Endocrine Control of Calcium Homeostasis

Abstract

Two endocrine glands of branchial origin are involved in regulation of calcium metabolism\p=m-\the parathyroids which secrete parathormone (parathyroid hormone) in response to hypocalcaemia, and the ultimobranchial glands which release calcitonin (thyrocalcitonin) during hypercalcaemia. The ultimobranchial glands are distinct organs in most vertebrates but in mammals the cells become embedded in the internal parathyroid and thyroid where they constitute the parafollicular 'C' cells. Calcitonin can be extracted from mammalian thyroid (hence the alternative name, thyrocalcitonin) but it is present in much higher concentrations in the ultimobranchial glands of lower vertebrates. In mammals, both hormones appear to act primarily on bone resorption and calcium mobilization from the skeleton, which is stimulated by parathormone and inhibited by calcitonin. Secretion or release of both hormones is determined by the level of calcium in the blood, and the two constitute a dual endocrine control comparable to that of insulin and glucagon in regulation of blood glucose. The ultimobranchials are the older glands phylogenetically for they are found in sharks and bony fishes, while the parathyroids appear first in amphibians.

Effect of Calcitonin on Calcitonin Cells, Parathyroid Glands and Serum Electrolytes in the House Shrew (<i>Suncus murinus</i>)

“…It also increases bone formation [Gaillard, 1967] and inhibits bone resorption [Copp, 1969;Hirsch and Mun son, 1969], Most of the investigations are devoted to the effect of calcitonin on phosphocalcic metabolism. They establish a direct action of calcitonin on the skeletal system and on the kidney.…”

Evidence for an important physiological role for calcitonin

“…Because plasma calcium levels remain in the normal range in the absence of any calcitonin-secreting tissue, this role was changed to one that characterized it as an antihypercalcemic hormone (3,4). This function was proposed because endogenous calcitonin secretion will prevent or decrease the hypercalcemia that follows calcium administration by intravenous infusion, intraperitoneal injection, or gavage (5,6).…”

Laboratory Tests in the Differentiation of Hyperparathyroidism from other Causes of Hypercalcaemia

“…The complexed fraction consists of calcium loosely complexed with, mainly, bicarbonate, citrate and phosphate. The physiologically active fraction, to which the parathyroid glands respond, is the ionised portion (Copp, 1969) and alterations in the state of calcium fractions in the plasma have, in the past, been considered to be of value in the differential diagnosis of hypercalcaemia (Fanconi and Rose, 1958). In our own studies where we have measured the total and ultrafiltrable, and the total and ionised plasma calcium concentrations, we have shown significant correlation between both of these variables with the total calcium concentration over a wide range of the latter (Wills and Lewin, 1970).…”