Comparison by three different radioimmunoassay systems of the polymorphism of plasma FSH in mares in various reproductive states

Abstract

FSH was measured in the pituitary, and in pituitary venous and jugular blood collected at frequent intervals from mares in various reproductive states, using 3 validated and highly specific radioimmunoassay systems based on different antibodies, 'o', 'h' and 'e'. In the pituitary, 4 forms of FSH were found which differed in isoelectric point and relative potency in the 3 assays. In jugular blood, mean FSH concentrations and short-term patterns depended on the assay used and the reproductive state of the mare. In pituitary venous blood, although FSH concentrations were greatly elevated above jugular values, the relationship amongst the 3 assays was similar to that in jugular blood. However, the pulsatility of FSH secretion at oestrus (1-2 pulses per h in 4 of 5 mares) could be observed only in pituitary blood. When the rate of folliculogenesis was slow (acyclicity, oestrus), FSH concentrations measured in assay 'o' were relatively high and showed smaller fluctuations than in assays 'h' and 'e'. By contrast, when folliculogenesis was rapid (transitional phase, dioestrus), FSH tended to be lowest in assay 'o' and pulses had a similar magnitude in all assays. These results suggest that plasma FSH is polymorphic and that changes in the circulating form may have functional importance. Furthermore, since FSH polymorphism affects immunoactivity, the choice of a radioimmunoassay for clinical or research use is difficult without further FSH structure-function studies to determine which form of the hormone should be measured in a given situation.