ovarian follicles in vitro with physiological concentrations of T and 5 beta-T indicated that these steroids could induce all stages of final oocyte maturation. Similar in vitro incubations showed that 17-PE and 17,20-PE were only effective on germinal vesicle breakdown. The principal conclusions are that T, 5 beta-T and the PEs can be considered as MISs in winter flounder and the PE pathway predominates during the final stages of oocyte maturation in winter flounder in contrast to progesterones which predominate in other fish species, mostly salmonids, studies to date.     

Plasma levels of C18-, C19- and C21-steroids in captive and feral female sea bass

Morphometric analysis of the gonads of sea bass Dicentrarchus labrax revealed that captive fish matured 1 month later than feral fish, but levels of gonadal steroids were identical in both groups at the same stage of sexual development. 17β-oestradiol (E₂) (up to 3 ng ml-1) and testosterone (T) (up to 4 ng ml-1) were highest during the gametogenetic period while 17,20β,21-trihydroxy-4-pregnen-3-one (17,20β,21-P) (free and sulphated) were maximal during the spawning period. Free 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) was very low and did not change (c. 0·5 ng ml⁻¹) while 17,20β-P-sulphate increased during the spawning period in both groups (up to 2 ng ml⁻¹). In contrast cortisol levels were higher in captive fish and increased during the spawning period (up to 100 ng ml⁻¹). These results suggest that captivity delays vitellogenesis and spawning in sea bass without affecting the final levels of the gonadal steroids and further indicates a role for cortisol in the latter period. The increased levels during the spawning period suggests a pheromonal role for 17,20β-P-sulphate and 17,20β,21-P-conjugates and the involvement of 17,20β,21-P in final ooccyte maturation.     

Steroidogenesis in ovarian follicles of chub mackerel, Scomber japonicus

We incubated different radiolabeled steroid precursors with intact chub mackerel ovarian follicles to clarify the synthetic pathways of steroid hormones during vitellogenesis and following final oocyte maturation (FOM). During vitellogenesis, estradiol-17beta (E2) was synthesized from pregnenolone via 17-hydroxypregnenolone, 17-hydroxyprogesterone, androstenedione, and testosterone. The physiological significance of the intermediate metabolites of E2 in the ovarian follicles was examined by comparing follicular steroidogenesis between gonochoric and hermaphroditic fish species. After vitellogenesis, the steroidogenic pathway shifted from E2 to maturation-inducing hormone (MIH) production owing to the inactivation of 17,20-lyase and the activation of 20 beta-hydroxysteroid dehydrogenase. Of the new steroids produced during FOM, 17,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P) was most effective at inducing germinal vesicle breakdown in vitro. Circulating levels of 17,20beta-P increased specifically around the time of germinal vesicle migration, while another FOM-specific 20beta-hydroxylated progestin, 17,20beta,21-trihydroxy-4-pregnen-3-one, was present at consistently low levels during FOM. These results indicate that 17,20beta-P is the MIH of chub mackerel.     

The effect of four C21-steroids on oocyte maturation in Siberian sturgeon (<Emphasis Type="Italic">Acipenser baeri</Emphasis> Brandt)

The effect of four C21-steroids, progesterone (P₄), 17,20β-dihydroxy-4-pregnen-3-one (17,20βP), 17,20β,21-trihydroxy-4-pregnen-3-one (20βS) and 11-desoxycortisol (S), on in vitro oocyte maturation in Siberian sturgeon (Acipenser baeri Brandt) females was demonstrated using short-term (5 and 30 min) exposures of ovarian follicles to steroid solutions followed by incubation in steroid-free medium. The study aimed to find out which of the four candidates for a maturationinducing steroid (P₄, 17,20βP, 20βS or S) induces a fastest germinal vesicle breakdown (GVBD) in oocytes of Siberian sturgeon. Dissolution of the oocyte nucleus or GVBD was taken as a criterion of oocyte maturation. Dose-response profiles of hormone activities as well as effects of the hormones under short-term exposures of follicles to their equal doses were compared. P₄ was found to be a most active GVBD inducer compared to other C21-steroids, S was the second in its activity, whereas 17,20βP and 20βS were less efficient. A comparison of the present and previously obtained data on the dynamics of C21-steroids in vivo and their effect on ovarian follicles in vitro indicates an important role of the above hormones, particularly P₄ and 20βS, in the regulation of the final stage of oocyte maturation in sturgeons.     

