When serum calcium levels rise, which of the following hormones is secreted?
A. Aldosterone
B. Renin
C. Parathyroid hormone
D. Calcitonin
Correct Answer: D. Calcitonin
When calcium levels rise, calcitonin is secreted from the thyroid; this hormone moves calcium from plasma into bone. In response to hypercalcemia, calcitonin is secreted by the parafollicular C cells. Calcitonin lowers serum calcium by decreasing renal calcium and phosphorus reabsorption and also by decreasing bone reabsorption. Calcitonin is not significant in overall calcium homeostasis, but it is an important therapeutic option.
Option A: Aldosterone enhances renal calcium reabsorption by two types of channels. Reabsorption by the distal nephron. The present in vitro experiments investigated the effect of the hormone on calcium (Ca2+) transport by the luminal membrane of the rabbit nephron, independent of any systemic influence.
Option B: Renin secretion is mainly dependent on cyclic AMP formation. Cyclic AMP availability is the net effect of positive adenylyl cyclase activity and competing degradative activity of calmodulin-activated phosphodiesterase. Increasing intracellular calcium concentrations decrease net cyclic AMP formation by dampening adenylate cyclase and enhancing phosphodiesterase activities.
Option C: Parathyroid hormone is secreted in response to lowered calcium levels; this hormone moves calcium from bone into plasma. The main cause of hypercalcemia is an excess parathyroid hormone (PTH). PTH-mediated causes include adenoma/hyperplasia of the gland, familial hypocalciuric hypercalcemia, and multiple endocrine neoplasia syndromes (type1, 2A).
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