Biomedical subjects
A Rijnberk
Publications and source records attributed to A Rijnberk.
Pulsatile secretion of alpha-MSH and the differential effects of dexamethasone and haloperidol on the secretion of alpha-MSH and ACTH in dogs.
This study was performed to determine whether, in the dog, there is at any time pulsatile release of alpha-MSH and whether secretion of ACTH from the pars intermedia (PI) contributes to the circulating concentrations of ACTH. The 24-h secretory profiles of alpha-MSH, ACTH, and cortisol were determined in eight dogs. Plasma samples were obtained at 10-min intervals via an indwelling jugular catheter during two 12-h periods. Pulsatile secretion of alpha-MSH was found in all dogs, with wide variations in peak height. Plasma alpha-MSH levels were usually low (mean 15 pmol/l), but brief, distinct periods of increased plasma alpha-MSH concentrations as high as 489 pmol/l were found. Analysis of pulse frequency revealed a mean of 4.75 significant alpha-MSH peaks/24 h. The highest alpha-MSH peaks were associated with definite changes in the plasma concentrations of ACTH. In separate studies, the influence of dexamethasone on the 6-h secretory profiles and on the haloperidol-stimulated secretion of alpha-MSH, ACTH, and cortisol was investigated. In these two studies, plasma ACTH was measured by a highly sensitive immunoradiometric assay. Dexamethasone pretreatment significantly suppressed the plasma concentrations of ACTH, cortisol, and alpha-MSH to 10.3%, 3.9%, and 74.6% respectively. Dexamethasone pretreatment also significantly reduced the haloperidol-stimulated secretion of ACTH and cortisol, but had no influence on the haloperidol-stimulated secretion of alpha-MSH. After the administration of haloperidol to the dexamethasone-pretreated dogs, there were small increases in the plasma concentrations of ACTH and cortisol, the latter being significant. These data demonstrate that alpha-MSH is secreted spontaneously in a pulsatile manner in the dog and suggest that the canine PI contributes to circulating ACTH concentrations.
Correlation between impairment of glucocorticoid feedback and the size of the pituitary gland in dogs with pituitary-dependent hyperadrenocorticism.
Under the assumption that the impaired inhibitory effect of glucocorticoids on cell division is an important determinant in the progression of corticotrophic adenomas, it is postulated that the magnitude of proliferation and the resistance to glucocorticoids are correlated. To test this hypothesis, 67 dogs with pituitary-dependent hyperadrenocorticism were studied to determine whether a correlation could be demonstrated between the effect of dexamethasone administration on the activity of the pituitary-adrenocortical axis and the size of the pituitary gland as estimated by computed tomography. The volumes of the pituitary glands as calculated from summations of subsequent images of pituitary areas, ranged from 11.8 to 3238.6 mm3. Among the three dimensions, the height of the pituitary was the most sensitive indicator of enlargement. Calculation of the pituitary height/brain area ratio (P/B ratio) allowed correction for the size of the dog. The P/B ratio had the highest discriminatory power in distinguishing enlarged (n = 41) from non-enlarged (n = 26) pituitaries. The effects of dexamethasone (0.1 mg/kg) on the plasma concentrations of cortisol and ACTH and on the urinary corticoid/creatinine (C/C) ratios were expressed as percentage changes from the initial values. For ACTH, cortisol and C/C ratios these figures for resistance to dexamethasone were significantly correlated with the dimensions of the pituitary, particularly the height, volume and P/B ratio. It is concluded that the magnitude of the expansion of pituitary corticotrophic adenomas is dependent upon the loss of restraint by glucocorticoids, i.e. the degree of insensitivity to glucocorticoid feedback.
Alterations in anterior pituitary function of dogs with pituitary-dependent hyperadrenocorticism.
