Evolution of Calcium Regulation in Lower Vertebrates

Calcium regulation in lower vertebrates appears to be a continuum.The predominant hypercalcemic hormone in reptiles, birds andmammals is parathyroid hormone, while the major hypercalcemiccontrol in fishes is a pituitary factor, probably prolactin.In the amphibians dual controlling mechanisms are at work, sothat both the pituitary and parathyroids exert their influence.Prolactin may still retain some hypercalcemic potency in thehigher vertebrate groups, either directly or indirectly by influencingthe secretion of other hypercalcemic hormones. On the otherhand, parathyroid hormone does not occur in, nor does it elevateblood calcium in fishes. It thus seems to be a new inventionof tetrapods, or possibly to have evolved from a pituitary factorof fishes. The ability to lower blood calcium seems to be veryimportant in seawater fishes, in which the corpuscles of Stanniusexert major control. In terrestrial forms, the corpuscles ofStannius are not present, and hypocalcemic factors assume aminor role in overall calcium regulation