Calcium ion uptake,somatotropin release,and fine structure of somatotrophs in cultures of the rat anterior pituitary upon the action of an oligopeptide (Boc-Gln-Leu-Lysinal)

Summary Cultured cells from the anterior pituitary glands of adult rats were treated with the tripeptide aldehyde proteinase inhibitor, Boc-Gln-Leu-Lys-H. The oligopeptide had a profound relesing effect on growth hormone, whereas the prolactin release remained unchanged at 10^–3 mol/l drug concentration after an incubation for 2 h. In the presence of the oligopeptide a time- and dose-dependent calcium influx into cultured cells has been shown which was proved to be almost completely antagonized with magnesium ions but not with Nifedipine. In addition, radioactive calcium ions could be detected in a number of cells by light microscopic autoradiography when cultures were treated with Boc-Gln-Leu-Lys-H for short periods. The selective Gel action of the oligopeptide on growth hormone producing cells has been demonstrated also in fine structural investigations: multigranular and single exocytotic profiles have been observed. Accordingly, we have postulated that Boc-Gln-Leu-Lysinal mimics the effects of the known ionophores. Its mode of action needs, however, further studies especially on isolated somatotrophs.     

The calcium-mediated promotion of mitotic activity in rat thymocyte populations by growth hormone, neurohormones, parathyroid hormone and prolactin

Growth hormone, oxytocin, parathyroid hormone, prolactin and lysine vasopressin strongly stimulate mitotic activity in rat thymocyte populations maintained in vitro. These hormones have no mitotic effect on cells maintained in calcium-free medium. It is concluded that they stimulate mitosis only indirectly by sensitising the mitotically competent segment of a thymocyte population to the action of calcium. The stimulatory action of calcium itself is opposed by low concentrations of the mucopolysaccharide chondroitin sulphate. However, the inhibitory action of chondroitin sulphate can be overcome by growth hormone. A possible common mechanism of action of these hormones on mitotically competent cells is discussed.     

Effects of testosterone on hormonal content and calcium-dependent basal secretion in female rat pituitary cells

In vivo and in vitro effects of elevated androgens on agonist-induced gonadotropin secretion have been addressed previously. Here we investigated the effects of testosterone on hormonal content and basal (in the absence of agonists) hormone release in pituitary lactotrophs, somatotrophs and gonadotrophs from female rats. Furthermore we tested the hypothesis that testosterone action is dependent on the pattern of spontaneous and Bay K 8644 (a L-type calcium channel agonist) -induced calcium signalling. Mixed anterior pituitary cells were cultured in steroid containing or depleted media, and testosterone (1pM to 10nM) was added for 48h. Cells were studied for their spontaneous and Bay K 8644-induced calcium signalling pattern and total hormone levels (release and hormonal content). In lactotrophs, somatotrophs and gonadotrophs testosterone did not affect the pattern of spontaneous calcium signalling. Bay K 8644-induced calcium signalling and hormone release were not affected by testosterone. In both steroid-depleted and -containing medium, testosterone inhibited prolactin (PRL), luteinizing hormone (LH) and growth hormone (GH) cellular content and release in a dose-dependent manner, with IC(50)s in a sub-nanomolar concentration range. These results indicate that testosterone inhibits basal hormone release from lactotrophs, somatotrophs and gonadotrophs without affecting intracellular calcium signalling. This action of testosterone is not dependent on the presence of other steroid hormones.     

Direct effect of a gonadotropin-releasing hormone agonist on the growth of canine mammary tumour cells

Gonadotropin-releasing hormone (GnRH) agonist exert "in vivo" an inhibitory action on the growth of hormone-dependent canine mammary tumours (Lombardi et al. [1999] J. Vet. Pharmacol Ther. 22(1):56-61). The present experiments have been performed "in vitro" in order to investigate the mechanisms involved in this direct antiproliferative action of GnRH agonists. In particular, the aim was to study whether these compounds might exert their antiproliferative effect by interfering with the stimulatory action of epidermal growth factor (EGF). To this purpose, the effects of GnRH agonist, Goserelin (GnRH-A), on the mitogenic action of EGF, on EGF-activated intracellular signaling mechanisms (intracellular calcium and nitric oxide production) as well as on ATP induced cell proliferation and signalling, and on the binding of EGF receptors have been evaluated in primary culture of canine mammary tumour cells. The results of these "in vitro" studies show that GnRH-A counteracts the mitogenic action of EGF and ATP, decreases the EGF/ATP-induced calcium signalling and reduces EGF binding, probably by means of NO-induced [Ca2+]i downregulation. These data suggest that GnRH agonists may inhibit the proliferation of the tumour cells by interfering with the stimulatory action of EGF.     

