Dimeric inhibin concentrations in mares with granulosa-theca cell tumors

Objective - To determine whether concentrations of dimeric inhibin (αβ_A) are greater in plasma and tumor fluid from mares with granulosa-theca cell tumors (GTCT), compared with concentrations in plasma and equine follicular fluid (eFF) from control mares. Animals - 6 mares with GTCT and 12 clinically normal mares. Procedure - The αβ_A immunoradiometric assay used 2 antibodies, one against each subunit of inhibin (α and β_A subunits). Tumor tissue, tumor fluid, and a single blood sample were collected at the time of surgical removal of the GTCT. A single blood sample was collected from 7 control mares during various stages of the estrous cycle. Five other control mares were ovariectomized when their ovaries contained growing follicles of 25 to 35 mm in diameter. A blood sample and eFF from the largest follicle were collected at the time of ovariectomy. Results - Mares with GTCT had significantly greater plasma concentrations of αβ_A (mean ± SEM, 0.86 ± 0.53 ng of recombinant human-αβ_A/ml), compared with control mares (0.14 ± 0.02 ng/ml). Concentrations of αβ_A in tumor fluid and eFF were similar. Concentrations of αβ_A were significantly lower after ovariectomy. Conclusions and Clinical Relevance - Dimeric inhibin concentration was higher in plasma from mares with GTCT than in plasma from control mares. Increased granulosa cell mass and loss of mechanisms regulating αβ_A release in mares with GTCT likely accounted for the increase in plasma concentra-tions. Measurement of αβ_A concentrations may be useful for identifying mares with GTCT.