Effects of age, season, and fertility status on plasma and intratesticular immunoreactive (IR) inhibin concentrations in stallions

Effect of age on androgens pattern in cyclic mares

Androgens are produced in both sexes. In females produced by the adrenal gland and the ovaries they play a crucial role in regulating ovarian function, estrogen synthesis and follicular growth. Age leads to a reduction in androgen concentrations, although, at present, these mechanisms are not elucidated in mares. The objective of this study was to evaluate the concentrations of testosterone (T), androstenedione (A4) and dehydroepiandrosterone (DHEA) in mares of different ages. Blood samples were drawn from seventy cyclic Spanish Purebred mares belonging to five age groups: 3-5 years, 6-9 years, 10-13 years, 14-16 years and > 16 years. The concentrations of T, A4 and DHEA were determined by EIA, validated specifically for horses. Mares aged 3-5, 6-9 and 10-13 years had higher T concentrations (P < 0.05) than mares aged >16 years, and mares aged 6-9 years had also higher concentrations than those 14-16 years old (P < 0.05). A4 concentrations were lower (P < 0.05) in mares >16 years old when compared with those of other age groups. DHEA concentrations were lower (P < 0.05) in mares 14-16 years and > 16 years old when compared with those of other age groups. DHEA was positively correlated with T (r = 0.61; P < 0.05) and A4 (r = 0.51; P < 0.05). Age induces reduction in androgens' synthesis in physiologically cyclic Spanish Purebred mares. These physiological variations must be duly considered for a correct and objective interpretation of the analytical data.

Endocrine changes induced by GnRH immunisation and subsequent early re-stimulation of testicular function with a GnRH agonist in stallions

CONTEXT

Resumption of testicular function after gonadotrophin-releasing hormone (GnRH) immunisation varies among individual animals and some stallions regain fertility only after a prolonged time.

AIMS

This study evaluated endocrine effects of GnRH immunisation and early subsequent re-stimulation with a GnRH agonist. We hypothesised that GnRH agonist treatment advances resumption of normal endocrine function in GnRH-vaccinated stallions.

METHODS

Shetland stallions were assigned to an experimental and a control group (n =6 each). Experimental stallions were GnRH-immunised twice, 4weeks apart. Each experimental stallion was hemicastrated together with an age-matched control animal when testosterone concentration decreased below 0.3ng/mL. Three weeks later, daily treatment with the GnRH agonist buserelin was initiated (4μg/day for 4weeks followed by 8μg/day). The remaining testicle was removed when testosterone concentration exceeded 0.5ng/mL in vaccinated stallions. Blood was collected for LH, FSH, oestradiol and anti-müllerian hormone (AMH) analyses, and testicular and epididymal tissue were conserved for real-time qPCR and histology.

KEY RESULTS

GnRH vaccination reduced blood concentrations of LH and FSH, with a structural deterioration of testicular tissue and disruption of spermatogenesis. Daily buserelin treatment for approximately 60days partially restored gonadotropin secretion and induced a recovery of the functional organisation of the testicular tissue with effective spermatogenesis.

CONCLUSIONS

Endocrine testicular function can be restored in GnRH-vaccinated stallions by daily low-dose buserelin treatment. The buserelin treatment protocol may potentially be improved regarding the dose, interval and duration.

IMPLICATIONS

Daily buserelin treatment can be recommended for treatment of GnRH-vaccinated stallions with prolonged inhibition of testicular function.

Characterisation of the testicular transcriptome in stallions with age-related testicular degeneration

BACKGROUND

Age-related testicular degeneration can be defined as the progressive deterioration of the testis that typically occurs in middle-aged or older males and that leads to diminished testicular function and subfertility. In the equine breeding industry, genetically valuable males maintain their value as breeding animals well into old age. Because testicular degeneration is common in middle-aged and older stallions, the disease often has a significant negative impact on a stallion's breeding career and leads to economic losses in the horse breeding industry.