Growth hormone-induced alteration of morphology and tubulin expression in 3T3 preadipose cells

Effects of growth hormone on morphology and cytoskeletal protein expression were examined in 3T3-F442A preadipocytes in serum-free medium. Between 2 and 5 days of culture 2 nM methionyl human growth hormone converted 3T3-F442A cells from a flat fibroblastic morphology to a rounded form with numerous membrane convolutions. Growth hormone treated cultures manifested a 30-40% reduction in cell volume. Growth hormone induced changes in morphology and volume preceded and were independent of lipogenesis. In cells treated with growth hormone, expression of alpha and beta-tubulin as determined by Western blotting was found to increase approximately 50% within 72 h as compared to untreated cells. After 7 days, tubulin levels in growth hormone treated cells were approximately 40% of control levels. This indicated that morphological changes and alteration of tubulin expression were signatures of growth hormone action on 3T3-F442A cells.     

Transport of hepcidin, an iron-regulatory peptide hormone, into retinal pigment epithelial cells via oligopeptide transporters and its relevance to iron homeostasis

Retinal pigment epithelial cells (RPE) express two transport systems (SOPT1 and SOPT2) for oligopeptides. Hepcidin is an iron-regulatory peptide hormone consisting of 25 amino acids. This hormone binds to ferroportin, an iron exporter expressed on the cell surface, and facilitates its degradation. Here we investigated if hepcidin is a substrate for SOPT1 and SOPT2 and if the hormone has any intracellular function in RPE. Hepcidin inhibited competitively the uptake of deltorphin II (a synthetic oligopeptide substrate for SOPT1) and DADLE (a synthetic oligopeptide substrate for SOPT2) with IC₅₀ values in the range of 0.4–1.7 μM. FITC-hepcidin was taken up into RPE, and this uptake was inhibited by deltorphin II and DADLE. The entry of FITC-hepcidin into cells was confirmed by flow cytometry. Incubation of RPE with hepcidin decreased the levels of ferroportin mRNA. This effect was not a consequence of hepcidin-induced ferroportin degradation because excessive iron accumulation in RPE, which is expected to occur in these cells as a result of ferroportin degradation, did not decrease but instead increased the levels of ferroportin mRNA. This study reveals for the first time a novel intracellular function for hepcidin other than its established cell surface action on ferroportin.     

Towards the understanding of tetrahydroquinolines action in Aedes aegypti: larvicide or adulticide?

Aedes aegypti is an important vector of arboviruses such as dengue, yellow fever, chikungunya and Zika. Among the various types of insecticides used to combat this vector, the insect growth regulators have been developed and recommended for control of their larvae. In this work compounds with proven regulatory action, tetrahydroquinolines will be studied. These regulators act on the hormones responsible for the insect development. Ecdysone, one of the main hormones involved in this process has a specific receptor (EcR), where tetrahydroquinolines derivatives can bind, disrupting the normal action of this hormone, because they have structure similar to hormone 20-hydroxyecdysone (20E). In addition, studies show that this class of compounds interacts strongly in the potassium channel activated by calcium (BK channel). Thus, the goal is to study the action of compounds (tetrahydroquinolines) as insecticides and evaluate their larvicidal action (action on the ecdysone receptor) or adulticide (action on the BK channel) through homology modelling techniques, molecular docking and molecular dynamics simulations and aiming to propose a compound that presents both actions (larvicide / adulticide).     

