Effects of differing oocyte-secreted factors during mouse in vitro maturation on subsequent embryo and fetal development.

PURPOSE: We hypothesised that varying native oocyte-secreted factor (OSF) exposure or using different recombinant OSF peptides would have differential effects on post-in vitro maturation (IVM) embryo and fetal development. METHODS: Mouse cumulus oocyte complexes (COCs) were treated with the purified mature domain of GDF9 and/or BMP15 or were co-cultured with denuded oocytes (DOs) from 0 h or 3 h of IVM. DOs were matured for 3 h as either intact COCs+/-FSH before denuding, or as DOs + FSH. COCs were fertilised and blastocyst development was assessed on days 5 and 6, and either differentially stained for ICM numbers or vitrified/warmed embryos were transferred to recipients to assess implantation and fetal rates. RESULTS: No improvement in embryo development was observed with the addition of GDF9 and/or BMP15 to IVM. In contrast, embryos derived from COCs co-cultured with DOs had significantly improved blastocyst rates and ICM numbers compared to controls (P < 0.05). The highest response was obtained when DOs were first added to COCs at 3 h of IVM, after being pre-treated (0-3 h) as COCs + FSH. Compared to control, co-culture with DOs from 3 h did not affect implantation rates but more than doubled fetal yield (21% vs 48%; P < 0.05). GDF9 Western blot analysis was unable to detect any differences in quantity or form of GDF9 (17 and 65 kDa) in extracts of DO at 0 h or 3 h. CONCLUSIONS: This study provides new knowledge on means to improve oocyte quality in vitro which has the potential to significantly aid human infertility treatment and animal embryo production technologies.

Application of specific ELISAs for BMP15 and GDF9 to cumulus cell extracts from infertile women.

Bone morphogenetic protein 15 (BMP15) and growth differentiation factor 9 (GDF9) are oocyte-specific paracrine factors which regulate ovarian cumulus cell (CC) functions. This study aimed to investigate if BMP15 and GDF9 bound to CCs can be characterized, quantified, and show an association with IVF outcomes in infertile women. BMP15 and GDF9 ELISAs were validated and applied to discarded CC extracts. Pooled CCs from individual patients were collected from 120 (cohort 1; BMP15 only) and 81 infertility patients (cohort 2; BMP15 and GDF9) undergoing superovulation. BMP15 and GDF9 levels expressed per CC DNA were correlated with maternal age, clinical and embryology data. Total BMP15 and GDF9 were highly correlated with each other (r = 0.9, p < 0.001). The GDF9:BMP15 ratio was unrelated to oocyte number or age. BMP15/CC DNA and GDF9/CC DNA were unaffected by the type of superovulation and were not related to oocyte/embryo outcomes.

Nutrient and sediment concentrations and corresponding loads during the historic June 2008 flooding in eastern Iowa.

A combination of above-normal precipitation during the winter and spring of 2007-2008 and extensive rainfall during June 2008 led to severe flooding in many parts of the midwestern United States. This resulted in transport of substantial amounts of nutrients and sediment from Iowa basins into the Mississippi River. Water samples were collected from 31 sites on six large Iowa tributaries to the Mississippi River to characterize water quality and to quantify nutrient and sediment loads during this extreme discharge event. Each sample was analyzed for total nitrogen, dissolved nitrate plus nitrite nitrogen, dissolved ammonia as nitrogen, total phosphorus, orthophosphate, and suspended sediment. Concentrations measured near peak flow in June 2008 were compared with the corresponding mean concentrations from June 1979 to 2007 using a paired t test. While there was no consistent pattern in concentrations between historical samples and those from the 2008 flood, increased flow during the flood resulted in near-peak June 2008 flood daily loads that were statistically greater (p < 0.05) than the median June 1979 to 2007 daily loads for all constituents. Estimates of loads for the 16-d period during the flood were calculated for four major tributaries and totaled 4.95 x 10(7) kg of nitrogen (N) and 2.9 x 10(6) kg of phosphorus (P) leaving Iowa, which accounted for about 22 and 46% of the total average annual nutrient yield, respectively. This study demonstrates the importance of large flood events to the total annual nutrient load in both small streams and large rivers.

Activins and inhibins in endocrine and other tumors.

