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Individual large luteal cells (LLC) derived from pregnant swine differ conspicuously in their ability to secrete the peptide hormone relaxin under basal and stimulated conditions – the phenomenon of functional heterogeneity. The purpose of this study was to quantitate knowledge of this phenomenon through use of a reverse haemolytic plaque assay, a technique that utilizes antibody-directed, complement-mediated erythrocyte lysis to detect hormone secretion by single LLCs in culture.

Measurement of individual plaque areas (an index of the amount of relaxin secreted) demonstrated an approximate 100-fold range in the amount of relaxin secreted by a single cell under basal conditions. This range was doubled by exposure to the phorbol ester, 4β-phorbol 12β-myristate 13-acetate (PMA; 50 nmol/l). Under basal conditions, 50 and 80% of the total amount of relaxin was secreted by approximately 10 and 30% of all LLCs respectively. The size of these fractions was not influenced by the time of incubation (1–8 h), or by the presence of either of two non-specific stimulatory relaxin secretagogues, PMA (50 nmol/l) or arachidonic acid (1 µmol/l). The unimodal frequency distribution of plaque areas (under basal or stimulated conditions) suggests that relaxin-secreting LLCs comprise a discrete functional population of secretory cells, at least under these experimental conditions.

We conclude that a remarkably small fraction of LLCs secretes the majority of relaxin, and that the size of this fraction was not influenced by time or secretagogues.

Journal of Endocrinology (1992) 134, 77–83