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Plasma Calcium (mg /100 ml ) FIGURE 14 Concentrations of immunoreactive PTH and calcitonin in pig plasma as a function of plasma calcium. (From Arnaud CD, Littledike T, Tsao HS. Simultaneous measurements of calcitonin and parathyroid hormone in the pig. In Taylor S, Foster GV, eds., Proceedings of the symposium on calcitonin and C cells. Heinemann, London, 1969.)

same concentration range, but their secretory responses are opposite, presumably because of differences in the transduction and secretory apparatus. Although little information is available concerning intracellular mechanisms in parafollicular cells, the relation between stimulation and secretion is more typical of endocrine cells than the regulation of PTH secretion.

In addition to responding directly to high concentrations of calcium, calcitonin secretion may also increase after eating. Gastrin, a hormone produced by gastric mucosal cells, stimulates parafollicular cells to secrete calcitonin. Other gastrointestinal hormones, including cholecystokinin, glucagon, and secretin, have similar effects, but gastrin is the most potent among these agents. Secretion of calcitonin in anticipation of an influx of calcium from the intestine is a feed-forward mechanism that may guard against excessive concentrations of plasma calcium by decreasing osteoclastic activity. This phenomenon is analogous to the anticipatory secretion of insulin after a carbohydrate-rich meal (see Chapter 41). Although the importance of this response in humans is not established, sensitivity of parafollicular cells to gastrin has been exploited clinically as a provocative test for diagnosing medullary carcinoma of the thyroid.

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