Inhibin and activin are members of the TGF beta superfamily of growth and differentiation factors. They were first identified as gonadal-derived regulators of pituitary FSH and were subsequently assigned multiple actions in a wide range of tissues. More recently, the inhibin alpha subunit was considered as a tumor suppressor based on functional studies employing transgenic mouse models. This review evaluates the functional and molecular evidence that the inhibin alpha subunit is a tumor suppressor in endocrine cancers. The evaluation highlights the discrepant results from the human and mouse studies, as well as the differences between endocrine tumor types. In addition, we examine the evidence that the activin-signaling pathway is tumor suppressive and identify organ-specific differences in the actions and putative roles of this pathway in endocrine tumors. In summary, there is a considerable body of evidence to support the role of inhibins and activins in endocrine-related tumors. Future studies will define the mechanisms by which inhibins and activins contribute to the process of initiation, promotion, or progression of endocrine-related cancers.

Shifting brain inhibitory balance and connectivity of the prefrontal cortex of adults with autism spectrum disorder.

Currently, there are no effective pharmacologic treatments for the core symptoms of autism spectrum disorder (ASD). There is, nevertheless, potential for progress. For example, recent evidence suggests that the excitatory (E) glutamate and inhibitory (I) GABA systems may be altered in ASD. However, no prior studies of ASD have examined the 'responsivity' of the E-I system to pharmacologic challenge; or whether E-I modulation alters abnormalities in functional connectivity of brain regions implicated in the disorder. Therefore, we used magnetic resonance spectroscopy ([1H]MRS) to measure prefrontal E-I flux in response to the glutamate and GABA acting drug riluzole in adult men with and without ASD. We compared the change in prefrontal 'Inhibitory Index'-the GABA fraction within the pool of glutamate plus GABA metabolites-post riluzole challenge; and the impact of riluzole on differences in resting-state functional connectivity. Despite no baseline differences in E-I balance, there was a significant group difference in response to pharmacologic challenge. Riluzole increased the prefrontal cortex inhibitory index in ASD but decreased it in controls. There was also a significant group difference in prefrontal functional connectivity at baseline, which was abolished by riluzole within the ASD group. Our results also show, for we believe the first time in ASD, that E-I flux can be 'shifted' with a pharmacologic challenge, but that responsivity is significantly different from controls. Further, our initial evidence suggests that abnormalities in functional connectivity can be 'normalised' by targeting E-I, even in adults.

Diabetic neuropathy in the mutant mouse [C57BL/ks(db/db)]: a morphometric study.

Detailed studies of peripheral nerves were undertaken in the mutant diabetic mouse of the [C57BL/ks(db/db)] strain using electrophysiologic and morphometric techniques. Electrophysiologic studies showed severely impaired motor nerve conduction velocity (MNCV), which developed promptly during the early phase of the diabetic syndrome. Morphometric changes occurred first after 20 wk of diabetes in both myelinated and unmyelinated fibers. There were both loss and shrinkage of myelinated fibers, most pronounced in the sural nerve and the ventral root. Changes appeared late in the dorsal root and in the peroneal and vagus nerves. Unmyelinated fibers showed both shrinkage and loss of axons, presumably involving sympathetic and afferent somatic fibers. Teased fiber studies and calculations of axon-myelin ratios confirmed our earlier suggestion that the neuropathy is primarily axonal in nature. The temporal discrepancy between functional and structural impairments in the present model strongly suggests a metabolic cause of the early neuropathy. This was further supported by the positive effect of insulin treatment on MNCV during the early phase of diabetes, whereas, during the late phase, treatment failed to show any effect.

Follistatin/activin-binding protein.

Follistatin, or follicle-stimulating hormone (FSH) suppressing protein, is a protein isolated from side fractions in the purification of inhibin and activin, which shares with inhibin the property of suppressing the secretion of FSH from the pituitary gonadotroph. This protein is structurally distinct from inhibin. Recent observations that follistatin is an activin-binding protein suggest that it may have a much wider biologic role, given the variety of systems in which activin has been shown to act.

Hormonal control of spermatogenesis.

FSH and testosterone (T) secretion are essential for the successful completion of spermatogenesis. Because there are no receptors for FSH or testosterone on germ cells, there are intermediate steps in this action, the nature of which are unknown. However, as the Sertoli cell contains receptors for both FSH and T, it is likely that these hormones exert their influence on germ cells by modulating Sertoli cell function. Both FSH and T exert synergistic actions on germ cells, but T has a specific action on the later stages of spermatid maturation. FSH, by its ability to stimulate Sertoli cell mitosis during testicular development, can influence the spermatogenic capacity of the adult testis.