Ovarian development and serum sex steroid and vitellogenin profiles in the female cultured sturgeon hybrid, the bester

Changes in serum levels of estradiol-17/J (E₂), testosterone (T), 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) and vitellogenin (VTG), in cultured adult female bester were examined in relation to ovarian development during a 1-year sampling period. Considerable variations in oocyte development were found among fish. Oocytes at previtellogenic stage (≥0–6mm in diameter) generally started to develop concomitantly with the degeneration of the first batch of oocytes. In vitellogenic individuals, ovaries were comprised of more advanced oocytes with diameter ranging from 0–6 to 2–6 mm and in the post-vitellogenic class, oocytes attained their largest size (>2–6mm) while the germinal vesicle was migrating towards the animal pole. Oocytes with a migrating nucleus were maintained during the winter period and massive degeneration started in April–May without germinal vesicle breakdown (GVBD) or ovulation occurring. Seasonal changes in E₂, T and VTG levels were well correlated with the advancement of oogenesis. Their levels increased during vitellogenesis, whereas in the post-vitellogenic (migratory nucleus) stage the levels of E₂ declined from 2–4 ng ml ^?1± to 1–2 ng ml ^?1 and VTG from 4–10 mg ml ^?1 to 0.–0.5 mg ml ^?1 while T levels remained high (50–60 ng ml ^?1). In contrast, serum levels of 17,20?-P were constantly low (less than 0.2 ng ml^?L) throughout the reproductive cycle. These results indicate that the time appropriate for induction of artificial reproduction would be from October–November to April–May when the oocytes are in the late Stages of the development.     

Hormonal regulation of the European sea bass reproductive cycle: an individualized female approach

Fifteen tagged female sea bass Dicentrarchus labrax were sampled weekly from September to April and plasma vitellogenin (VTG), testosterone (T), 17β-estradiol (E₂), and two potential maturation inducing steroids (MISs): 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) and 17,20β,21-trihydroxy-4-pregnen-3-one (20βS) assayed. An oocyte sample was obtained via intraovarian cannulation at each sampling time from every female and the stage of development of the most advanced clutch of oocytes determined and related to VTG and hormone plasma levels for each female. The mean number of ovulations per female was 1·75+0·25 when those females that did not present ovulations were excluded and up to 4 ovulations detected in some females. The highest plasma levels of T ( c. 6 ng ml^-1) were observed during postvitellogenesis and the beginning of maturation while maximum plasma levels of E2 (>5 ng ml^-1) were obtained during late vitellogenesis. VTG plasma levels increased throughout vitellogenesis peaking ( c. 2·5 mg ml^-1) at postvittelogenesis. For the first time significant changes of plasma progestogens were detected in European sea bass during the sexual cycle. The highest plasma level of 17,20βP ( c. 1·1 ng ml^-1) was observed during postvitellogenesis while the highest level of 20βS ( c. 1·4 ng ml^-1) coincided with final maturation. These results suggest that 17,20βP and 20βS play a role in the early and final maturation, respectively, in the European sea bass.     

Regulation of ovarian steroidogenesis in vitro by follicle-stimulating hormone and luteinizing hormone during sexual maturation in salmonid fish

The regulation of ovarian steroidogenesis in vitro by coho salmon FSH and LH was investigated in intact coho salmon follicles and isolated follicular layers at various stages of oocyte maturation, from late vitellogenesis until the completion of germinal vesicle breakdown (GVBD). In granulosa layers from all stages, LH, but not FSH, stimulated 17alpha,20beta-dihydroxy-4-pregnen-3-one (17, 20beta-P) production. In theca-interstitial layers from all stages, FSH and LH stimulated steroid production, LH being more potent than FSH. The basal steroid output of intact follicles was significantly lower than that of isolated follicular layers, and their response to FSH and LH also differed. First, the intact follicles produced 17alpha-hydroxyprogesterone in response to FSH during the central germinal vesicle stage while theca-interstitial layers did not. Second, estradiol-17beta production was not inhibited by LH during final oocyte maturation in intact follicles, as observed for granulosa layers. Our results indicate that LH is the determining factor regulating the production of the maturation-inducing steroid, 17,20beta-P, and the induction of GVBD in the salmonid ovary. In summary, we have provided evidence for maturation-associated changes in the effects of FSH and LH in the salmonid ovary, which further supports the hypothesis that FSH and LH have distinct functions in the teleost ovary.     

