Endocrinology of calcium metabolism in amphibians, with emphasis on the evolution of hypercalcemic regulation in tetrapods.

Amphibians are the most diverse group as regards the endocrinology of Ca homeostasis because in them only "true" parathyroid glands made first appearance but its involvement varies greatly according to their phyletic affinities and habitats. Presence of ultimobranchial gland, parathyroid and renal 25-hydroxyvitamin-D-1-hydroxylase enzyme in anurans represents their tetrapod pattern of Ca regulation. Among "higher" urodeles, both parathyroid and pituitary are involved in hypercalcemic regulation whereas in "lower" urodeles (some of them are "aparathyroid" too), pituitary alone appears to discharge this function. The latter situation is analogous to the fish. Apodans do possess ultimobranchial and parathyroid glands but the problem of their Ca metabolism has not yet been explored. This review deals exhaustively with the endocrinology of Ca metabolism in amphibians. Also, the evolution of hypercalcemic regulation in tetrapods will be discussed in the light of recent developments.