Voltage gated calcium channels in molluscs: classification,Ca2+ dependent inactivation,modulation and functional roles

Molluscan neurons and muscle cells express transient (T-type like) and sustained LVA calcium channels, as well as transient and sustained HVA channels. In addition weakly voltage sensitive calcium channels are observed. In a number of cases toxin or dihydropyridine sensitivity justifies classification of the HVA currents in L, N or P-type categories. In many cases, however, pharmacological characterization is still preliminary. Characterization of novel toxins from molluscivorousConus snails may facilitate classification of molluscan calcium channels. Molluscan preparations have been very useful to study calcium dependent inactivation of calcium channels. Proposed mechanisms explain calcium dependent inactivation through direct interaction of Ca²⁺ with the channel, through dephosphorylation by calcium dependent phosphatases or through calcium dependent disruption of connections with the cytoskeleton. Transmitter modulation operating through various second messenger mediated pathways is well documented. In general, phosphorylation through PKA, cGMP dependent PK or PKC facilitates the calcium channels, while putative direct G-protein action inhibits the channels. Ca²⁺ and cGMP may inhibit the channels through activation of phosphodiesterases or phosphatases. Detailed evidence has been provided on the role of sustained LVA channels in pacemaking and the generation of firing patterns, and on the role of HVA channels in the dynamic changes in action potentials during spiking, the regulation of the release of transmitters and hormones, and the regulation of growth cone behavior and neurite outgrowth. The accessibility of molluscan preparations (e.g. the squid giant synapse for excitation release studies,Helisoma B5 neuron for neurite and synapse formation) and the large body of knowledge on electrophysiological properties and functional connections of identified molluscan neurons (e.g. sensory neurons, R15, egg laying hormone producing cells, etc.) creates valuable opportunities to increase the insight into the functional roles of calcium channels.     

甲状旁腺素对成骨样细胞增殖的调节作用

甲状旁腺素（ＰＴＨ）是调节钙磷代谢的经典激素,有报道ＰＴＨ对其靶细胞－成骨细胞有促增殖分化作用。经多层次、多水平的实验研究证实,ＰＴＨ对成骨样细胞ＲＯＳ１７／２．８确有促增殖作用。（１）细胞计数、ＭＴＴ［３－（４,５－ｄｉｍｅｔｈｙｌｔｈｉａ－ｚｏｌ－ｚ－ｙｉ）２,５－ｄｉｐｈｅｎｙｌｔｅｔｒａｚｏｌｉｕｍｂｒｏｍｉｄｅ］测定及ＳＲＢ（ｓｏｄｉｕｍｒｈｏｄａｍｉｎｅＢ,ＳＲＢ）染色均显示经ＰＴＨ（１０－９ｍｏｌ／Ｌ）处理的细胞,其数目明显增加;（２）３Ｈ－ＴｄＲ参入增加;（３）与增殖相关的原癌基因（ｃ－ｆｏｓ、ｃ－ｊｕｎ、ｃ－ｋｉ－ｒａｓ和ｃ－ｍｙｃ）的表达增强;（４）成骨细胞特征性蛋白－碱性磷酸酶活性降低．这些结果不仅表明该激素具有非经典样作用,同时意味着激素也参与其靶细胞增殖分化的调节作用     

Evidence for thyroid hormone-dependent and independent glucocorticoid actions in cultured cells. Studies on the induction of growth hormone and glutamine synthetase in GH1 cells and tyrosine aminotransferase in Reuber H-35 cells.

The induction of growth hormone synthesis and mRNA by thyroid hormone in cultured GH1 cells is mediated by the thyroid hormone nuclear receptor. In addition, the regulation of the growth hormone response by glucocorticoid is highly dependent on the action of thyroid hormone. To clarify whether thyroid hormone has a general influence on glucocorticoid action in GH1 cells, the glucocorticoid induction of growth hormone and glutamine synthetase was simultaneously examined. In contrast to the growth hormone response, the induction of glutamine synthetase by glucocorticoid was not influenced by thyroid hormone. Both responses appear to be modulated by the glucocorticoid receptor, and thyroid hormone had no influence on nuclear-associated glucocorticoid receptor levels. These results suggest that the thyroid hormone control of glucocorticoid induction of growth hormone may be a selective process, and the nuclear associated receptors for both thyroid and glucocorticoid hormones interrelate to control the growth hormone response.     

Insulin-like growth factor II increases cytoplasmic free calcium in competent Balb/c 3T3 cells treated with epidermal growth factor