Specific roles for the asparagine-linked carbohydrate residues of recombinant human follicle stimulating hormone in receptor binding and signal transduction.

FSH comprises two distinct subunits, both of which contain asparagine-linked carbohydrate residues, located at positions 52 and 78 on the alpha-subunit and positions 7 and 24 on the beta-subunit. These carbohydrate chains have been shown to regulate the biological activity of FSH, including signal transduction and receptor binding. However, the specific roles of the individual carbohydrate chains have been poorly defined. Using site-directed mutagenesis we disrupted the consensus sequences for glycosylation and expressed the mutated cDNAs in Chinese Hamster Ovary cells. Specifically deglycosylated FSH variants were secreted from all clonal cell lines expressing the mutated FSH cDNAs except for the cell line that lacked all four glycosylation sites. Analysis of the singly or doubly deglycosylated FSH mutants revealed that removal of the carbohydrate residue at position 78 on the alpha-subunit significantly increased the receptor binding affinity of human FSH by 72%. Removal of the other carbohydrate residues had no significant effect on receptor binding. The carbohydrate residue at position 52 on the alpha-subunit was found to play an essential role in signal transduction as its removal resulted in a significant decrease in potency to 26% of wild type levels. The other individual carbohydrate residues appear to play a minor role in signal transduction, although removal of each residue results in reduced maximal response. The removal of both alpha-subunit carbohydrates resulted in a significant decrease in biopotency, to 41% of wild type levels; whereas, the removal of both beta-subunit carbohydrate chains resulted in a significant increase in biopotency, to 216% of wild type levels. These studies have allowed the identification of site-specific roles for the carbohydrate residues of human FSH. Our data suggest that the carbohydrate residues play a greater role in determining the biological activity of FSH than has been suggested in similar studies of other glycoprotein hormones.

Inhibin/activin and ovarian cancer.

Inhibin and activin are members of the transforming growth factor beta (TGFbeta) family of cytokines produced by the gonads, with a recognised role in regulating pituitary FSH secretion. Inhibin consists of two homologous subunits, alpha and either betaA or betaB (inhibin A and B). Activins are hetero- or homodimers of the beta-subunits. Inhibin and free alpha subunit are known products of two ovarian tumours (granulosa cell tumours and mucinous carcinomas). This observation has provided the basis for the development of a serum diagnostic test to monitor the occurrence and treatment of these cancers. Transgenic mice with an inhibin alpha subunit gene deletion develop stromal/granulosa cell tumours suggesting that the alpha subunit is a tumour suppressor gene. The role of inhibin and activin is reviewed in ovarian cancer both as a measure of proven clinical utility in diagnosis and management and also as a factor in the pathogenesis of these tumours. In order to place these findings into perspective the biology of inhibin/activin and of other members of the TGFbeta superfamily is also discussed.

Changes in the proteins of wool following treatment of sheep with epidermal growth factor.

Administration of epidermal growth factor (EGF) extracted from mouse submaxillary gland to Merino sheep resulted in a temporary inhibition of the activity of the wool follicles. Subsequently, either complete discontinuities appeared in the fibers resulting in shedding of the entire fleece, or incomplete, in which case the fleece was retained but bore a zone of weakness. The protein composition of the first sample of wool harvested from 1 sheep following infusion for 66 hr with 27.5 mg EGF (0-2 weeks posttreatment) was similar to pretreatment wool. This represented wool fibers which were already present in the follicles at the beginning of infusion. Thereafter, the composition of the wool changed progressively, reaching a maximum divergence from the control in the 3-4 week regrowth period followed by a return to normal by about 10 weeks. Over this period the content of high-sulfur proteins first rose from an initial 19% to a maximum of 30%, then returned to 19%, while the high-tyrosine protein content initially decreased from 12% to 5% and then slowly increased to 12%. In addition to changes in overall protein composition, two-dimensional electrophoresis revealed alterations in the proportions of some individual protein components. These changes were similar to those observed with many other wool growth inhibitors. Smaller doses of EGF (5.8 and 2.9 mg but not 1 mg) had similar effects on wool composition but these were of lower magnitude and there was a delay in reaching a maximum response. Even after 16-18 weeks the wool from these treated sheep differed slightly in composition from the pretreatment samples.

A syndrome of acute severe muscle necrosis in intensive care unit patients.

