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 ±â€¯4.1 pg/mL) was present at 4.7-fold higher concentrations in serum than inhibin-A (overall mean 23.0 ±â€¯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.

Reversible downregulation of the hypothalamic-pituitary-gonadal axis in stallions with a novel GnRH antagonist.

The GnRH antagonist, acyline, has not yet been investigated in the stallion. Our study aimed to: (1) evaluate the downregulation of the stallion hypothalamic-pituitary-gonadal axis by acyline through assessment of seminal parameters, testicular volume, and sexual behavior; (2) assess hormonal response of acyline-treated stallions to GnRH stimulation; and (3) verify reversibility after treatment. Stallions were assessed pretreatment and subsequently treated (every five days) for 50 days: acyline (n = 4; 330 µg/kg acyline) or control (n = 4, vehicle). The stallions were then monitored for 62 days after the last day of treatment. Treatment-induced declines (P < 0.05) in FSH, LH, testosterone, and estrone sulfate. Gonadotropins and testosterone returned to control values within 9 days, and estrone sulfate by 14 days, after discontinuation of treatment. Acyline-treated stallions failed to respond with a rise in FSH, LH, and testosterone after exogenous GnRH stimulation (gonadorelin) at Day 46 of treatment compared to pretreatment stimulation and control stallions. Decreases (P < 0.05) were observed in total sperm numbers and motility (week 2) in acyline-treated stallions, as well as total seminal plasma protein (week 2) and testicular volume (week 5). Over the course of the study, the time to erection, time to ejaculation, and number of mounts increased (P < 0.0001) across both groups of stallions; however, there was no effect of treatment or treatment by time interactions on these parameters. Testicular volume, and most seminal parameters regained normal levels within 62 days after treatment ended; on follow-up, sperm output of acyline-treated stallions was regained within 7 months after the end of experiment. In conclusion, acyline reversibly suppresses the stallion hypothalamic-pituitary-gonadal axis.

Decreasing pH of mammary gland secretions is associated with parturition and is correlated with electrolyte concentrations in prefoaling mares.

The objectives of this study were to determine pH of the mammary gland secretions and the corresponding electrolyte concentrations in prefoaling mares. Pregnant mares (seven primiparous and seven multiparous) were monitored daily from 310-320 days of gestation until parturition. Prefoaling mammary gland secretions were collected, and pH was immediately determined with a pH meter and pH strip test. An aliquot of prefoaling mammary secretions was frozen and stored until further analyses. After parturition, samples from day -4 to 0 (day of foaling) were thawed and electrolyte concentrations (ie, Ca(2)(+), Mg(2)(+), Na(+), K(+) and Cl(-)) were determined with an automated analyser. Data were analysed via a mixed model with the mare as a random effect. Correlations were determined between pH and electrolyte concentrations by the Pearson product-moment for each pair. There was significant reduction in pH of mammary secretions on the day of foaling (P<0.0001), and most mares (11/14) with a pH ≤7 foaled within 24 hours. There was high correlation between the two pH methods (r=0.93). Additionally, there were significant (P<0.05) increases in Ca(2+) and K(+) concentrations, and significant decreases in Na(+) and Cl(-) concentrations from one day before to the day of foaling. The pH of mammary secretions was highly and significantly (P<0.001) correlated with Na(+) (r=0.87), Cl(-) (r=0.85), Ca(2+) (r=-0.88); and K(+)(r=-0.80) concentrations, and moderately correlated with Mg(2+) (r=-0.58). Daily evening pH measure of the mammary gland secretions can predict foaling in most mares.