For the purpose of obtaining an integral picture of anterior pituitary function in canine pituitary-dependent hyperadrenocorticism (PDH), 47 dogs with PDH and eight control dogs received combined administration of four hypophysiotropic hormones (CRH, GHRH, GnRH and TRH) and measurements were made of ACTH, cortisol, GH, LH, PRL and TSH. Basal plasma levels in 47 dogs with PDH were higher for ACTH, cortisol and PRL, lower for GH, and not different for LH (n = 25 noncastrated dogs) and TSH compared with controls (n = 8). In dogs with PDH the responses to combined hypophysiotropic stimulation, measured as increment and area under the curve (AUC), were not different for ACTH, lower for GH and TSH (increments and AUC) and higher for cortisol (increments), LH (AUC, n = 25 noncastrated dogs) and PRL (increments and AUC) than in controls. We conclude that pituitary function is altered in several respects in dogs with PDH. 1) In spite of persisting hypercortisolemia and the neoplastic transformation of the corticotropic cells, these cells usually remain responsive to combined hypophysiotropic stimulation. 2) Basal plasma GH concentrations and GH responsiveness in the combined stimulation test are decreased, probably as a result of the glucocorticoid-induced increase in somatostatin tone. 3) Plasma PRL concentrations and the PRL response to stimulation are increased, probably as a result of cosecretion with ACTH by the transformed corticotropic cells. 4) Despite the well known effect of glucocorticoids of decreasing circulating concentrations of gonadal steroids and thyroxine, the basal plasma concentrations of LH and TSH remain unchanged and there is a tendency to hyperresponsiveness to stimulation for LH and hyporesponsiveness for TSH. The most likely explanation for these changes is a dual effect of glucocorticoids: a direct effect on the gonads and thyroids and/or the transport and metabolism of their secretory products, and an influence on the sensitivity of the feedback control at the hypothalamic-pituitary level.
Residual pituitary function after transsphenoidal hypophysectomy in dogs with pituitary-dependent hyperadrenocorticism.
Pituitary function was assessed before and after transsphenoidal hypophysectomy in 39 dogs with pituitary-dependent hyperadrenocorticism (PDH). Anterior pituitary function was investigated using combined administration of four hypophysiotropic releasing hormones (corticotropin-releasing hormone (CRH), GHRH, GnRH, and TRH) with measurements of ACTH, cortisol, GH, LH, prolactin (PRL), and TSH Pars intermedia function was assessed by measurements of basal plasma alpha-MSH concentrations and adrenocortical function by baseline urinary corticoid/creatinine ratios. At eight weeks after hypophysectomy basal plasma ACTH, cortisol, GH, LH, PRL, and TSH concentrations were significantly lower than before surgery. In seven dogs with elevated alpha-MSH concentrations, the values returned to the normal level after surgery. In the combined anterior pituitary function test there were no plasma GH, LH, PRL, and TSH responses to stimulation, whereas plasma ACTH and cortisol responses were small but significant. Remission of hyperadrenocorticism was obtained in 35 dogs and recurrences occurred in 3 of these within 16 months postoperatively. At 8 weeks after hypophysectomy, these 3 dogs were not discernible, with respect to residual pituitary and adrenocortical function, from the 32 dogs with persisting remission. Urinary corticoid/creatinine ratios in the latter group of dogs did not increase during 22 months after hypophysectomy. In contrast to the presurgical findings, at 8 weeks after hypophysectomy there were significant positive correlations between baseline urinary corticoid/creatinine ratios and basal levels and responses for ACTH, indicating return to normal function of the pituitary-adrenocortical axis. It is concluded that among the adenohypophyseal cells present in the sella turcica after hypophysectomy, the corticotropes have a distinct behavior. Much more so than the other cell types, the unaffected corticotropes tend to remain functional, or a repressed reserve fraction of corticotropes may become functional. This may be due to the removal of the hypothalamic influence of a postulated corticotropin-release inhibiting factor or a diminished inhibitory influence of a postulated paracrine factor. The corticotropes may maintain normocorticism, but may also lead to mild recurrence after relatively long periods of remission.
Progestin-induced hypersecretion of growth hormone: an introductory review.