Oocyte maturation-inducing steroids in protandrous black porgy, Acanthopagrus schlegeli

The study objectives aimed to investigate the maturation-inducing steroid (MIS) in marine protandrous black porgy, Acanthopagrus schlegeli. The characteristics of oocyte maturation were also described. Females were injected with two successive doses of LHRH analog (LHRH-A, 10 and 50 microg/kg of fish). The ovarian tissue was obtained at 6-h intervals for in vitro oocyte maturation. Both 17,20 beta-dihydroxy-4-pregnen-3-one (DHP) and 17,20 beta,21-trihydorxy-4-pregnen-3-one (20 beta-S) were the most effective steroids to induce in vitro maturation (e.g. germinal vesicle breakdown, GVBD) in oocytes cultured for either 24 h or 1 min. 20 beta-S had a better potency than DHP in inducing oocyte maturation. 17-hydroxyprogesterone, 11-deoxycortisol, and 20 beta-21-dihydroxy-4-pregnen-3-one also significantly induced oocyte maturation at high concentrations. The process of oocyte maturation (after the injection of LHRH analog) was founded to be divided into four stages: hormone-insensitive stage (insensitive to gonadotropin and MIS); MIS-insensitive (respond to gonadotropin, but not MIS); MIS-sensitive (respond to MIS); and spontaneous stage (GVBD in the hormone-free condition), respectively. Cycloheximide blocked GVBD at the MIS-insensitive stage, control (hormone-free), and hormone-induced GVBD at the MIS-sensitive stage in a dose-dependent effect.     

Thein vitro effect of theophylline on 17α, 20ß-dihydroxy-4-pregnen-3-one-induced germinal vesicle breakdown in the catfish,Clarias batrachus

The effects of theophylline (a phosphodiesterase inhibitor) and cAMP on 17α, 20ß-dihydroxy-4-pregnen-3-one-induced germinal vesicle breakdown was investigatedin vitro in catfish (Clarias batrachus) oocytes. Folliculated oocytes incubated with 17α, 20ß-dihydroxy-4-pregnen-3-one at the concentration of 1 μg/ml induced 93.2 ± 2.23% germinal vesicle breakdown. When the oocytes were prestimulated with 17α,20ß-dihydroxy-4-pregnen-3-one for 6 h and then treated with different concentrations of theophylline, there was a significant drop in the frequency of germinal vesicle breakdown at the concentrations 2.0, 1.5 and 1.0 mM. However, theophylline was found to be incapable of inhibiting germinal vesicle breakdown at its lowest concentration (0.5 inM). In the time course study, significant inhibition of germinal vesicle breakdown was recorded when 1 mM theophylline was added up to 30 h of 17α,20ß-dihydroxy-4-pregnen-3-one Stimulation but the inhibitory effect of theophylline gradually (time dependent manner) declined if the stimulatory time of 17α,20ß-dihydroxy-4-pregnen-3-one was increased. A similar inhibition of germinal vesicle breakdown was also recorded with various concentrations of cAMP. Except 0.5 mM, all the higher concentrations of cAMP significantly inhibited 17α,20ß-dihydroxy-4-pregnen-3-one induced germinal vesicle breakdown.     

