A plasma-membrane progesterone receptor in human sperm is switched on by increasing intracellular free calcium

Recent studies have revealed the presence of an atypical progesterone receptor on the plasma membrane of a relatively small subpopulation of human sperm cells. In this study, we show that the receptor is present in many cells in a latent form and that the receptor function in these cells is switched on asynchronously by increasing intracellular free Ca2+ levels. Under physiological conditions, this process occurs during sperm capacitation, but it can be mimicked by a short sperm exposure to the Ca(2+)-mobilizing agent thapsigargin. These observations suggest a role of Ca2+ as a modulator of nongenomic steroid effects on the cell surface.