Existence of multiple molecular forms of luteinizing hormone in rat: differences in immunological and biological activities between stored and circulating forms

Abstract

Abstract. LH from rat anterior pituitary and serum was characterized with respect to its molecular size, and immunological and biological activity. Pituitary extracts prepared by homogenization followed by high speed centrifugation and chromatography on Ultrogel (AcA 44, 1.5 × 80 cm) columns showed two peaks of radioimmunoassayable lutropin (LH) activity, a major peak 1 and a minor peak 2. On the basis of their elution from Ultrogel columns, peak 1 had a larger molecular size (Kav = 0.44) than peak 2 (Kav = 0.53). The biological activities of the two peaks were assessed by testing the ability of the two peaks to stimulate cAMP production and progesterone synthesis using immature rat ovarian cells. Both cAMP production and progesterone synthesis by ovarian cells were stimulated by peak 1 and peak 2 in a dose dependent manner, but differed in the degree of stimulation. At optimal concentrations, stimulation of cAMP production by peak 2 was 1.45-fold higher than peak 1. Similarly, stimulation of progesterone production by peak 2 was 2-fold higher than that produced by peak 1. The molecular size of the LH released from anterior hemipituitaries in response to a gonadoliberin (LRH) agonist, [Des-gly-NH210]LRH ethylaminde, was also determined. Chromatography of control medium as well as the agonist treated medium on the Ultrogel columns showed a single peak of LH activity at a position which corresponded to peak 1 found in the pituitary extract. However, the elution profile of serum LH from animals injected with LRH agonist was found to be similar to that of peak 2 of the pituitary extract. Incubation of the pituitary extract with serum resulted in a shift in the position of elution of peak 1 towards peak 2. From these studies it is concluded that the anterior pituitary contains a major high mol wt form of LH with lower biological activity and a minor low mol wt form with higher biological activity, but only the low mol wt form appears in the circulation.