OBJECTIVE

Because testicular degeneration is a tissue autologous disease in the horse, the objective of this study was to use whole-transcriptome sequencing to compare the testicular transcriptomes of normal, fertile stallions to those of stallions affected by age-related testicular degeneration in order to better understand the pathophysiology of the disease.

STUDY DESIGN

Cross sectional.

METHODS

Testicular tissue samples from clinical castrations or euthanasia were collected from normal healthy (n = 3) or older subfertile (n = 4) stallions. Samples were processed and sequenced on an Illumina HiSeq™ 2000 Sequencing System. Bioinformatic analysis of the data was performed in R/RStudio, and the transcriptomes were compared between the two groups. Genes were considered to be differentially expressed between healthy and diseased tissue if they demonstrated at least a ±1.5× fold change difference and had a false discovery rate-adjusted P value <0.05. Gene ontology analysis was performed using Ingenuity® IPA.

RESULTS

Analyses of differential expression of individual genes, as well as computer-based gene ontology analysis, identified upregulation of cytokine-mediated inflammatory pathways in testes from stallions affected with testicular degeneration. This upregulation of inflammation was associated with upregulation of cell survival pathways, inhibition of apoptotic pathways and increases in collagen formation.

MAIN LIMITATIONS

There are unavoidable confounding factors (e.g. differences in breed, management, environment, age) that could create non disease-related genetic variation between our normal and affected samples. In addition, there are practical limitations to applying computer-based gene ontology analysis to equine samples. Gene ontology software relies on published information (mostly non-equine), and some biological processes (e.g. apoptosis and inflammation) are more commonly studied than others and so are over-represented in the literature and therefore more likely to be identified by computer algorithms. Caution should be taken when interpreting the data, as alterations in gene expression can be the cause of disease processes or can be the result of disease processes.

CONCLUSIONS

These results suggest that chronic, low-grade inflammation may be involved in the pathophysiology of age-related testicular degeneration in stallions.

Functions of somatic cells for spermatogenesis in stallions

Spermatogenesis and testis development are highly structured physiological processes responsible for post-pubertal fertility in stallions. Spermatogenesis comprises spermatocytogenesis, meiosis, and spermiogenesis. Although germ cell degeneration is a continuous process, its effects are more pronounced during spermatocytogenesis and meiosis. The productivity and efficiency of spermatogenesis are directly linked to pubertal development, degenerated germ cell populations, aging, nutrition, and season of the year in stallions. The multiplex interplay of germ cells with somatic cells, endocrine and paracrine factors, growth factors, and signaling molecules contributes to the regulation of spermatogenesis. A cell-to-cell communication within the testes of these factors is a fundamental requirement of normal spermatogenesis. A noteworthy development has been made recently on discovering the effects of different somatic cells including Leydig, Sertoli, and peritubular myoid cells on manipulation the fate of spermatogonial stem cells. In this review, we discuss the self-renewal, differentiation, and apoptotic roles of somatic cells and the relationship between somatic and germ cells during normal spermatogenesis. We also summarize the roles of different growth factors, their paracrine/endocrine/autocrine pathways, and the different cytokines associated with spermatogenesis. Furthermore, we highlight important matters for further studies on the regulation of spermatogenesis. This review presents an insight into the mechanism of spermatogenesis, and helpful in developing better understanding of the functions of somatic cells, particularly in stallions and would offer new research goals for developing curative techniques to address infertility/subfertility in stallions.