In the 1970s acromegalic features were reported in some dogs used in long-term toxicity studies of progestins. In 1980 confirmation that progestagen administration can lead to increased circulating growth hormone (GH) concentrations was obtained. This phenomenon appeared not to be confined to exogenous progestins, for an excess of GH was also found in bitches during the luteal phase of the oestrous cycle. In bitches with a progestin-induced excess of GH, GH secretion could neither be inhibited nor stimulated by well-known regulatory neurohormones, indicating autonomous secretion. Because it could not be attributed to a neoplasm and was reversible, an extra-pituitary site of GH production was investigated. The progestin-induced GH was found to originate from the mammary gland. This phenomenon seems to play a role in the mammary development that occurs during the luteal phase of the oestrous cycle. The increase in cell proliferative activity may also be responsible for the susceptibility of the mammary gland to neoplastic transformation. The discovery of mammary GH in the dog has recently become of wider importance now that expression of the GH gene has also been demonstrated in other species, namely, humans and cats.
Effects of progestin administration on the hypothalamic-pituitary-adrenal axis and glucose homeostasis in dogs.
The effects of medroxyprogesterone acetate (MPA) and proligestone (PROL) on the hypothalamic-pituitary-adrenocortical axis and glucose homeostasis were studied in two groups of eight ovariohysterectomized beagle bitches. In addition, the binding characteristics of MPA and PROL for the progesterone and glucocorticoid receptor were investigated. The administration of both progestins resulted in suppression of the hypothalamic-pituitary-adrenal axis. Whereas basal plasma concentrations of adrenocorticotrophic hormone (ACTH) were only moderately affected, the basal plasma concentrations of cortisol and the cortisol:creatinine ratio in urine were significantly decreased after the first administration of both progestins. In the group given MPA the increase of ACTH after stimulation with corticotrophin-releasing hormone (CRH) remained normal but it was suppressed in the group treated with PROL. In both treatment groups the increase of cortisol after stimulation with CRH was lower. After cessation of progestin administration both basal and stimulated plasma ACTH concentrations returned to pretreatment concentrations within a few weeks. In contrast, it took 6 month to restore the basal plasma cortisol concentrations and cortisol:creatinine ratios in urine. Paradoxically, the stimulated cortisol concentrations returned to normal shortly after the cessation. Histological examinations revealed a severe atrophy of the zona fasciculata and reticularis of the adrenal gland in all treated dogs and a steroid-induced hepatopathy in 50% of them. During the first half of the progestin treatment, glucose homeostasis was maintained by increased plasma concentrations of insulin in both groups. After prolonged treatment the response to a glucose load became impaired. None of these parameters improved during the 6 month recovery period. Histological changes in the pancreas, characteristics of diabetes mellitus, were found in two dogs of each group. Most probably, the glucocorticoid action of the progestins is not the sole explanation for the insulin resistance since progestin treatment resulted in a concomitant increase in plasma concentrations of growth hormone which has diabetogenic properties. Experiments in vitro confirmed the strong glucocorticoid component of MPA and PROL. The inhibition constants (Ki) of PROL for both the progesterone receptor (PR) and the glucocorticoid receptor (GR) were approximately then times higher than those of MPA. Nonetheless, the ratios of the Ki for the GR and PR indicated that the specificity of MPA and PROL was only slightly different but considerably smaller than that of progesterone. It is long-term treatment with high doses of these progestins may result in a iatrogenic Cushing's syndrome and diabetes mellitus.
Lack of association of progestin-induced cystic endometrial hyperplasia with GH gene expression in the canine uterus.
