Different subcellular storage sites for decidual- and pituitary-derived prolactin: possible explanation for...

The intracellular sites of storage for human decidual- and pituitary-derived prolactins have been determined by measuring the hormones in subcellular fractions following differential and density gradient centrifugations. In decidual tissue, 78 +/- 4% (mean +/- SE, n = 5) of the prolactin was detected in the post-microsomal supernatant, while 98% of the prolactin in the pituitary was detected in mitochondrial and microsomal particulate fractions. Following centrifugation on sucrose density gradients, decidual prolactin was distributed diffusely throughout the gradients, while greater than 90% of the pituitary prolactin sedimented as a single peak with a mean density of 1.22 g/cm3. These studies indicate that, unlike pituitary prolactin, decidual prolactin is not stored in secretory granules. Differences in the intracellular storage sites of decidual and pituitary prolactins may explain observed differences in the regulation of prolactin secretion by decidual and pituitary tissues.