RESUMO
The interaction of LH and its receptor was investigated by ultrastructural analysis of ferritin-LH (FELH) binding to isolated rat luteal cells in the absence and presence of prostaglandin F2 alpha (PGF2 alpha), an inhibitor of LH-stimulated cAMP production. FELH, with a molar ratio of FE to LH of 1:1, bound specifically to LH receptors, either singly or in small groups (microaggregates), at intervals on luteal cell surfaces. FELH elicited a dose-dependent increase in progesterone production, and its binding increased with increased FELH concentration. The number of LH receptors per cell, estimated from particle counts, was about 6.2 +/- 0.6 X 10(4), similar to estimates from Scatchard analysis of [125I]iodo-hCG binding. Microaggregate size increased in parallel with FELH binding. Only partial aggregation was seen at concentrations of FELH that elicited near-maximal progesterone secretion. Aggregation continued to increase at FELH concentrations beyond that required to elicit maximal progesterone secretion. In the presence of PGF2 alpha, FELH-stimulated progesterone production was attenuated, and FELH binding decreased from 2.9 +/- 0.4 X 10(4) to 2.1 +/- 0.3 X 10(4) receptors/cell. PGF2 alpha did not alter microaggregate size on cells labeled with FELH at 4 C when membrane fluidity was already reduced, but did substantially reduce microaggregate size at 37 C. We conclude that FELH binds initially at random sites on membranes of isolated luteal cells and that as binding increases, receptors aggregate into small groups. Furthermore, microaggregates are related in part to receptor occupancy and possibly also to levels of cAMP or activation of the adenylate cyclase mechanism.