Changes in circulating concentrations of testosterone and estrone sulfate after human chorionic gonadotropin administration and subsequent to castration of 2-year-old stallions

Reproductive steroids testosterone (T) and estrone sulfate (E1S) are used as diagnostic markers for cryptorchidism in horses. The human chorionic gonadotropin (hCG) stimulation test is used as a diagnostic aid because administration of this hormone results in greater incremental differences in circulating steroid concentrations. Thoughts regarding optimal sampling times following hCG administration, however, are inconsistent. Additionally, determination of half-life of these steroids is important in postsurgical samples to confirm complete removal of testicular tissue. Objectives of this study, therefore, were to determine optimal sampling periods for peak T and E1S after hCG administration and half-life of these steroids after castration. Eight pony stallions were randomly assigned to control or treatment groups (5000 IU hCG). Blood samples were collected following hCG administration. Subsequently, stallions were castrated and blood samples were collected post-castration. The T concentrations were greatest at 72 h after hCG and were greater (P < 0.02) in samples from hCG-treated than control animals: 9,903.4 ± 384 and 784.0 ± 192 pg/mL, respectively (Mean ± SEM). The T concentrations were also greater at 1, 12, 24, 48 and 96 h. The E1S concentrations did not change after administration of hCG. The T response to hCG administration was biphasic with a maximal response between 48-96 h after administration. Half-lives of T and E1S were 1.1 and 0.7 h, respectively, and concentration of T and E1S was similar to that of geldings at 24 h post-castration, which, therefore, should be considered an optimal time to ensure complete castration has occurred.

Declining testicular function in the aging stallion: Management options and future therapies

Declining fertility in association with declining testicular function is commonly seen as stallions age and can be the cause of significant economic losses in the equine breeding industry. This manuscript describes how to clinically recognize the signs of age-related declining testicular function (testicular degeneration) and also provides mare and stallion management strategies for improving reproductive outcomes. Finally, the current understanding of the pathophysiology of the disease is presented, including the results of recent studies that are beginning to uncover the underlying causes for age-related declines in testicular function in stallions. These new findings provide a basis for possible future treatments that could delay the effects of aging on the testis.

Azoospermia in male dromedary: Clinical findings, testicular biopsy, serum follicle stimulating hormone and seminal biomarkers

Azoospermia is not an uncommon infertility problem in the male dromedary (Camelus dromedarius). Azoospermia was investigated via clinical findings, testicular biopsy as well as the evaluation of follicle stimulating hormone (FSH), concentration of camel testis protein (TEX101) and camel epididymis-specific extracellular matrix protein (ECM1) in seminal fluids. Azoospermic male camels (AZOO group, n = 28) that had been detected to be infertile as a result of lack of resulting pregnancies after repeated mating's for at least one season were included in this study. Clinical examination, semen analysis and testicular biopsy sampling were conducted for each individual animal. Blood samples were collected from the AZOO and from reference fertile males (FERT group, n = 8) for the assay of FSH hormone and semen biomarkers (TEX101 and ECM1). There were bilaterally normal-sized testes in 42.8%, bilaterally small-sized testes in 35.7%, bilaterally large-sized testes in 7.1%, no testicles in 7.1% and only one testicle in 7.1% of azoospermic animals. Sertoli cell-only syndrome (SCO) and maturation arrest were observed in 78.6% and 21.4% of the animals, respectively. There were greater concentrations of FSH in the AZOO group compared with the FERT group (P  = 0.01). In conclusion, azoospermia in dromedary camels is mainly associated with spermatogenic defects and greater serum FSH concentrations. Seminal biomarkers, therefore, might be feasible indicators for identifying azoospermia in the male dromedary camels and the condition of non-obstructive azoospermia was seemingly prevalent in the male dromedary camels in the present study.

Inhibin-A and Inhibin-B in stallions: Seasonal changes and changes after down-regulation of the hypothalamic-pituitary-gonadal axis

