Pituitary and prolactin influences on calcium regulation in the mud puppy, Necturus maculosus

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.

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.

Effects of parathyroid hormone on cyclic-AMP concentrations of in vitro Necturus maculosus gastric antrum

The effects of bovine parathyroid hormone (bPTH1-84) on the stimulation of intracellular cyclic-AMP [cAMP] were investigated in an in vitro preparation of Necturus maculosus antral mucosa. When the antrum was exposed to 1, 5, 10, or 100 nM bPTH1-84, there was an approximately 2-fold nonlinear increase in tissue [cAMP] over basal values. The pretreatment of the antral mucosa with 1 mM isobutylmethylxanthine (IBMX, a phosphodiesterase inhibitor) increased with detectability of mucosal [cAMP]. The addition of 1, 5, 10, or 100 nM bPTH1-84 to tissues pretreated with IBMX resulted in an approximately 3.5-fold linear increase in mucosal [cAMP] over basal values. The time course of the generation of mucosal cAMP to 10 nM bPTH1-84 resulted in a small but significant transient increase at 2.5 min after the addition of bPTH1-84 but no change in the medium [cAMP]. In tissues pretreated with 1 mM IBMX the response to 10 nM bPTH1-84 was a large biphasic increase of [cAMP] at 2.5 min that progressively declined to near basal values by 15 min. There was also a significant sustained increase in the [cAMP] in the bathing medium at 2.5 min of tissues pretreated with IBMX followed by 10 nM bPTH1-84. These results suggest the presence of an adenylate cyclase that can be activated by a mammalian bPTH1-84 in elevating intracellular cAMP levels in the N. maculosus antral mucosa.

Prolactin and calcium metabolism: influence of hyperprolactinemia on immunoreactive parathyroid hormone levels in man and in the rat

Serum prolactin, parathyroid hormone, P and Ca serum levels were measured in 15 patients (12 women and 3 men) with hyperprolactinemia, and in 6 normal male volunteers who underwent a TRH test (100 micrograms by rapid iv injection) in order to obtain a short-term pharmacologically-induced hyperprolactinemia. A pituitary gland graft under the kidney capsule was carried out on 26 Sprague-Dawley male rats, which became hyperprolactinemic since the transplanted pituitary was stripped of the inhibitory hypothalamic control. Another group of 10 rats was injected with L-sulpiride (0.1 mg/kg). The serum PTH levels in patients and in subjects with induced hyperprolactinemia were within the normal range and there was no correlation between serum PRL and PTH levels. The same occurred both in transplanted and L-sulpiride injected rats. Our results suggest that prolactin does not modify PTH secretion in vivo and therefore, in contrast with previous data, it should not be considered a physiologically relevant secretagogue for parathyroid hormone.

Cardiac actions of bovine parathyroid hormone fragment (1-34) in some lower vertebrates

Only recently have the cardiac actions of parathyroid hormone and its N-terminal (1-34) fragment (bPTH(1-34] been examined. Parathyroid hormone was found to exert a positive chronotropic effect on the heart tissue of several mammals and one amphibian in vivo and/or in vitro. The purpose of the present study was to examine in vitro the heart tissue of several lower vertebrates, particularly aquatic vertebrates, for responsiveness to bPTH(1-34) and, for comparative purposes, isoproterenol. Heart tissue (atrium or the entire heart) from all the animals studied (trout, mudpuppy, bullfrog tadpole, and adult bullfrog) responded to isoproterenol in a dose-dependent fashion with increased heart rate and contractile force. Only the atria from the adult bullfrog, however, responded in a similar manner to the administration of bPTH(1-34). In all cases propranalol (10(-6) M) was able to block both chronotropic and inotropic effects of isoproterenol but had no effect on the cardiac stimulation induced by bPTH(1-34) in the adult bullfrog atrium. The data suggests that cardiac responsiveness to bPTH(1-34) is associated with a terrestrial as opposed to an aquatic existence.

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.

Effect of parathyroidectomy on plasma calcium levels in the seal salamander, Desmognathus monticola

The parathyroid glands of Desmognathus monticola are located in the subdermal connective tissue of the neck. Although they are smaller than previously described urodele parathyroid glands, their histological structure is similar. Parathyroidectomy caused a decrease in plasma calcium concentrations within 4 days following the operation. By Days 12 and 21, however, the plasma calcium levels of parathyroidectomized animals were not significantly different from those of sham-operated animals. Thus, the parathyroid gland of this species contains a hypercalcemic principle, but the recovery from hypocalcemia suggests that some hypercalcemic mechanism other than the parathyroid system must be involved in calcium homeostasis.