Distribution of immunoreactive calcitonin in the rat pituitary gland

Calcitonin inhibits testosterone and luteinizing hormone secretion through a mechanism involving an increase in cAMP production in rats

Effects of calcitonin peptides, including human calcitonin (hCT), salmon calcitonin (sCT), and calcitonin gene-related peptide (CGRP), on the secretion of testosterone and luteinizing hormone (LH) in male rats were studied. Male rats were injected intravenously with human chorionic gonadotropin (hCG), calcitonin peptides, or hCG plus calcitonin peptides. Blood samples were collected at several intervals following hormone challenge. In an in vitro experiment, testis blocks were incubated with hCG (0, 0.05, 0.5, or 5 IU/ml) or hCG (0.5 IU/ml) plus calcitonin peptides (0-10(-9) or 10(-6) M) at 34 degrees C for 30 minutes. Both medium and plasma samples were extracted by ether and analyzed for testosterone by radioimmunoassay (RIA). The concentration of calcium in each plasma sample was measured by an automatic calcium analyzer. The anterior pituitary gland (AP) was incubated with or without calcitonin peptides (0-10 nM) at 37 degrees C for 30 minutes. They were then incubated with gonadotropin releasing hormone (GnRH, 10 nM) for a further 30 minutes. The concentration of LH in AP medium was measured by RIA. The accumulation of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) in both testicular tissues and APs were measured by RIA. A single intravenous injection of calcitonin peptides decreased the basal and hCG-stimulated levels of plasma testosterone gradually from 60 to 180 or 360 minutes after challenge. The plasma calcium was not altered by the injection of calcitonin peptides and/or hCG. Administration of calcitonin peptides in vitro resulted in a dose-dependent inhibition of both basal and hCG-stimulated release of testosterone.(ABSTRACT TRUNCATED AT 250 WORDS)

Bioactive peptides in anterior pituitary cells

The anterior pituitary (AP) has been shown to contain a wide variety of bioactive peptides: brain-gut peptides, growth factors, hypothalamic releasing factors, posterior lobe peptides, opioids, and various other peptides. The localization of most of these peptides was first established by immunocytochemical methods and some of the peptides were localized in identified cell types. Although intracellular localization of a peptide may be the consequence of internalization from the plasma compartment, there is evidence for local synthesis of most of these peptides in the AP based on the identification of their messenger-RNA (mRNA). In several cases the release of the peptide from the AP cell has been shown and regulation of synthesis, storage and release have also been described. Because the amount of most of the AP peptides is very low (except for POMC peptides and galanin), endocrine functions are not expected. There is more evidence for paracrine, autocrine, or intracrine roles in growth, differentiation, and regeneration, or in the control of hormone release. To demonstrate such functions, in vitro AP experiments have been designed to avoid the interference of hypothalamic or peripheral hormones. The strategy is first to show a direct effect of the peptide after adding it to the in vitro system and, secondly, to explore if the endogenous AP peptide has a similar action by using blockers of peptide receptors or antisera immunoneutralizing the peptide.

The effects on anterior pituitary hormone secretion of salmon calcitonin in healthy volunteers

The reports of the effect of calcitonin on pituitary function are confusing and often refer to uncontrolled studies. We have now carried out a double-blind placebo-controlled trial of intravenous and subcutaneous salmon calcitonin on anterior pituitary function in 17 healthy volunteers. Visual analogue scores for the nausea and vomiting seen after salmon calcitonin correlated with the rise in ACTH and, secondarily, cortisol. Calcitonin had no effect on growth hormone, prolactin, thyrotrophin, luteinizing hormone or follicle stimulating hormone. It is concluded that the stimulation of ACTH secretion following a single dose of salmon calcitonin is probably the result of the stress of nausea rather than a direct effect on the pituitary.

Null cell adenomas of the pituitary gland. An immunohistochemical study

Fourteen null cell adenomas of the pituitary gland were examined immunohistochemically with antisera against three general neuroendocrine markers and 22 hormones. All cases showed positive immunostaining for neuron-specific enolase, ten cases for synaptophysin, and six cases expressed chromogranin immunoreactivity. Hormone immunoreactivity was detected in a few cells in ten of the 14 cases studied and the number of hormones demonstrated in each case was one or two. Thyroid-stimulating hormone was detected in five of the 14 cases, gastrin in four, beta-endorphin in two, calcitonin gene related peptide in one, prolactin in one, and follicle-stimulating hormone in one.

