The by-pass of tissue hormone stores during the secretion of newly synthesized parathyroid hormone.

Bovine parathyroid gland slices were incubated in Krebs' buffer with [3-H] leucine in order to assess the biosynthesis and secretion of parathyroid hormone (PTH). After incubation the particulate structures of the tissue were extracted with sodium deoxycholate to yield primarily newly synthesized (radioactive) PTH in the extract and preexisting granule PTH in the residue. In pulse-chase experiments radioactive PTH entered the granule fraction at a time when total tissue radioactive hormone was declining, which indicates that some newly-synthesized PTH was packaged into these granules. The specific radioactivity of the PTH in the incubation medium was from 20-fold to more than 80-fold greater than that of the granule PTH of the tissue, but was similar in magnitude to that of the tissue's newly synthesized hormone. Secretion of newly synthesized PTH was greater during incubation in buffer with 1.25 mM Ca than it was at 2.5 mM Ca which indicated that the hormone release was subject to physiological control. The PTH content of the tissue granule fraction was lower following incubation at 1.25 mM Ca than at 2.5 mM Ca. The possibility was excluded that the higher specific radioactivity of PT1 of incubation medium compared to that of the secretory granule fraction resulted from a diffusion gradient of [3-H] leucine into the tissue slices. These data indicate that a major portion of the newly synthesized PTH was secreted without prior equilibration with the hormone in the pool of secretory granules.