Interactions of Gonadotropins with Corpus Luteum Surface Membranes. V. Differential Effects of Digitonin on the Buoyant Densities of Light and Heavy Rat Ovarian Membrane Fractions*

Abstract

Previous studies have indicated that rat luteal cells at certain stages of development can be fractionated so as to obtain two plasma membrane fractions with different densities and different profiles of marker enzymes. The light membrane fractions (density 1.13) contain the majority of hCG-binding sites and little or no cyclase enzyme, while the heavy membranes (density 1.17) contain the majority of cyclase enzyme and lesser quantities of hormone-binding sites. These membrane fractions were further compared with respect to their susceptibility to perturbation by digitonin. The buoyant density of luteal cell light membrane fractions, as marked by [125I]iodo-hCG binding, Mg2+-dependent ATPase, and 5'-nucleotidase, were highly perturbable by digotonin (delta density, greater than 0.05), while adenylate cyclase activity and phosphodiesterase activity associated with this fraction were only slightly perturbed (delta density, less than 0.02). The buoyant density of luteal cell heavy membrane fractions, as marked by adenylate cyclase, ATPase, and nucleotidase, was not significantly perturbed by digotonin. The hCG binding associated with the heavy membrane fraction was not perturbed by digitonin. From these studies, we conclude that the adenylate cyclase activity associated with light membrane fractions is due to contamination by heavy membranes, while the hCG-binding activity in heavy membrane fractions is intrinsic to that membrane. Except for the lysosomal marker (glucuronidase), which was solubilized by digitonin, the detergent had no significant effect on the density of mitochondrial, Golgi, GERL (Golgi, endoplasmic reticulum, and lysomal), or endoplasmic reticulum membranes. Plasma membranes from isolated granulosa cells and ovaries obtained 24 h after priming with PMS gonadotropin-hCG behaved as heavy membranes (density, 1.17) which contained hCG-binding sites, adenylate cyclase, nucleotidase, and Mg2+-dependent ATPase. These were not significantly perturbed by digitonin. The appearance of light membranes and the segregation of adenylate cyclase from the majority of hCG-binding sites is a development feature of the luteal cell.