Paracrine interactions in the anterior pituitary

The topographical affinity between certain cell types in rat anterior pituitary as well as the presence of biogenic amines, neuropeptides, growth and tissue factors in specific cell types suggest participation of paracrine control mechanisms in the regulation of anterior pituitary hormone secretion. Due to the recent advances in the separation of pituitary cell types and the development of three-dimensional cell cultures, direct experimental evidence for control by intercellular messengers has become available. The stimulation of PRL release from superfused pituitary cell aggregates by LHRH has been shown to be mediated by gonadotrophs. Gonadotrophs appear to secrete a factor with PRL-releasing activity. Gonadotrophs also modulate the stimulation of PRL release by angiotensin II. Interaction of somatotrophs with an unknown small-sized cell type strongly amplifies the GH response to adrenaline, GRF and VIP. The latter phenomenon requires the permissive action of glucocorticoids. Some of these in vitro observations can be correlated with recently reported in vivo actions of LHRH, PRL and angiotensin II and with pathophysiological changes in the pituitary.

Immunoreactive calcitonin in brain regions and pituitary of sheep

In this study we have investigated the presence of immunoreactive calcitonin in the central nervous system and pituitary of sheep. The calcitonin concentrations were determined radioimmunologically by two different antibodies. We have demonstrated calcitonin in extracts of areas of the central nervous system, whole pituitary, thyroid gland and plasma of 21 sheep. The concentrations were (ng/g wet weight, mean values +/- SE): thyroid 16.0 +/- 4.4, pituitary 2.03 +/- 0.34, reticular formation 1.64 +/- 0.25, substantia nigra 1.53 +/- 0.46, dentate nucleus 1.11 +/- 0.27, putamen 1.05 +/- 0.35, hippocampus 0.97 +/- 0.17, fornix 0.96 +/- 0.15, anterior thalamus 0.92 +/- 0.28, mammillary body 0.88 +/- 0.12, cerebellum 0.86 +/- 0.09, caudate nucleus 0.84 +/- 0.11, posterior hypothalamus 0.83 +/- 0.19, epiphysis 0.75 +/- 0.25, thalamus centralis 0.71 +/- 0.10, almond nucleus 0.69 +/- 0.16, medulla oblongata 0.67 +/- 0.15, anterior hypothalamus 0.66 +/- 0.20, precentral gyrus 0.66 +/- 0.16, globus pallidus 0.63 +/- 0.31, postcentral gyrus 0.36 +/- 0.08 and plasma (ng/ml) 0.058 +/- 0.013. Our results demonstrate that immunoreactive calcitonin is present in the central nervous system (CNS) of sheep, compatible with a neurotransmitter function for this hormone.

High serum calcitonin levels in heroin addicts

An involvement of calcitonin in the mechanism of pain perception has recently been hypothesized. In order to collect information about the relationship between this hormone and well known analgesic substances such as opioids, we have studied the serum levels of calcitonin in a group of heroin addicts, finding higher average concentrations than in normal subjects of matched age and sex. In these addicts there were no severe signs of impaired renal or hepatic function, or alterations of the serum levels of calcium and phosphate. So we think that opioids, in a direct or indirect way, can stimulate the secretion of calcitonin.

Pituitary immunoreactive calcitonin-like material: lack of evidence for cross-reactivity with pro-opiomelanocortin

