Ovine serum and pituitary isoforms of luteinising hormone during the luteal phase

The relative abundance of the different isoforms of pituitary and circulating luteinising hormone (LH) in ewes, at different times after the administration of gonadotrophin-releasing hormone ( GnRH), during the luteal phase of the oestrous cycle was investigated. Sixteen ewes on Day 9 of their cycle were divided into four groups ( n= 4). The control group (T-0) received saline solution; the remaining animals received 100 mu g GnRH (i. m.) 30, 90 or 180 min (T-30, T-90 and T-180, respectively) before serum and pituitary gland collection. Luteinising hormone polymorphism was analysed by chromatofocusing ( pH 10.5 - 3.5). The LH eluted from each chromatofocusing was grouped on the basis of the following three criteria: ( 1) according to the pH of elution ( pH >= 10 - 3.5); ( 2) as either a basic ( pH >= 7.5), neutral ( pH 7.4 - 6.5) and acidic ( pH <= 6.4) elution of LH of serum and hypophyseal origin; and ( 3) on the basis of distinct isoforms, of which 10 ( A - J) were identifiable in hypophyseal extracts and four ( A - D) were found in the serum. In general, the most abundant forms of LH in both the pituitary and serum, at all times, were basic. However, that proportion was greater in hypophyseal extracts ( 84 +/- 3%, 81 +/- 4%, 82 +/- 3% and 83 +/- 2% at T-0, T-30, T-90 and T-180, respectively) than in serum (5 +/- 5%, 48 +/- 10% and 54 +/- 6% at T-30, T-90 and T-180, respectively). Neutral and acidic LH made up a larger proportion of the total LH in sera ( neutral: 17 +/- 4%, 20 +/- 6% and 23 +/- 3% at T-30, T-90 and T-180, respectively; acidic: 32 +/- 8%, 32 +/- 11% and 23 +/- 6% at T-30, T-90 and T-180, respectively) than in the pituitary extracts ( neutral: 4.0 +/- 0.7%, 10 +/- 4%, 7 +/- 2% and 5.0 +/- 0.5% at T-0, T-30, T-90 and T-180, respectively; acidic: 12 +/- 3%, 11 +/- 2%, 12 +/- 2% and 12 +/- 2% at T-0, T-30, T-90 and T-180, respectively) at all times. These data reveal that the relative composition of the LH present in the pituitary gland and the LH secreted into the circulation is different, with more neutral and acidic isoforms being secreted. The pattern of circulating LH isoforms changes between 30 and 180 min after GnRH peak induction, with a greater proportion of isoform C ( eluting between pH 7.0 and 6.5) at T-180 compared with T-30 and T-90.