To determine the role of calcium in the action of insulin-like growth factor II (IGF-II), we have examined the effect of multiplication stimulating activity, the rat IGF-II, on cytoplasmic-free calcium concentration, [Ca2+]c, in aequorin-loaded Balb/c 3T3 cells. IGF-II does not cause any change in [Ca2+]c in quiescent cells. By contrast, IGF-II induces changes in [Ca2+]c in platelet-derived growth factor(PDGF) - pretreated competent cells: when competent cells are incubated with epidermal growth factor (EGF) for 10 min, subsequent IGF-II induces an immediate increase in [Ca2+]c. Without EGF treatment, IGF-II does not cause any increase in [Ca2+]c. The priming action of EGF is time dependent, requiring approximately 10 min for the maximum effect. The IGF-II-mediated increase in [Ca2+]c is totally dependent on extracellular calcium and is blocked by lanthanum. When DNA synthesis in PDGF-treated competent cells is assessed by measuring [3H]thymidine incorporation, IGF-II by itself has only a small effect. Likewise, a brief treatment with EGF results in only a small increase in [3H]thymidine incorporation. By contrast, in competent cells briefly treated with EGF, IGF-II causes a marked stimulation of [3H]thymidine incorporation. These results indicate that IGF-II increases [Ca2+]c in competent Balb/c 3T3 cells treated with EGF by stimulating calcium influx and that IGF-II-stimulated calcium influx may be related causally to its action on cell proliferation.     

Alteration in surface substructure of frog urinary bladder by calcium ionophore, verapamil and antidiuretic hormone

The calcium antagonist verapamil and the calcium ionophore A23187 have been shown to inhibit the hydro-osmotic actions of antidiuretic hormone (ADH) presumably by different mechanisms. Presently, urinary bladders of the frog (Rana pipiens) were examined under SEM following exposure to calcium ionophore A23187, verapamil and ADH in the presence and absence of an osmotic gradient. Cells exposed to ADH show marked changes in surface substructure which is accompanied by an expansion of microridges, cell borders and the appearance of microvilli in the granular cells. The microvilli are pronounced and appear at the junction of microridges. In the presence of an osmotic gradient, ADH induces granular cell swelling and some cells show a blistering effect. Calcium ionophore, in the absence of an osmotic gradient, induced pronounced morphological changes in the granular cells, where the microvilli become prominently visible as 'finger-like' projections. This effect may be due to the action of calcium in promoting elongation of microtubules. Cells exposed to ionophore plus ADH are indistinguishable from ionophore alone. The most apparent effect of verapamil on surface substructure was on the elevation of the mitochondrial-rich cells above the surrounding granular cells. These cells show some degree of separation from the granular cells and are accentuated in tissues exposed to verapamil plus ADH. The present observations suggest that these agents, verapamil and calcium ionophore, have marked effects on cellular morphology. These actions are mediated through alterations in calcium movements and reflect the relative importance of cellular calcium in transepithelial water flow and the actions of antidiuretic hormone.     

Androgen action on hepatic vitellogenin synthesis in the eel, Anguilla japonica is suppressed by an androgen receptor antagonist

Involvement of additional hormones other than estrogen in the control of vitellogenin (Vg) synthesis has been suggested in fish. However, no satisfactory explanation on the mechanism of the action of these hormones has been reported. In this study, we have exploited the possibility of androgen receptor mediation during the androgen action on the pathway of Vg synthesis. Hepatocytes were prepared from sexually immature Japanese eel Anguilla japonica and treated with estradiol-17beta (E2), 17alpha-methyltestosterone (MT), growth hormone, tamoxifen or flutamide, or in combination of these. Spent culture media were analysed by SDS-PAGE for Vg detection. Results from the chemical treatments demonstrated the necessity of E2 as the primary factor for Vg synthesis and requirement of additional hormones for the full expression of Vg. The effects of E2 and MT were effectively blocked by tamoxifen, an estrogen receptor antagonist and flutamide, an androgen receptor antagonist, respectively, indicating ER-mediated estrogen action and AR-mediated androgen action on Vg synthesis in this species.     

Calcium influx: an intracellular message of the mitogenic action of insulin-like growth factor-I