Four septic patients and one asthmatic patient are described who developed a severe paralytic disorder in an intensive care unit (ICU), associated with a rise in serum creatine kinase and a severe necrotizing myopathy. All cases had received non-depolarizing muscle blocking agents and large intravenous doses of glucocorticoids. Three patients developed myoglobinuria. No improvement or very little improvement in muscle function was noted in the four fatal cases. The single survivor recovered his strength after 6 months. This syndrome ("necrotizing myopathy of intensive care") provides one of the differential diagnoses for ICU-acquired weakness. The myopathy appears to have several interdependent causes and it is proposed that these should be classified as myonecrosis "priming" factors (glucocorticoids, myotropic infections, sepsis) and "triggering" factors (non-depolarizing muscle blocking agents).

The effect of follicle-stimulating hormone-suppressing protein or follistatin on luteinizing bovine granulosa cells in vitro and its antagonistic effect on the action of activin.

The time- and dose-dependent effects of bovine FSH-suppressing protein (FSP)/follistatin and human recombinant activin A (hr-Act) on oxytocin (OT) and progesterone (P) production, markers of luteinization, were studied in mature and immature bovine granulosa cells (GC), using three forms of FSP (31, 35, and 39 kDa) and a FSP pool consisting of 35, 39, and 45 kDa forms. FSP alone had no detectable effect on OT and P production when added to cultures of fully differentiated bovine GC. On the other hand, all FSP forms (10-100 ng/ml) enhanced and prolonged OT and P production of immature GC induced by bovine LH (10 ng/ml). Overall, 35 kDa FSP was more effective than the other forms tested. Hr-Act alone had a dose-dependent inhibitory effect on OT and P production on LH-stimulated immature GC. All four forms of FSP (30 or 100 ng/ml) added to cultures treated with hr-Act, reversed the inhibitory effect of hr-Act, with a significant increase (25%) above control levels using the 35 and 39 kDa FSP forms. In conclusion, FSP enhanced and prolonged the luteinization process, as indicated by OT and P production induced in immature GC by bovine LH, and was able to antagonize the inhibitory effect of hr-Act in this system. These studies suggest a physiological role for activin and FSP, as modulators of folliculogenesis and luteinization in the ovary. We propose that activin and FSP act in an autocrine fashion on GC in the ovarian follicle to regulate folliculogenesis and luteinization.

Effects of activin and follicle-stimulating hormone (FSH)-suppressing protein/follistatin on FSH receptors and differentiation of cultured rat granulosa cells.

The aim of this study was to investigate the actions of both activin and FSH-suppressing protein (FSP)/follistatin either alone or in combination on FSH receptor number and on the responsiveness of granulosa cells to FSH and LH. Granulosa cells were harvested from diethylstilbestrol-treated immature Sprague-Dawley rats and cultured 48 h in serum-free medium with or without treatment. Activin treatment alone (3-100 ng/ml) resulted in a 4-fold increase in FSH receptor number with no change in binding affinity. This effect of activin was inhibited 31% by FSP (100 ng/ml) treatment which alone had no effect on FSH receptor number. Treatment with activin (100 ng/ml) prevented FSH-induced down-regulation of FSH receptor number, whereas at lower concentrations (3-30 ng/ml) activin enhanced down-regulation of FSH receptor number by 20% (P less than 0.05). In contrast, FSP alone prevented FSH-induced down-regulation by increasing FSH receptor number up to 40-50%. Pretreatment of granulosa cells with activin, but not FSP, for 24 h increased the responsiveness of cells to FSH (20 ng/ml) and LH (40 ng/ml) shown by increases in aromatase activity, progesterone, and immunoreactive inhibin production over and above control in a manner which depended upon activin doses. We conclude that 1) activin enhancement of FSH action on rat granulosa cells may be mediated in part via regulation of FSH receptor number, and 2) the effects of FSP on granulosa cells are likely to be due to its activin binding properties.

Heterogeneity of rat pituitary gonadotropins on electrofocusing; differences between sexes and after castration.

