Effects of hypophysectomy and replacement therapy with pituitary homogenates or ovine prolactin on serum calcium, sodium, and magnesium concentrations in bullfrog tadpoles

Regulation of epithelial calcium transport by prolactin: from fish to mammals

Among the reported ∼300 biological actions, the established role of prolactin (PRL) is to act as a vertebrate hypercalcemic hormone that regulates epithelial calcium transport in several organs, such as the gills, intestine, and kidney. In fish, PRL stimulates the branchial calcium transport by increasing the activity of Ca(2+)-ATPase. Although this calciotropic hormone also induces hypercalcemia in amphibians, reptiles and birds, little has been known regarding the underlying mechanism. In contrast, the effects of PRL on the epithelial calcium transport in mammals are well documented. In rodents, PRL has been shown to stimulate the renal tubular calcium reabsorption and intestinal calcium absorption, the latter of which is mediated by the PRL-induced upregulation of calcium transporter gene expression and activities. Recently, we demonstrated that the duodenal calcium absorption in lactating rats was markedly enhanced by the suckling-induced PRL surge, presumably to provide calcium for milk production. The cellular and molecular mechanisms of the PRL-stimulated calcium transport in mammals have been elaborated in this review.

Amphibian parathyroids: morphological and functional aspects

Amphibians living partially or totally in a terrestrial environment are the first tetrapods to possess parathyroid glands. Purely aquatic amphibians and amphibian larvae lack these endocrine glands. The parathyroids develop at the time of metamorphosis. The parathyroid glands in caecilians consist of a single cell type, that of urodeles may be composed of basal (supporting) cells and suprabasal (chief) cells, and that of anurans of small and large chief cells. Parathyroid glands of caecilians and anurans lack connective tissue, blood vessels, and nerves. The parathyroid cells become activated in response to decreased blood calcium concentration and undergo changes indicating increased parathyroid hormone secretion. Increased blood calcium concentration suppresses secretory activity. Usually, parathyroidectomy elicits hypocalcemia in most amphibians. Such operations have no effect in lower urodeles. Parathyroid hormone administration provokes hypercalcemia in most amphibians. The parathyroids of caecilians have not been studied in detail. The urodeles and anurans exhibit seasonal changes in the parathyroid glands. These changes may be initiated by environmental stimuli such as light, temperature, or alterations in blood calcium levels caused by natural hibernation.

The occurrence of chronic hypocalcemia following parathyroidectomy in the green frog, Rana clamitans

The plasma calcium concentrations of parathyroidectomized male specimens of the green frog, Rana clamitans, were monitored for 44 or 62 weeks. Plasma total and ionized calcium levels of the parathyroidectomized animals decreased 31-53% within 3 days, continued to fall to approximately 50% of the preoperative levels by Weeks 3-9, and remained at these low levels for the duration of the experiments. Plasma sodium, pH, and hematocrit levels were unchanged. The chronic hypocalcemia resulted in tetanic spasms in all of the long-term parathyroidectomized animals and in the death of most of the long-term parathyroidectomized animals. Thus, in the green frog the presence of the parathyroid gland is required for maintenance of blood calcium levels and for survival.

Calcium-binding protein (28,000 Mr calbindin-D28k) in kidneys of the bullfrog Rana catesbeiana during metamorphosis

A protein of approximately 28,000 relative molecular mass (Mr) cross-reacting with antiserum against the 28,000-Mr rat renal calcium-binding protein (calbindin-D28k) has been localized in the kidney of a salientian amphibian, Rana catesbeiana. Cells reactive for calbindin-D28k were found in the distal tubule at all stages of metamorphosis by the unlabeled antibody peroxidase-antiperoxidase technique. Adult kidneys appeared to have more calbindin-D28k-positive cells. The renal corpuscle, neck, and proximal tubule were negative. An immunoreactive 28,000-Mr band that comigrated with the band of calbindin-D28k was visualized by the immunoblot technique. The finding of the 28,000-Mr calbindin-D in the anamniotic kidney demonstrates that this calcium-binding protein (CaBP) is phylogenetically older than our previous studies of higher vertebrates had revealed (Rhoten et al., 1985). Although the function of calbindin-D28k in the distal nephron is unknown, this CaBP can now be presumed to have functional significance in the mesonephric as well as the metanephric kidney.

Effects of hypophysectomy and prolactin on calcium regulation in the red-spotted newt, Notophthalmus viridescens

Hypophysectomized male red-spotted newts exhibited hyponatremia but not hypocalcemia. Parathyroidectomy and a combination of hypophysectomy and parathyroidectomy caused plasma calcium levels to fall, but control levels were reestablished within 12 days. Injections of ovine prolactin had no effect on the plasma calcium or sodium levels of sham-operated and parathyroidectomized newts conditioned in tap water or in a 0.6% sodium chloride solution containing 10 meq calcium/liter. These data indicate that the pituitary gland is not involved in the hypercalcemic regulation of the red-spotted newt.