Abstract
Secretion of parathyroid hormone (PTH) is regulated in part by a classical "stimulus-secretion" pathway responsive to catecholamines. The primary physiological modulator of PTH exocytosis in parathyroid cells, however, is extracellular free Ca2+. Ca(2+)-modulated PTH release exhibits several characteristics suggestive of constitutive secretion. The aim of this work was to obtain further information about the possible intracellular origins of Ca(2+)-modulated exocytosis in parathyroid cells. Freshly dissociated bovine parathyroid cells labeled with [35S]sulfate synthesized a soluble chondroitin/dermatan sulfate proteoglycan (M(r) approximately 90-150 K) that was secreted into the medium. The export of [35S]sulfated proteoglycan satisfied several criteria that generally define constitutive release: 1) export is detected in the medium shortly (7-15 min) after a 5-min pulse, 2) there is minimal intracellular storage after equilibrium labeling (because of combined processes of rapid release and intracellular degradation), and 3) there is insensitivity to stimulation with isoproterenol, a known secretagogue in parathyroid cells. Nevertheless, the increase in extracellular Ca2+ from 0.5 to 2.0 mM reduced the export of the [35S]sulfated proteoglycan from 60% of total labeled to 30%. In addition, a secreted pool of immunoreactive PTH and [35S]sulfated proteoglycan was modulated by external Ca2+ to the same degree and sensitivity, although isoproterenol was more effective in stimulating the release of PTH than that of proteoglycan. Together, our experimental results show that in the parathyroid cell extracellular Ca2+ modulates negatively the export of both PTH and proteoglycan, a putative marker for constitutive secretion. We further suggest that a portion of newly synthesized PTH also enters this pathway, whereas another portion proceeds to an isoproterenol-releasable compartment from which the proteoglycan is largely excluded.