Anterior pituitary extracts from intact and 4 week postcastration male and female rats were electrofocused in sucrose density gradients within the pH range 3.5-10. Column fractions were combined to cover this pH range in 0.5 pH units and assayed for LH by in vitro bioassay and RIA and for FSH by radioreceptor assay and RIA. The pH distribution of bioactive LH was altered after castration in both sexes, with the proportion of recovered activity in the alkaline pH range increasing (P less than 0.01) from 52-57% in the intact animal to 71-73% after castration. In addition, significantly more bioactivity was recovered in the pH range 7-9.5 with the male (37%) than with either of the female (diestrous, 30% or proestrous, 28%) groups (P less than 0.05). FSH receptor binding activity was located in the pH region 3.5-6.5. Significantly less receptor binding activity was recovered in the pH range 3.5-4.5 with the female groups (39% and 37% diestrous and proestrous, respectively) than the male group (61%; P less than 0.05, P less than 0.01). The distribution of immunoreactive LH and FSH was similar to that observed with the LH in vitro bioassay and FSH radioreceptor assay. It is concluded that the charge distribution of pituitary gonadotropins is altered according to the sex of the animal and after castration. These findings provide further evidence that the type of gonadotropin produced by the pituitary is under endocrine control.

Spermiation failure is a major contributor to early spermatogenic suppression caused by hormone withdrawal in adult rats.

Spermiation is the process by which mature sperm are released from the Sertoli cell into the lumen of the seminiferous tubule. Previous studies have shown that FSH and LH/testosterone suppression causes a significant increase in the degeneration of mature elongated spermatids. The purpose of this study was to investigate the extent to which spermiation failure contributes to the overall failure of spermatogenesis during hormone suppression. We used in vivo models to selectively suppress either FSH, by passive immunization, and or testosterone, by administration of SILASTIC brand (Dow Corning) testosterone and estradiol implants to suppress LH and testicular testosterone production. Stereological quantitation of the number of step 17-18 spermatids before spermiation and the number of step 19 spermatids retained within the epithelium after spermiation showed that 2% of spermatids failed to spermiate in control animals, and 11% and 14% of spermatids failed to spermiate after 1 week of FSH inhibition or testosterone suppression, respectively. After 1 week of combined FSH and testosterone withdrawal, 50% of the spermatids in the testis failed to be released. A time course of testosterone suppression showed that after 4-5 weeks over 90% of spermatids failed to spermiate. We conclude that spermiation is highly sensitive to hormone suppression, with T and FSH acting synergistically to support spermiation, and that spermiation inhibition is a potential target for contraception.

Inhibin/activin beta-subunit monomer: isolation and characterization.

Using an activin RIA that showed limited cross-reaction with inhibin, activin immunoactivity was monitored throughout the isolation of activin from bovine follicular fluid and side-fractions during the isolation of human recombinant inhibin. Two peaks of activin immunoactivity were identified in both materials and isolated to homogeneity by dye affinity chromatography, hydrophobic interaction and gel permeation chromatography, and reverse phase HPLC. The purified proteins in all four peaks had terminal amino acid sequences identical to those of the inhibin/activin beta-subunit. The molecular masses determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing (and reducing) conditions were 25 and 15 and 15 and 15 kilodaltons (kDa) for each pair of proteins from both sources. Based on these criteria, the bovine and human recombinant 25-kDa proteins correspond to the inhibin/activin beta A-subunit dimer (activin-A), while the 15-kDa proteins correspond to the inhibin/activin beta A-subunit monomer. The activity of the monomer was 17% of the activity of the dimer in the activin RIA. Based on this level of cross-reaction and the proportion of monomer to dimer immunoactivity found after reverse phase HPLC of bovine follicular fluid, it is estimated that the levels of monomer in bovine follicular fluid are 25-60% those of the dimer. The biological activities of the human recombinant activin monomer and dimer were investigated in two different cell culture systems. In a rat pituitary cell system the activity of the activin monomer was 19% of the activity of the dimer in stimulating FSH release, while in rat thymocyte cultures the activity of the monomer was 45% the activity of the dimer in suppressing lectin-stimulated [3H]thymidine uptake. It is concluded that the beta A-subunit monomer is found in bovine follicular fluid at a level 25-60% that of the beta A-subunit dimer (activin-A). The monomer displays in vitro responses similar to those of the dimer, although the monomer is less active (18-45%) than the dimer. It is unclear if dimerization of the monomer is a necessary prerequisite for biological activity.

Ovine anterior pituitary production of follistatin in vitro.