When G0-arrested BALB/c 3T3 cells were treated sequentially with platelet-derived growth factor and epidermal growth factor, cells became responsive to insulin-like growth factor-I (IGF-I). In these primed competent cells, 1 nM IGF-I elicited an approximately 3-fold increase in the calcium influx rate. IGF-I-induced calcium influx was relatively slow in onset and continued for at least 2 h in the presence of IGF-I. When a single Ca2+ channel current was studied by the patch-clamp technique using the cell-attached mode, inward currents with unitary conductance of 19 pS were observed in the presence of 1 nM IGF-I in the patch pipette. IGF-I-sensitive inward current was independent of membrane potential and was activated by a high concentration of insulin. Accordingly, 1 nM IGF-I caused a gradual increase in cytoplasmic free calcium concentration measured by fura2. The action of IGF-I on calcium influx was dependent on extracellular calcium, and IGF-I did not stimulate calcium influx when extracellular calcium concentration was reduced to 10 microM. Both cobalt and tetramethrin blocked the action of IGF-I on calcium influx without affecting the binding of 125I-IGF-I. In primed competent cells, IGF-I-stimulated [3H]thymidine incorporation was dependent on extracellular calcium and was attenuated by cobalt and tetramethrin. When cell-bound 125I-IGF-I was cross-linked by use of disuccinimidyl suberate, a 130-kDa protein was radiolabeled. Affinity labeling of the 130-kDa protein, presumably the alpha-subunit of the IGF-I receptor, was blocked by excess amount of unlabeled IGF-I. These results suggest that relatively low concentrations of IGF-I stimulate calcium influx in primed competent BALB/c 3T3 cells by activating a calcium-permeable cation channel via the IGF-I receptor and that calcium influx may be a critical intracellular message of the progression activity of IGF-I.     

Prolactin and growth hormone in the regulation of the immune system

Evidence implicating prolactin (PRL) and growth hormone (GH) in the regulation of the immune system has been reviewed. Hypophysectomized animals have deficiencies in both cell-mediated and humoral immunological functions and either PRL or GH corrects these deficiencies. Animals administered bromocryptine, a drug that specifically blocks PRL release, have impaired immune responses similar to hypophysectomized animals, and again both PRL and GH correct these deficiencies. Genetically dwarf animals, which lack both PRL and GH, are also immunocompromised, and once again PRL and GH can correct the deficiencies. In dwarf animals, however, fewer studies have examined PRL actions. In growth-deficient children, immune function is not dramatically altered and basal secretion of GH has been reported. Very few clinical studies have examined whether PRL secretion is also deficient, and this may explain why a clear loss in immune function is not evident in growth-deficient children. In a number of species, including man, both PRL and GH stimulate thymic function and increase the secretion of thymulin, a thymic hormone. No studies, however, have reported on the effects of PRL and GH on other thymic hormones. A number of studies have reported in vitro effects of PRL and GH on cells involved with immunity, and the presence of high-affinity PRL and GH receptors have been observed on a number of these cells. The action of GH on the proliferative response of cells involved with immunity in vitro appears to be mediated by the production of insulin-like growth factor I. The effect of PRL on insulin-like growth factor I production by these cells has not been examined. One of the most consistent findings from in vitro studies is that prolactin antisera blocked a number of immune reactions. This led to the discovery that cells involved with immunity appear capable of producing PRL and GH, but the physiological significance of these observations have not been explored. There is a great need to identify the cell types responding to PRL and GH and this should be a goal of future investigations. There is also a need for investigators to be aware that both PRL and GH are involved in the regulation of the immune system and to design experiments to elucidate where each functions in the maturation cascade of cells involved with immunity. From the evidence available, it is apparent that PRL and GH have an important function in the immune system and future investigations should be directed toward elucidating their site(s) of action.     

Bradykinin-induced oscillations of cell membrane potential in cells expressing the Ha-ras oncogene

Products of ras genes are putative elements of growth factor signal transduction. However, the mechanism of action of these proteins in normal and malignant growth is as yet obscure. To test for functional consequences of ras oncogene expression, electrophysiological experiments were performed on NIH-3T3 fibroblasts transfected with a transforming Ha-ras MMTV-LTR construct expressing the oncogene on treatment with dexamethasone (+ras). Transfected cells in the absence of dexamethasone (-ras) and nontransfected cells in the presence of dexamethasone (oras) served as controls. In -ras and oras, bradykinin induces a single, transient hyperpolarization. In +ras, bradykinin elicits oscillations of cell membrane potential throughout the presence of the hormone by activation of calcium-sensitive K+ channels. The oscillations of cell membrane potential are abolished in the absence of extracellular calcium. As evident from fura 2 fluorescence, bradykinin leads to a transient increase of intracellular calcium both in the presence and absence of extracellular calcium. Oscillations of intracellular calcium could be observed in +ras cells, if bradykinin was applied at reduced extracellular sodium concentration possibly to impair calcium extrusion via the sodium/calcium exchange. Bradykinin induces oscillations of cell membrane potential similarly in -ras cells loaded with GTP[S], a nonhydrolyzable analogue of GTP. Thus, the altered response of ras oncogene expressing cells to bradykinin relates to the GTP binding property of the ras protein. It is concluded that in cells expressing ras oncogene but not in other fibroblasts bradykinin mimicks the effect of growth factors on the cell membrane.     