Effect of 5-Hydroxytryptamine on Steroidogenesis and Oocyte Maturation in Pre-ovulatory Follicles of the Medaka Oryzias latipes

The effect of 5-hydroxytryptamine (5-HT) on steroidogenesis and oocyte maturation in pre-ovulatory follicles of the medaka Oryzias lalipes was examined using in vitro culture system. The earliest breakdown of the germinal vesicle of intrafollicular oocytes occurred about 17 hr after the beginning of incubation in the presence of 5-HT at concentration of 10 ng/ml or more. 5-HT induced oocyte maturation in a dose-dependent manner. Cyanoketone inhibited this stimulation. The concentration of 5-HT required to induce oocyte maturation corresponded to that required to enhance the production (secretion) of estradiol-17β and 17α,20β-dihydroxy-4-pregnen-3-one by pre-ovulatory follicle cells. At a concentration of 1 μg/ml, the follicle had to be exposed to 5-HT for at least 4 hr for oocyte maturation accompanied by ovulation to occur. These results indicate that 5-HT induces in vitro maturation of medaka oocytes by stimulating 17α,20β-dihydroxy-4-pregnen-3-one production by pre-ovulatory follicular cells.     

Serotonin inhibition of steroid-induced meiotic maturation in the teleost Fundulus heteroclitus: Role of cyclic AMP and protein kinases

The transduction of the serotonin (5-HT) signal in Fundulus heteroclitusovarian follicles leading to the inhibition of oocyte meiosis reinitiation (oocyte maturation) in vitro induced by the naturally occurring maturation-inducing steroid 17α,20β-dihydroxy-4-pregnen-3-one (17,20βP) was investigated. Steroid-induced oocyte maturation was inhibited by 5-HT in a dose-dependent manner; maximum inhibition (90%) was observed with 10⁻⁴ M 5-HT. Groups of follicle-enclosed oocytes were cultured in the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) and treated with increasing doses of 5-HT. Serotonin was found to slightly increase the levels of follicular 3′,5′-cyclic adenosine monophosphate (cAMP) in a dose-dependent manner; 10⁻⁴ M 5-HT induced approximately a 3-fold increase in cAMP with respect to the controls. The changes in cAMP were then evaluated in follicles treated with 17,20βP in IBMX-free culture media in the presence or absence of 10⁻⁴ M 5-HT. The exposure of follicles to 17,20βP alone produced a small and transient reduction in cAMP (40%) within 1–3 hr of steroid stimulation, and these early changes in cAMP appeared associated with a high incidence of germinal vesicle breakdown (80% GVBD) by 24 hr of incubation. Under these conditions, treatment of follicles with 5-HT also increased significantly the production of cAMP, and when 5-HT was combined with 17,20βP, the steroid-mediated reduction in cAMP was prevented and the levels of GVBD inhibited by 95%. Meiosis also was reinitiated with either the protein kinase A (PKA) inhibitor H8 or the protein kinase C (PKC) activator PMA, and the 5-HT inhibitory action on GVBD was found to be 100-fold reduced or completely ineffective, respectively. Preincubation of follicles with the PKC inhibitor GF109203x abolished PMA-induced GVBD in a dose-dependent manner, whereas this inhibitor had no effect on 17,20βP-triggered meiotic maturation, indicating that activation of PKC is apparently sufficient but not necessary to reinitiate meiosis. Taken together, these findings suggest that 5-HT may inhibit 17,20βP-induced meiotic reinitiation through the activation of a cAMP-PKA transduction pathway and that PKC possibly induces oocyte maturation by a different pathway than the steroid and thus is not affected by 5-HT. Mol. Reprod. Dev. 49:333–341, 1998. © 1998 Wiley-Liss, Inc.     

Sex steroids in plasma of lake whitefish Coregonus clupeaformis during spawning in Lake Erie

Lake whitefish, Coregonus clupeaformis, were collected from the Western basin of Lake Erie during spawning. Free and conjugated (sulfated and glucuronidated) steroids including testosterone (T), 11-ketotestosterone (11-kT), estradiol-17beta (E2) and 17,20beta-dihydroxy-4-pregnen-3-one (17,20betaP) were measured in the plasma by radioimmunoassay. In males, the progression of spermiation was characterized by a significant decrease in plasma free steroids, whereas the levels of conjugated steroids remained similar and low, except for sulfated and glucuronidated testosterone. Plasma sex steroids did not correlate with the density or the motility of the spermatozoa. In females, the concentration of plasma T was significantly higher in preovulating than in ovulating females. The levels of E2 and 17,20betaP in ovulating lake whitefish exhibited large variations ranging from below detection limit to 0.9 ng ml(-1) and from 0.2 to 13 ng ml(-1), respectively. Analysis of conjugated steroids revealed high levels of glucuronidated and sulfated 17,20betaP and glucuronidated T in females ovulating in December. However, no significant differences in the proportion of the conjugated steroids were observed.     