In mammals, calcitonin (CT) is synthesized, stored, and secreted by intrathyroidal C cells. Several reports have suggested the presence of immunoreactive CT in the pituitary gland. We have studied the rat pituitary gland using a radioimmunoassay for CT and have also found immunoreactive CT-like material. Assay of extracts of whole rat pituitary glands was performed using a radioimmunoassay for human CT, which gave identical dilution curves with synthetic human CT (hCT), synthetic rat CT (rCT), and mouse and rat thyroid extracts, but not with a variety of other pituitary and hypothalamic peptides. Immunoreactive CT (iCT) content of extracts of whole pituitary glands ranged from 6 to 72 pg/mg wet weight of tissue (60-840 pg/whole pituitary gland), whereas iCT was not measureable (less than 5 pg/mg tissue) in similar extracts of hypothalamus and cerebral cortex. Gel filtration studies of pituitary extracts showed a peak of iCT, which eluted with 125I-rCT and diluted in parallel with rCT. To investigate whether the pituitary iCT was related to pro-opiomelanocortin, extracts of ACTH-producing AtT20/D16 cells from mice, which contain the ACTH precursor in large quantities, were examined and no iCT was found. Immunohistochemical studies of rat pituitary glands with peroxidase-antiperoxidase and immunofluorescent techniques showed positive staining for CT in cells in the pars anterior, but not in the pars intermedia of pars nervosa; this staining was not eliminated when the antiserum was absorbed with CT under conditions that completely obliterated staining of rat thyroid glands. Double staining demonstrated essentially two distinct populations of cells, one positive for CT and another positive for ACTH, with less than 1% of the cells positive for both ACTH and CT. Immunoreactive CT-like material was present in the pituitary glands of rats thyroparathyroidectomized 18 days before they were killed, but was diminished. Biosynthetic labeling in vitro of rat pituitary glands with 3H-leucine showed incorporation into prolactin; there was no incorporation into CT. No in vitro secretion of iCT by whole rat pituitary glands either basally or after high K+ stimulation was observed. We conclude that: (1) a substance that has certain immunologic and size characteristics of CT is present in minute amounts in the pituitary gland of rats; (2) this material is not a part of the ACTH precursor; and (3) positive immunohistochemical staining in pituitary glands may not be specific for authentic CT.

Extraction of a substance with calcitonin-like immunoreactivity from pituitary glands of intact and thyroidectomized rats

We have previously demonstrated the presence of calcitonin (CT)-like immunoreactivity in the pituitary glands of a number of vertebrate species. Subsequent work by ourselves and others has confirmed this finding by immunohistology and radioimmunoassay (RIA) of pituitary extracts. However, no data have been reported on the site of biosynthesis of the CT-like pituitary peptide, or the relationship of pituitary levels of the substance to experimental alterations of systemic CT levels. In the present study, we measured the content of CT-like peptide in rat pituitaries by using gel filtration chromatography and an RIA, which recognizes rat and human calcitonin. The CT-like peptide has an elution position on gel filtration similar to that of rat and human calcitonin. Its RIA binding appears indistinguishable from that of rat thyroidal CT. Various conditions for extraction were evaluated, and homogenization in acetic acid at 100 degrees C gave the best results. The CT-like content of rat pituitaries was generally 75-125 pg/gland. Presence of CT-like peptide in rat pituitaries was not abolished by prior thyroidectomy. We conclude that the CT-like pituitary peptide is not synthesized in the thyroid, but is similar to thyroidal CT in the characteristics studied.

Presence of immunoreactive calcitonin in the hypothalamus and pituitary lobes of rats

We have found calcitonin-like immunoreactive material in extracts of hypothalami from six-month old, male rats. The level of this immunoreactivity, 0.21 ng/hypothalamus, is substantial considering out lower limit of detection of 0.006 ng of rat calcitonin. However, the hypothalamus contains less calcitonin-like immunoreactive material than either the anterior lobe (1.16 ng) or neurointermediate lobe (0.81 ng) of the pituitary. Taken together these three sources of calcitonin-like immunoreactive material contain less than one thousandth the immunoreactivity found in the thyroid. The exact nature of the calcitonin-like immunoreactive material found in these extrathyroidal sites and its physiological role, if any, remain to be discovered. The recent report of the occurrence of calcitonin receptors in the hypothalamus and other brain regions in conjunction with our finding of calcitonin-like immunoreactivity in the hypothalamus suggests that calcitonin-like molecules may be active within the brain.

Differential immunostaining of adenohypophysial cells with antisera to ACTH and beta-endorphin

Three antisera, raised in rabbits against human beta-endorphin were denoted Y-10, Y-18 and R-230 and tested for their ability to immunohistochemically stain cells in teh adenohypophysis of the rat and dog. By radioimmunoassay Y-10 was highly specific towards beta-endorphin with minimal cross-reactivity against beta-LPH while Y-18 and R-230 cross-reacted with both beta-endorphin and beta-LPH on an equimolar basis. In the rat pituitary, Y-18 and R-230 antisera stained cells identified as corticotrophs. With the dog, however, beta-endorphin-staining cells in the anterior pituitary and infundibulum did not necessarily co-stain for ACTH-like material. When the beta-endorphin antiserum, Y-10, was applied to rat anterior pituitary tissues, the subsequent positive immunocytochemical staining was associated not only with corticotrophs but also with somatotrophs. The findings are consistent with (a) a differential processing of the 31K pro-opiocortin and (b) the presence in rat somatotrophs of determinants that cross-react immunologically with some beta-endorphin antisera.