The biological function of inhibin is mediated by two heterodimers, inhibin-A and inhibin-B. The relative importance of inhibin-A and -B in male reproductive function varies considerably across species with inhibin-B predominating in many species, whereas inhibin-A appears relatively more important in rams. Research reported to date in stallions has examined total or immunoreactive (ir) inhibin which does not distinguish the two heterodimers. Therefore, the objective of this study was to characterize changes in inhibin-A and inhibin-B concentrations in stallions: 1) across season for a period of one year, and 2) after downregulation of the hypothalamic-pituitary-gonadal (HPG) axis. In Study one, serum samples were obtained monthly from five stallions for a period of one year. Serum concentrations of inhibin-A, inhibin-B, testosterone and estrone sulfate were determined by ELISA. In Study two, stallions were treated with the GnRH antagonist, acyline (n = 4; 330 mg/kg acyline IM) or vehicle control (n = 4; vehicle alone) every five days for 50 days. Plasma concentrations of inhibin-A and -B were determined by ELISA at Days 0, 6, 12, 22, 37, 59, 80, 87 and 104 after initiation of acyline treatment. Testis volume was determined by ultrasonography at weekly intervals. In Study 1, both inhibin-A and inhibin-B showed seasonal changes in concentration with highest concentrations in increasing day length and lowest concentrations in short day lengths. Inhibin-B (overall mean 107.8 ± 4.1 pg/mL) was present at 4.7-fold higher concentrations in serum than inhibin-A (overall mean 23.0 ± 0.7 pg/mL). In Study 2, plasma concentrations of inhibin-B but not inhibin-A were significantly downregulated by administration of the GnRH antagonist, acyline. When the HPG axis was downregulated by acyline, testis volume was strongly correlated with inhibin-B (r = 0.73; P < 0.05) but not inhibin-A (r = 0.22; P = 0.20). In summary, inhibin-B appears to be the predominant form of inhibin in the stallion which undergoes seasonal regulation along with other reproductive parameters and is co-regulated with other endocrine parameters of the HPG axis.

Impotentia generandi in male dromedary camels: FSH, LH and testosterone profiles and their association with clinical findings and semen analysis data

Impotentia generandi (IG) is a major problem in male dromedary camels. The objective of this research was to characterize the FSH, LH and testosterone profiles and their association with clinical findings and semen characteristics in IG-male dromedaries. Semen was collected by electroejaculation from camels with IG (n = 17) and from a fertile group (FERT, n = 5) and was evaluated for motility, viability, abnormality and concentration. According to their sperm counts, the IG-camels were categorized into three groups: those with normal sperm concentration (IG-NC, n = 8), those with oligospermia (IG-OLIGO, n = 5) and those with azoospermia (IG-AZOO, n = 4). Jugular blood was collected from all camels for analysis of FSH, LH and testosterone (T) concentrations. Results showed that the FSH, LH and T levels were higher in the IG-OLIGO and IG-AZOO groups than in the FERT and IG-NC groups (P < 0.05). The IG-camels with small testes showed lower sperm counts (P < 0.01) and greater FSH and T levels (P < 0.05) than the IG-camels with normal-sized testes. Inverse relationships were observed between the sperm counts and both the FSH and the LH levels (P < 0.05). In conclusion, the IG dromedary camels with oligo- and azoospermia were characterized by high serum levels of FSH, LH and T, indicating a condition of spermatogenic failure. In addition, inverse correlations were observed between these hormones and testicular size and sperm count.

Evaluation of the chemiluminescent enzyme immunoassay system for the measurement of testosterone in the serum and whole blood of stallions

Testosterone (T) concentration is a useful indicator of reproductive function in male animals. However, T concentration is not usually measured in veterinary clinics, partly due to the unavailability of reliable and rapid assays for animal samples. In this study, a rapid chemiluminescent enzyme immunoassay system (CLEIA system) that was developed for the measurement of T concentration in humans use was validated for stallion blood samples. First, serum T concentrations were measured using the CLEIA system and compared with those measured by a fluoroimmunoassay that has been validated for use in stallions. The serum T concentrations measured by the two methods were highly correlated (r = 0.9865, n = 56). Second, to validate the use of whole blood as assay samples, T concentrations in whole blood and in the serum were measured by the CLEIA system. T concentrations in both samples were highly correlated (r = 0.9665, n = 64). Finally, to evaluate the practical value of the CLEIA system in clinical settings, T concentrations were measured in three stallions with reproductive abnormalities after the administration of human chorionic gonadotropin (hCG). Two stallions with small or absent testes in the scrotum showed an increase in T production in response to hCG administration and one stallion with seminoma did not. In conclusion, the CLEIA system was found to be a rapid and reliable tool for measuring T concentrations in stallions and may improve reproductive management in clinical settings and in breeding studs.