Growth hormone (GH) is produced in progestin-induced hyperplastic ductular mammary epithelia in dogs. Progestins also induce the development of cystic endometrial hyperplasia (CEH) in this species. The study reported here investigated whether GH gene expression could also be demonstrated in progestin-induced hyperplastic epithelium in the canine uterus. Eight beagle bitches were treated with 10 mg medroxyprogesterone acetate (MPA) kg-1 body mass s.c. at intervals of 3 weeks, for a total of five times in four dogs (group I) and for a total of 13 times in the other four dogs (group II). Blood samples were collected twice during each 3 week period for measurement of plasma concentrations of GH, insulin-like growth factor I (IGF-I) and IGF-II. At the end of the series of injections uterine tissue was obtained by ovario-hysterectomy. Histological examination confirmed that CEH was present in all uteri after MPA treatment; the changes in the dogs of group I were less marked than those in group II. Immunohistochemical examination of the uterine tissues showed that immunoreactive(i) GH was present in a number of uteri with CEH. iGH was usually located in the cytoplasm of glandular epithelial cells. However, reverse transcriptase PCR using GH-specific primers failed to demonstrate mRNA encoding GH in the uterine tissue of all dogs. It is concluded that local production of GH is not involved in progestin-induced hyperplasia of uterine epithelial cells in dogs.
Assessment of a combined anterior pituitary function test in beagle dogs: rapid sequential intravenous administration of four hypothalamic releasing hormones.
A combined anterior pituitary (CAP) function test was assessed in eight healthy male beagle dogs. The CAP test consisted of sequential 30-second intravenous administrations of four hypothalamic releasing hormones in the following order and doses: 1 microgram of corticotropin-releasing hormone (CRH)/kg, 1 microgram of growth hormone-releasing hormone (GHRH)/kg, 10 micrograms of gonadotropin-releasing hormone (GnRH)/kg, and 10 micrograms of thyrotropin-releasing hormone (TRH)/kg. Plasma samples were assayed for adrenocorticotropin, cortisol, GH, luteinizing hormone (LH), and prolactin (PRL) at multiple times for 120 min after injection. Each releasing hormone was also administered separately in the same dose to the same eight dogs in order to investigate any interactions between the releasing hormones in the combined function test. Compared with separate administration, the combined administration of these four hypothalamic releasing hormones caused no apparent inhibition or synergism with respect to the responses to CRH, GHRH, and TRH. The combined administration of these four hypothalamic releasing hormones caused a 50% attenuation in LH response compared with the LH response to single GnRH administration. The side effects of the combined test were confined to restlessness and nausea in three dogs, which disappeared within minutes after the administration of the releasing hormones. It is concluded that with the rapid sequential administration of four hypothalamic releasing hormones (CRH, GHRH, GnRH, and TRH), the adenohypophyseal responses are similar to those occurring with the single administration of these secretagogues, with the exception of the LH response, which is lower in the CAP test than after single GnRH administration.
Thyroid-stimulating hormone responses after single administration of thyrotropin-releasing hormone and combined administration of four hypothalamic releasing hormones in beagle dogs.
Thyrotropin (TSH) responses were determined in eight healthy male beagle dogs after a single administration of thyrotropin-releasing hormone (TRH) and the combined administration of four hypothalamic releasing hormones, i.e., corticotropin-releasing hormone, growth hormone-releasing hormone, gonadotropin-releasing hormone, and TRH. In both tests, TRH was administered in a dose of 10 micrograms/kg. Basal TSH concentrations ranged from 0.07 to 0.27 microgram/l (mean +/- SE, 0.14 +/- 0.02 microgram/l). The administration of TRH, alone or in the combined test, resulted in a prompt and significant increase in TSH with mean (+/-SE) plasma TSH peaks of 1.26 +/- 0.22 micrograms/l at 10 min and 0.85 +/- 0.17 microgram/l at 30 min, respectively. The area under the curve (0-120 min) was significantly lower in the combined test than in the single TRH test, whereas the increments were not significantly different. It is concluded that measurements of TSH responses to TRH alone and in combination with other releasing hormones can be used for the assessment of pituitary thyrotropic cell function. In the combined test, the TSH response is slightly lower than that in the single test.
New insights in the molecular mechanism of progestin-induced proliferation of mammary epithelium: induction of the local biosynthesis of growth hormone (GH) in the mammary glands of dogs, cats and humans.