Gonadotropin-induced steroidogenic shift towards maturation-inducing hormone in Japanese yellowtail during final oocyte maturation

Intact ovarian follicles, obtained from untreated and human chorionic gonadotropin (HCG) treated Japanese yellowtail Seriola quinqueradiata during different maturational stages, were incubated with radioactive [3H]pregnenolone, [³H]17‐hydroxyprogesterone or [¹⁴C] androstenedione and steroid metabolites identified by thin layer chromatography (TLC) followed by recrystallization to constant specific activity. In untreated late vitellogenic (0 h) follicles, androstenedione was the major product with smaller amounts of testosterone and oestradiol‐17α. In post‐vitellogenic (12 h post‐injection) intact follicles, androstenedione predominated, and although testosterone and oestradiol‐17α were not produced, there were small amounts of 17, 20β‐dihydroxy‐4‐pregnen‐3‐one (17,20β‐P) and 17,21‐dihydroxy‐4‐pregnene‐3, 20‐dione (11‐deoxycortisol). In HCG‐treated fish, a steroidogenic shift resulted in the disappearance of testosterone and oestradiol‐17 coinciding with the appearance of 17, 20β‐P. During early and late final oocyte maturation FOM (24 and 36 h post‐injection), there was a five‐ to seven‐fold increase in the production of 17, 20β‐P, whereas production of 11‐deoxycortisol remained almost the same. During FOM, in addition to 17,20β‐P, its 5β‐reduced metabolite, 17,20β‐dihydroxy‐5β‐pregnan‐3‐one (5β‐17,20β‐P) was synthesized, suggesting a decrease in maturation‐inducing 17,20β‐P activity. 17, 20β,21‐Trihydroxy‐4‐pregnen‐3‐one (20β‐S) was not synthesized by ovarian fragments in Japanese yellowtail at any maturational stage. The metabolites identified on TLC during FOM were tested to evaluate their maturation‐inducing activity in an in vitro bioassay. Of the steroids tested, 17,20β‐P was the most effective inducer of germinal vesicle breakdown (GVBD), followed by 5β‐17,20β‐P. Timely synthesis of 17,20β‐P immediately prior to and during FOM as well as its great potency in inducing GVBD in vitro supports the evidence for a physiological role of 17,20β‐P as a maturation‐inducing hormone in Japanese yellowtail.     

In vitro hormone induction of final oocyte maturation in striped bass (Morone saxatilis) follicles is inhibited by blockers of phosphatidylinositol 3-kinase activity

Oocyte germinal vesicle breakdown (GVBD) was induced in striped bass ovarian fragments when tissues were incubated with 100-nM recombinant human insulin-like growth factor-I (rhIGF-I), 25-IU human chorionic gonadotropin (hCG) ml(-1), or 290 nM of the maturation-inducing steroid (MIS), 17,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S). Inhibitors of phosphatidylinositol 3-kinase (PI 3-K), wortmannin (100 nM) and LY 294002 (50 microM), inhibited GVBD induced by these hormones. Furthermore, the inhibitors attenuated hCG-induced steroid hormone synthesis. Previous studies report that gap junction uncouplers inhibit GVBD induced by hCG, but not by rhIGF-I, in striped bass. We show that 20beta-S-induced GVBD is also attenuated by 1 mM 1-heptanol or 1-octanol without being affected by incubation with 3 mM ethanol. Thus, the effects of inhibiting PI 3-K activity on GtH and MIS actions are similar to effects of uncoupling gap junctions. These data suggest that PI 3-K activity is required for GtH- MIS- and IGF-I induction of GVBD in striped bass. Our data are also consistent with the notion that a ligand that regulates PI 3-K activity, possibly an IGF, participates in maintenance of gap junctional communication required for maximal GtH and MIS action.