Xenogeneic transplantation of equine testicular cells into seminiferous tubules of immunocompetent rats

The objectives were to develop a transplantation assay for equine testicular cells using busulfan-treated prepubertal immunocompetent rats as recipients, and to determine if putative equine spermatogonial stem cells (SSCs) could be enriched by flow cytometric cell sorting (based on light scattering properties), thereby improving engraftment efficiency. Four weeks after transplantation of frozen/thawed PKH26-labeled equine testicular cells, 0.029 ± 0.045% (mean ± SD) of viable donor cells transplanted had engrafted. Donor cells were present in seminiferous tubules of all recipient rats forming chains, pairs, mesh structures, or clusters (with two to >30 cells/structure). Cells were localized to the basal compartment by the basement membrane. Although equine cells proliferated within rat seminiferous tubules, no donor-derived spermatogenesis was evident. Furthermore, there was no histologic evidence of acute cellular rejection. No fluorescent cells were present in control testes. When equine testicular cells were sorted based on light scattering properties, the percentage of transplanted donor cells that engrafted was higher after injection of cells from the small, low complexity fraction (II; 0.169 ± 0.099%) than from either the large, high complexity fraction (I; 0.046 ± 0.051%) or unsorted cells (0.009 ± 0.007%; P < 0.05). Seminiferous tubules of busulfan-treated prepubertal immunocompetent rats provided a suitable niche for engraftment and proliferation, but not differentiation, of equine testicular cells. Sorting equine testicular cells based on light scattering properties resulted in a 19-fold improvement in colonization efficiency by cells with high forward scatter and low side scatter, which may represent putative equine SSCs.

Immunolocalization of estrogen receptor alpha, estrogen receptor beta and androgen receptor in the pre-, peri- and post-pubertal stallion testis

In various species, androgens and estrogens regulate the function of testicular Leydig, Sertoli, peritubular myoid, and germ cells by binding to their respective receptors and eliciting a cellular response. Androgen receptor (AR) is expressed in Sertoli cells, peritubular myoid cells, Leydig cells and perivascular smooth muscle cells in the testis depending on the species, but its presence in germ cells remains controversial. Two different estrogen receptors have been identified, estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ), and their localization and function in testicular cells varies depending on the species, developmental stage of the cell and type of receptor. The localization of AR in an immature and mature stallion has been reported but estrogen receptors have only been reported for the mature stallion. In the present study, the localizations of AR and ERα/ERβ were investigated in pre-pubertal, peri-pubertal and post-pubertal stallions. Testes were collected by routine castration from 21 horses, of light horse breeds (3 months-27 years). Animals were divided into the following age groups: pre-pubertal (3-11 months; n=7), peri-pubertal (12-23 months; n=7) and post-pubertal (2-27 years; n=7). Testicular tissue samples were fixed and embedded, and the presence of AR, ERα and ERβ was investigated by immunohistochemistry (IHC) using procedures previously validated for the horse. Primary antibodies used were rabbit anti-human AR, mouse anti-human ERβ and rabbit anti-mouse ERα. Sections of each region were incubated with normal rabbit serum (NRS; AR and ERα) or mouse IgG (ERβ) instead of primary antibody to generate negative controls. Androgen receptors were localized in Leydig, Sertoli and peritubular myoid cells of all ages. Estrogen receptor alpha was localized in Leydig and germ cells of all ages but only in pre- and peri-pubertal Sertoli cells and post-pubertal peritubular myoid cells. Estrogen receptor beta was localized in Leydig and Sertoli cells of all ages but in only pre-pubertal germ cells and absent in peritubular myoid cells of all ages. Taken together, the data suggest that estrogen regulates steroidogenesis by acting through ERα and ERβ in the Leydig cells and promotes gametogenesis by acting through ERβ in the Sertoli cells and ERα in the germ cells. In contrast androgen receptors are not found in germ cells throughout development and thus are likely to support spermatogenesis by way of a paracrine/autocrine pathway via its receptors in Leydig, Sertoli and peritubular myoid cells.

