Evidence for the existence of a luteal cell type that is steroidogenic and releases relaxin.

Secretion of relaxin from cultured luteal cells derived from pregnant sows was detected by a reverse hemolytic plaque assay. In this method, luteal cells are cultured in monolayers together with protein-A-conjugated ovine red blood cells. In the presence of porcine relaxin anti-serum and complement, relaxin-releasing cells become surrounded by an area of hemolysis--a plaque--which can be microscopically visualized. After fixation, these same luteal cells in monolayers were stained for the presence of 3 beta-hydroxysteroid dehydrogenase, an enzyme marker for steroidogenic cells. Cells could then be classified by their ability to form plaques (relaxin-releasing cells) and/or steroidogenic capability (positive staining). Dual-secretors (large luteal cells that were steroidogenic and released relaxin) could be identified in dispersed luteal cells derived from pigs at all stages of pregnancy examined (Day 22-112 of gestation, n = 9; term is Day 114 +/- 2 days). In addition, luteal cells were detected that were either steroidogenic only or released relaxin, and finally, cells that appeared to possess neither endocrine capability. Frequency analysis of functional subtypes indicated approximately equal representation of each in the first half of pregnancy, but an apparent fall in relaxin-releasing cells in the preparturient period. It is suggested that dual-secretors may represent one mechanism that allows the corpus luteum to express multiple endocrine function during pregnancy without the requirement for increased cell numbers.