Rat pituitary cells express messenger RNA for the activin-binding protein, follistatin (FS), and rat and bovine pituitary cell cultures secrete FS into the medium. In the present study, a previously validated, heterologous RIA for ovine FS was employed to investigate FS synthesis, secretion, and regulation in cultures of ovine anterior pituitary cells. The validity of the RIA was confirmed by the finding that FS immunoreactivity in ovine pituitary cell culture-conditioned medium diluted in parallel with purified bovine FS, and fractionation of the conditioned medium resulted in the coelution of activin-binding activity with the FS immunoactivity. The concentration of endogenous ovine FS achieved in the culture medium (0.08-0.6 nM) was in the range over which bovine FS suppresses FSH secretion in these cultures (IC50 = 0.5 nM). To characterize the relationship between endogenous FS and FSH secretion, dispersed ovine pituitary cells were preincubated with 10% fetal bovine serum for 2 days, then cultured between days 2-5 in the presence of a chemically defined serum substitute. Under these conditions, FS was continuously secreted at a rate of 12.1 +/- 1.8 ng/10(6) cells.day (mean +/- SEM; n = 18), whereas FSH was secreted at 64 +/- 13 ng/10(6) cells.day (n = 7). The secretion of FS and FSH changed in a reciprocal way as culture conditions were altered either by maintaining exposure of the cells to fetal bovine serum or by plating the cells at a 6- to 10-fold higher seeding density. Under the latter circumstance, for instance, FS secretion during the 3-day test period decreased to 47 +/- 14% (n = 10) and FSH secretion increased to 137 +/- 6% (n = 6) of the respective values in cultures of dispersed cells. FS secretion was increased nearly 3-fold (P < 0.05) in a dose-dependent manner by continuous exposure of ovine pituitary cells between days 2-5 to recombinant human activin A (1-10 nM), which concomitantly increased FSH secretion. Recombinant human inhibin A (0.003-10 nM); the synthetic glucocorticoids, RU28362 and dexamethasone (each 1-100 nM); the sex steroids, testosterone (1-100 nM), 17 beta-estradiol (0.001-5 nM), and progesterone (4-2500 nM); and the vitamin A derivative, retinoic acid (0.3-32 microM), each inhibited FSH secretion from these cultures, but only the last agent significantly (P < 0.05) increased FS secretion. Inhibin prevented the stimulation of FSH secretion by activin A without affecting its stimulation of FS secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Highly polarized secretion of inhibin by Sertoli cells in vitro.

We have studied the regulation of inhibin secretion by rat Sertoli cells grown on extracellular matrix-impregnated porous filters in a twin chamber assembly. Previous studies have established that rat Sertoli cells cultured under these conditions reproduce the morphological and functional polarization observed in the Sertoli cell in situ. Sertoli cells isolated from 18- to 22-day-old Wistar rats were cultured for up to 8 days with daily changes of fully defined supplemented Eagle's Minimum Essential Medium (MEM). Rat inhibin was measured by RIA and pituitary cell bioassay, and transferrin by RIA. Inhibin measured by immunoassay or bioassay was always readily detectable in the upper, but not the lower, chamber. Inhibin secretion into the upper chamber exhibited a dose-dependent stimulation of up to 3.7-fold by ovine FSH, with a medium effective dose of 2.2 micrograms/liter and a constant bio- to immunoreactive ratio (3.6 +/- 0.4). Apically directed secretion accounted for over 80% of inhibit output under basal conditions and over 94% with FSH stimulation. Insulin also stimulated upper chamber inhibin secretion at a high dose (5 mg/liter) but not at lower doses or in conjunction with FSH exposure of Sertoli cells. Testosterone augmented FSH-induced stimulation of inhibin secretion, but was ineffective without FSH exposure. In contrast to inhibin secretion, for which FSH is the principal regulator, transferrin secretion by Sertoli cells is more evenly bidirectional (overall mean upper to lower chamber ratio of 1.5) and requires exposure to other stimuli (insulin, retinoic acid, and testosterone) in addition to FSH to achieve maximal secretion. Both submaximal and maximal FSH stimulation of inhibin output were augmented by a phosphodiesterase inhibitor, isobutylmethylxanthine, and these effects were fully reproduced by forskolin, which suggests the involvement of cAMP in the vectorial secretion of inhibin. The marked polarization of Sertoli cell inhibin secretion in vitro could not be explained by restricted transmembrane passage of inhibin. It is, therefore, suggested that the bulk of inhibin secretion by the immature rat Sertoli cell in vivo may be directed primarily into the seminiferous tubular lumen. Thus, in addition to its role in endocrine negative feedback signaling to the pituitary, inhibin may also have important functions in seminiferous tubular function and the support of spermatogenesis.