Localization of LH receptors on luteal cells with a ferritin--LH conjugate.

In order to study the distribution of LH (HCG) receptors on luteal cells ferritin was coupled to ovine LH with glutaraldehyde and purified by gel chromatography. The conjugate (FELH) competed with 125I-hCG for binding to isolated luteal membranes and stimulated a dose-dependent release of progesterone (P) from isolated luteal cells which was inhibited by PGF2 alpha. FELH was distributed as single molecules or in small clusters at intervals on the surfaces of luteal cells labeled at 37 degrees C, 4 degrees C or with formaldehyde prefixation. Capping or preferential labeling at one site was not observed. The general distribution of LH (hCG) binding sites at 37 degrees C was confirmed by light-microscopic autoradiography. The distribution at 4 degrees C or with prefixation was more diffuse than at 37 degrees C suggesting that FELH binding induces small changes in receptor aggregation. Binding of FELH was specific since excess hCG reduced FELH binding to luteal cells. In cells labeled at 4 degrees C, rinsed and warmed to 37 degrees C FELH was observed along cell surfaces and within some coated vesicles and a few lysosomes within minutes suggesting that receptor internalization is a rapid and possibly continual process.