In contrast to the protective, anti-proliferative, action of progestins on the development of endometrium cancer, progestins may have local stimulatory and inhibitory effects on the proliferation of mammary epithelium. Until now there was no final molecular explanation of this discrepancy. Prolonged treatment of dogs with depot medroxyprogesterone acetate (DPMA) or with proligestone (PROL) results in enhanced plasma concentrations of growth hormone (GH), insulin-like growth factor (IGF)-I, IGF-II and IGF-binding proteins, together with the development of benign mammary tumours. The stimulated plasma GH levels do not have the typical pulsatile secretion pattern, and are not sensitive to stimulation with GHRH or to inhibition with somatostatin. The autonomous secretion can be inhibited by the anti-progestin RUU-486. The source of progestin-induced plasma GH levels has been demonstrated to be the canine mammary gland where progestins induce the expression of the gene encoding GH. The expression of the GH gene is restricted to focal areas of hyperplastic epithelium as shown by immunohistochemistry, and is predominantly located in single positive epithelial cells with an intermediate position between luminal- and myo-epithelium. Progestin-induced fibroadenomatous changes in the mammary gland of cats are also associated with locally enhanced GH expression. In both normal, benign and malignant mammary tumours of humans GH mRNA expression has been demonstrated by RT-PCR. The presence of GH mRNA is associated with the presence of immunoreactive GH as shown by immunohistochemistry. Sequence analysis revealed 100% homology to the pituitary expressed GH gene. In malignant mammary tumours of humans and dogs GH expression is also found in specimens negative for progesterone receptors as measured by ligand binding. It is concluded that the gene encoding GH is expressed in the mammary gland of a variety of species, including man. This appears to represent a contribution to the molecular explanation of the action of progestins on proliferation of mammary epithelium. It needs, however, to be proven whether this local biosynthesis of GH in the mammary gland is the cause of the local stimulatory effect of progestins on the proliferation of mammary epithelium.
Proliferation of the murine corticotropic tumour cell line AtT20 is affected by hypophysiotrophic hormones, growth factors and glucocorticoids.
In pituitary-dependent hyperadrenocorticism (Cushing's disease), the disturbed regulation of ACTH secretion is associated with neoplastic transformation of corticotropic cells. As these two phenomena are almost indissolubly connected, it is of prime importance to elucidate the factor(s) that induce corticotropic cell proliferation. Here we report on the effects of hypophysiotrophic hormones and intrapituitary growth factors on the proliferation and hormone secretion of the murine corticotropic tumour cell line AtT20/D16v, as measured by DNA content, and ACTH concentration in culture media. In addition, sensitivity to the inhibitory effect of cortisol was assessed under various conditions. Corticotropin releasing hormone (CRH) and vasopressin (AVP) induced proliferation of AtT20-cells. In contrast to that caused by AVP, the CRH-induced proliferation was associated with increased ACTH secretion, which could be inhibited by cortisol. Insulin-like growth factor-I (IGF-I), epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) also stimulated the proliferation of AtT20-cells. The proliferation of AtT20-cells was significantly inhibited by cortisol in all tests. The IGF-I-induced proliferation was the least sensitive to inhibition by cortisol. The growth factors did not stimulate ACTH secretion but IGF-I differed in that it prevented the inhibition of basal ACTH secretion by cortisol. Additional experiments (Western ligand blot analysis) concerning the relative insensitivity of IGF-I induced proliferation to inhibition by cortisol revealed that IGF-I increased the concentration of a 29 kDa IGF binding protein (IGFBP) in the culture medium. The concentration of the 29 kDa IGFBP was slightly decreased by cortisol.(ABSTRACT TRUNCATED AT 250 WORDS)
Polyglandular deficiency syndrome in a boxer dog: thyroid hormone and glucocorticoid deficiency.
