Inhibin concentrations in mares with granulosa cell tumors

The hormone-producing equine granulosa cell tumor (GCT) may secrete high levels of inhibin. Measurement of inhibin concentrations may be useful in the diagnosis and conformation of mares with GCT. Inhibin may be measured using RIA, which recognizes dimeric αβ_A-inhibin as well as the monomeric (free) inhibin α-subunit, or using a two-site immunoradiometric assay (IRMA) specific for αβ_A-inhibin. The objective of this study was to examine concurrent relationships among α-inhibin (as measured using RIA), αβ_A-inhibin (as measured using IRMA), and other hormones (testosterone, estradiol, LH, FSH) in mares with GCT. Hormone concentrations were measured in single serum or plasma samples obtained from 22 mares with GCT and from 31 normal cycling mares. One GCT mare had blood samples collected at 12-h intervals for 21 days, and at 15-min intervals for two 6-h periods during that time. Results showed that in GCT mares α-inhibin was increased to a greater extent, was more uniformly elevated, and had a less variable secretory pattern than did αβ_A-inhibin. Concentrations of α-inhibin and tumor mass were positively correlated (P < 0.01). Concentrations of LH were higher (P < 0.02) in GCT mares than control mares and were positively associated with testosterone concentrations (P = 0.05). Concentrations of FSH tended to be lower in GCT than control mares and were inversely related with αβ_A-inhibin in GCT mares. Testosterone and estradiol concentrations were variable. It was concluded that immunoreactive <-inhibin reflected detection of both αβ_A-inhibin and free α-subunit. Free α-subunit was evidently secreted at a relatively steady rate dependent upon mass of the GCT, whereas secretion of αβ_A-inhibin was more responsive to FSH regulation. Determination of α-inhibin using RIA appeared to be a more reliable indicator of the presence of a GCT than specific measurement of αβ_A-inhibin using IRMA.