Unifying concepts in stimulus-secretion coupling in endocrine cells and some implications for therapeutics
Stimulus-secretion coupling (SSC) in endocrine cells remains underappreciated as a subject for the study/teaching of general physiology. In the present article, we review key new electrophysiological, electrochemical, and fluorescence optical techniques for the study of exocytosis in single cells that have made this a fertile area for recent research. Based on findings using these techniques, we developed a model of SSC for adrenal chromaffin cells that blends features of Ca2+ entry-dependent SSC (characteristic of neurons) with G protein receptor-coupled, Ca2+ release-dependent, and second messenger-dependent SSC (characteristic of epithelial exocrine cells and nucleated blood cells). This model requires two distinct pools of secretory graunules with differing Ca2+ sensitivities. We extended this model to account for SSC in a wide variety of peripheral and hypothalamic/pituitary-based endocrine cells. These include osmosensitive magnocellular neurosecretory cells releasing antidiuretic hormone, stretch-sensitive atrial myocytes secreting atrial natriuretic peptide, K+-sensitive adrenal glomerulosa cells secreting aldosterone, Ca2+-sensitive parathyroid chief cells secreting parathyroid hormone, and glucose-sensitive ?- and -cells of pancreatic islets secreting insulin and glucagon, respectively. We conclude this article with implications of this approach for pathophysiology and therapeutics, including defects in chief cell Ca2+ sensitivity, resulting in the hyperparathyroidism of renal disease, and defects in biphasic insulin secretion, resulting in diabetes mellitus.