Muresan Z, MacGregor R R
Department of Anatomy and Cell Biology, University of Kansas Medical Center, Kansas City 66160-7400.
Mol Biol Cell. 1994 Jul;5(7):725-37. doi: 10.1091/mbc.5.7.725.
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.
甲状旁腺激素（PTH）的分泌部分受对儿茶酚胺有反应的经典“刺激-分泌”途径调控。然而，甲状旁腺细胞中PTH胞吐作用的主要生理调节因子是细胞外游离Ca2+。Ca(2+)调节的PTH释放表现出一些提示组成型分泌的特征。这项工作的目的是获取更多关于甲状旁腺细胞中Ca(2+)调节的胞吐作用可能的细胞内起源的信息。用[35S]硫酸盐标记的新鲜解离的牛甲状旁腺细胞合成了一种可溶性硫酸软骨素/硫酸皮肤素蛋白聚糖（Mr约为90 - 150K），并分泌到培养基中。[35S]硫酸化蛋白聚糖的输出满足通常定义组成型释放的几个标准：1）在5分钟脉冲后不久（7 - 15分钟）在培养基中检测到输出；2）平衡标记后细胞内储存极少（由于快速释放和细胞内降解的联合过程）；3）对异丙肾上腺素刺激不敏感，异丙肾上腺素是甲状旁腺细胞中已知的促分泌剂。尽管如此，细胞外Ca2+从0.5 mM增加到2.0 mM使[35S]硫酸化蛋白聚糖的输出从总标记物的60%降至30%。此外，免疫反应性PTH和[35S]硫酸化蛋白聚糖的分泌池受到细胞外Ca2+的相同程度和敏感性的调节，尽管异丙肾上腺素在刺激PTH释放方面比蛋白聚糖更有效。总之，我们的实验结果表明，在甲状旁腺细胞中，细胞外Ca2+对PTH和蛋白聚糖的输出均产生负调节作用，蛋白聚糖是组成型分泌的一个假定标志物。我们进一步表明，新合成的PTH的一部分也进入该途径，而另一部分进入异丙肾上腺素可释放区室，蛋白聚糖在很大程度上被排除在该区域之外。
