Comparison of the microheterogeneity of horse LH and FSH in the pituitary with that secreted into pituitary venous blood at oestrus.

For aqueous extracts of pituitary glands of oestrous mares, luteinizing hormone (LH) profiles were found to be similar to each other and to earlier work after chromatofocussing (CF) and isoelectricfocussing (IEF). After CF, both LH and follicle-stimulating hormone (FSH) in pituitary extracts focussed in multiple peaks in the acidic range, with 86% of LH and 80% of FSH found between pH 4 and 6. By contrast, in pituitary venous plasma, only 18% of the LH focussed in this range, whereas a significantly greater proportion (P less than 0.01) eluted above pH 7 than occurred in pituitary extracts (37% vs 2%, respectively). For pituitary venous FSH, there was only a slight shift in the distribution of isoforms compared with the pituitary extract, with a rise in the percentage of strongly acidic molecules in pituitary venous plasma (pH less than 3.65; 34% vs 16%). These results show that at oestrus, horse LH (which differs from that of other species because it has a heavily sialylated C-terminal extension to the beta-subunit, as does eCG), is much more alkaline when secreted as opposed to when it is stored in the pituitary. The authors of this report suggest that this modification is made after entry into a preferentially released pool of LH. Modulation of the forms of LH and FSH that are secreted may play a role in regulating target tissue responses.