Xenografting restores spermatogenesis to cryptorchid testicular tissue but does not rescue the phenotype of idiopathic testicular degeneration in the horse (Equus caballus)

Spermatogenesis from many mammalian species occurs in fragments of normal testis tissue xenografted to mice. Here we apply xenografting to the study of testicular pathology. Using the horse model, we investigated whether exposure to a permissive extratesticular environment in the mouse host would rescue spermatogenesis in cryptorchid testicular tissue or in tissue affected by idiopathic testicular degeneration (ITD). In cryptorchid tissue, where the extratesticular environment is abnormal, xenografting induced spermatogenesis up to meiosis in a subpopulation of seminiferous tubules. Thus, spermatogonia survive and partially retain their potential to differentiate in cryptorchid horse testes. In contrast, the primary defect in equine ITD is hypothesised to be tissue autologous. In support of this, xenografting did not restore spermatogenesis to tissue affected by ITD, thus confirming that the testis itself is primarily diseased. This outcome was not affected by supplementation of exogenous gonadotropins to the mouse host or by reconstitution of a normal reproductive regulatory axis supplied by functional porcine testicular xenografts. These studies demonstrate the usefulness of xenografting for the study of testicular pathology.

INHBA-associated markers as candidates for stallion fertility

The inhibin beta A (INHBA) gene was chosen as candidate for stallion fertility and analysed for intragenic markers to find associations with pregnancy rate per oestrus. Intragenic single nucleotide polymorphisms (SNPs) were developed in order to perform an association and haplotype analysis using the least square means (LSM) of the pregnancy rate per oestrus for stallions as well as breeding values (BVs) for the embryonic and paternal component of the pregnancy rate per oestrus. The polymorphisms were genotyped in 161 Hanoverian warmblood stallions. Insemination records from approximately 20,000 Hanoverian warmblood mares were used to calculate LSM for stallions and to predict the paternal and embryonic component of BVs for the pregnancy rate per oestrus. We demonstrated significant associations of single markers and haplotypes with the LSM and the embryonic and paternal component of BVs for the pregnancy rate per oestrus. This is the first report on INHBA as an associated candidate gene with the LSM of stallions and the paternal and embryonic component of BVs for the pregnancy rate per oestrus.

Immunolocalization of estrogen and androgen receptors and steroid concentrations in the stallion epididymis

The presence of steroids and their receptors throughout development, specifically androgen receptor (AR), estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta), in the epididymis of a high estrogen producing species like the stallion has not been determined. Epididymal and testicular samples were collected for analysis of testosterone and estradiol-17beta (E(2)) concentrations and for immunolocalization of AR, ERalpha and ERbeta. The concentration of testosterone in the testis and epididymis were not different among age groups (P>0.05). AR was localized in the principal cells of the caput, corpus and cauda in all four age groups. This lack of change in testosterone concentration and receptor localization suggests that testosterone is important for both development and maintenance of epididymal function. There was an age-related increase in E(2) concentrations in all regions of the epididymis (P<0.05), suggesting that E(2) is also important for adult function. ERbeta was localized in the principal cells of the caput, corpus and cauda in all four age groups, but the localization of ERalpha was regional and age dependent. In peri-pubertal animals, ERalpha immunostaining was most prominent and estradiol was similarly present in all three epididymal regions; this suggests that estradiol also plays a key role in the maturation of the stallion epididymis during the pubertal transition when sperm first arrive in the epididymis. In conclusion, these results suggest that the stallion epididymis is regulated by both androgens and estrogens throughout development and that estradiol is more important to epididymal function in the stallion than previously believed.