Priming of calcium mobilization in human neutrophils by granulocyte-macrophage colony-stimulating factor: Evidence for an involvement of phospholipase D-derived phosphatidic acid

Human neutrophils pre-incubated with granulocyte-macrophage-colony-stimulating factor (GM-CSF) exhibit an enhanced mobilization of calcium in response to secondary stimuli such as chemotactic factors. The mechanisms underlying this priming effect of GM-CSF were examined. It was first demonstrated that the additional calcium mobilized by chemotactic factors in GM-CSF-treated cells was derived from intracellular stores and was associated neither with an increased permeability to calcium nor with production of inositol 1,4,5-trisphosphate. These results indicated that GM-CSF called upon a novel mechanism in order to enhance the mobilization of calcium in human neutrophils. The growth factor has recently been shown to prime phospholipase D leading to an enhanced activation by chemotactic factors and an augmented production of phosphatidic acid. Furthermore the ability of exogenous phosphatidic acid to mobilize calcium in cell types other than neutrophils has been previously demonstrated. Therefore, we examined the potential involvement of phospholipase D in the priming of the calcium response by GM-CSF in human neutrophils. Inhibition of the production of the fMet-Leu-Phe-stimulated production of phosphatidic acid by ethanol or wortmannin had only marginal effects on the concurrent mobilization of calcium. However, the priming of the mobilization of calcium by GM-CSF was greatly decreased in cells treated with either ethanol or wortmannin. These results provide strong support for the hypothesis that the production of phosphatidic acid, which is enhanced in GM-CSF-treated cells, is linked to an increased mobilization of intracellular calcium. These results may have relevance to the mechanism of action of GM-CSF in mature haematopoeitic cells as well to the mitogenic activity of other growth factors.     

Pituitary cell type-specific electrical activity, calcium signaling and secretion

All secretory anterior pituitary cells exhibit spontaneous and extracellular calcium-dependent electrical activity, but differ with respect to the patterns of firing and associated calcium signaling and hormone secretion. Thus, somatotrophs and lactotrophs fire plateau-bursting action potentials spontaneously and without coupling to calcium release from intracellular stores, which generate calcium signals of sufficient amplitude to keep steady hormone release. In these cells, both spontaneous electrical activity and basal hormone secretion can be further amplified by activation of Gq/11 and Gs-coupled receptors and inhibited by Gi/o/z-coupled receptors. In contrast, gonadotrophs fire single, high-amplitude spikes with limited ability to promote calcium influx and exocytosis, whereas activated Gq/11-coupled receptors in these cells transform single-action potential spiking into the plateau-bursting type of electrical activity and trigger periodic high-amplitude calcium signals and exocytosis of prestored secretory vesicles. Here, we review biochemical and biophysical aspects of spontaneous and receptor-controlled electrical activity, calcium signaling, and hormone secretion in pituitary cells.     

Aspects of Parathyroid Physiology in Mammals

This report summarizes the parameters of parathyroid physiologyin the mammal. Emphasis is given to the part played by thishormone in the processes of growth and remodelling in bone aswell as to its function in the maintenance of a relatively constantlevel of ionic calcium in the extracellular fluid. The importanceof phosphate ions has been shown, particularly with respectto their role in the rate of Ca-transport through the extracellularfluid compartments, and their influence on the rate of parathyroidsecretion. In addition, the inter-relationship of the newlydiscovered hormone, thyrocalcitonin, is considered. The latterhormone, by suppressing resorption of bone, may have an influencein stabilizing plasma levels of calcium; it also has a moderate,but long range effect on the rate of bone remodelling. Finally,an attempt has been made to relate the vast amount of work donewith these two hormones in mammals to their phylogenetic developmentin vertebrates. It is suggested that the parathyroids developedat the time when the specialization of bone in vertebrates produceda solid structure which was incompatible with the fluid calciumconcentration needed for the maintenance of many physiologicalfunctions. The parathyroid hormone, by its action to increasethe transfer of calcium from bone against a concentration gradient,permitted these vertebrates to maintain the higher fluid calciumion concentration needed for the normal functioning of the animal.