A comparison of the effects of equine luteinizing hormone (eLH), equine growth hormone (eGH) and human recombinant insulin-like growth factor (hrIGF-I) on steroid production in cultured equine Leydig cells during sexual maturation

There are several hormones and local testicular factors involved in the initiation and control of steroidogenesis and spermatogenesis during puberty. GH and its mediator, IGF-I, increase substantially during puberty, and in addition to LH, these growth-promoting hormones can have direct effects upon testicular function. The objective of this work was to investigate the effects of eLH, eGH and hrIGF-I upon Leydig cells derived from testes of colts and stallions representing different stages of development. Testes were obtained from 48 light horse colts and stallions at the time of routine castration, horses were categorized according to age group (prepubertal, pubertal and postpubertal) and a Leydig cell enriched preparation was utilized for cell culture. Cells derived from all 48 horses were treated with doses of eLH, and a subset of 21 horses received doses of eGH and hrIGF-I. Cells were plated at a concentration of 1 x 10(6) cells/ml and incubated for 24 h at 32 degrees C. Production of testosterone and estradiol was measured by validated RIA. Leydig cells from prepubertal colts secreted greater basal amounts of testosterone but lesser basal amounts of estradiol compared with the other age groups (p < 0.001). Pubertal stallions exhibited the greatest relative response to eLH (p < 0.05). Neither eGH nor hrIGF-I elicited a steroidogenic response over baseline concentrations in any of the three age groups.

Endocrine and paracrine control of sperm production in stallions

The specific nature and relative contribution of the major hormones involved in regulation of reproductive function of the stallion are not well defined nor have paracrine or autocrine factors been identified. Over the last 12 years, our laboratory has been engaged in characterizing the hypothalamic-pituitary-testicular axis (HPT) in stallions. A number of endocrine factors and mechanisms important for normal reproductive function have been investigated. Studies investigating poor fertility in stallions suggest that a closer look at the testicular level is warranted. For a complete understanding of intratesticular control mechanisms including cell-to-cell interactions in the stallion, studies on the actions of paracrine/autocrine factors such as growth factors, inhibin, activin, and oxytocin are needed. In other species, paracrine/autocrine systems appear to be important in modulating endocrine control of testicular function and spermatogenesis.

The effects of age, season and fertility status on plasma and intratesticular insulin-like growth factor I concentration in stallions

The purposes of this study were to establish the basal plasma and testicular insulin-like growth factor-I (IGF-I) values for stallions ranging in age from 6 months to 23 years and to determine if IGF-I could be used as a marker for declining fertility. Blood and testes were obtained from 28 light horse stallions and colts. Of the 28 stallions, 22 were considered fertile and were categorized by age (<2 y, 5 to 10 y, 11 to 15 y, and 16 to 23 y); 12 age-matched stallions were grouped as to fertility status (fertile, subfertile, infertile); and all 28 stallions were grouped as to season of castration (breeding season vs. non-breeding season). In colts less than 2 years of age, IGF-I concentrations in plasma and testicular extracts were higher (P < 0.01) than in the other age groups and were higher in the breeding season than in the non-breeding season (P < 0.01). No significant differences in plasma or testicular extract concentrations of IGF-I were found among fertility groups. The results of this study demonstrate that plasma and testicular IGF-I levels are high in stallions younger than 2 years of age and then decline and plateau in stallions older than 5 years of age, suggesting that IGF-I may be involved in testicular development. The results allude to a possible seasonal effect on IGF-I production. However, it is difficult to separate true seasonality and the effect of age as only those stallions less than 2 years old exhibited variation between seasons. The IGF-I does not appear to have a direct relationship with declined fertility in the stallions tested, suggesting that IGF-I may not be a reliable biomarker for the diagnosis of subfertility and infertility.