Primary hypothyroidism and partial primary adrenocortical deficiency (isolated glucocorticoid deficiency) were diagnosed in an 8-year-old spayed female boxer dog, presented because of progressive symmetrical truncal alopecia, lethargy, and intolerance to cold. The diagnosis was based upon the combination of low, non-TSH-responsive concentrations of plasma thyroxine and low urinary excretion of corticoids together with high plasma concentrations of ACTH. Normal suppressibility of ACTH concentrations by a low dose of dexamethasone indicated an intact feedback system. Plasma growth hormone levels were elevated, most probably because somatostatin release was depressed by the glucocorticoid deficiency. The dog improved during oral replacement therapy with thyroxine until death ensued after 9 months as a result of intercurrent disease. Autopsy revealed thyroid atrophy and lymphocytic adrenalitis with complete destruction of the zona fasciculata and zona reticularis of the adrenal cortex. The combination of primary hypothyroidism and primary adrenocortical deficiency in this dog is identical to the entity known as type II polyglandular autoimmunity or Schmidt's syndrome in humans. The adrenocortical insufficiency remained confined to glucocorticoid deficiency during the observation period; on no occasion did electrolyte concentrations in the plasma reach values suggestive of mineralocorticoid deficiency.
Results of adrenalectomy in 36 dogs with hyperadrenocorticism caused by adreno-cortical tumour.
A total of 38 adrenocortical tumours were removed from 36 dogs with hyperadrenocorticism. The surgical approach was by way of a unilateral flank laparotomy (32 dogs; 14 left and 18 right), a bilateral flank laparotomy (3 dogs) or a midline celiotomy (1 dog). Two dogs were euthanized during surgery because their tumours could not be resected. Eight dogs died from post-operative complications. Pancreatic necrosis with peritonitis was the most common cause of death. Eight of the 26 dogs that survived had signs of recurrence of hyperadrenocorticism. Unsuppressible hyperadrenocorticism was found in four dogs; one dog had probably pre-existent pituitary-dependent hyperadrenocorticism, and adrenocortical function could not be re-examined in the remaining three dogs. Among the 37 tumours examined microscopically expansion of neoplastic tissue into blood vessels was found in 22 of them. Four adrenal glands with adrenocortical tumours also contained phaeochromocytomas. Necropsy was performed in eight dogs. Metastases were found in the lungs of two dogs and in the lungs and liver in one dog. In combination with the data of previous reports, it is suggested that histological findings in surgery specimens are not good predictors for the clinical outcome.
Expression of the gene encoding growth hormone in the human mammary gland.
Progestins cause a syndrome of growth hormone (GH) excess and enhanced mammary tumorigenesis in the dog. This has been regarded as being specific for the dog. Recently we reported that progestin-induced GH excess originates from foci of hyperplastic ductular epithelium of the mammary gland in the dog. In the present report we demonstrate by reverse-transcriptase PCR and immunohistochemistry that a main factor involved in tissue growth, i.e. GH, is also expressed in normal and neoplastic human mammary glands. The gene expressed in the human mammary gland proved to be identical to the gene encoding GH in the pituitary gland. The role of progesterone in the GH expression of the human mammary gland needs, however, to be proven. It is hypothesized that this locally produced hGH may play a pathogenetic role in breast cancer.
Growth hormone: its clinical relevance.
Explore the source record for details and available documents.
Production of antibodies to biosynthetic human growth hormone in the dog.
Explore the source record for details and available documents.
Progestin-induced growth hormone excess in the dog originates in the mammary gland.
In the dog endogenous progesterone and synthetic progestins may incite overproduction of GH, resulting in acromegaly and insulin resistance. This progrestin-induced excessive GH secretion is characterized by disappearance of the pulsatile secretion pattern and insensitivity to both stimulation with GHRH and inhibition with a somatostatin analog. This progestin-induced GH hypersecretion is not associated with neoplastic transformation at the pituitary level. These observations were the impetus for a search of a possible extrapituitary site of GH production. In four ovariohysterectomized dogs elevated plasma GH levels (46.5 +/- 7.7 micrograms/liter; mean +/- SEM) were induced by administration of synthetic progestins. In these dogs hypophysectomy did not led to a significant decrease in plasma GH levels. Analysis of the GH content of various tissue homogenates revealed that the highest GH immunoreactivity was found in extracts of the mammary gland. Ectopic production of GH in the mammary gland was confirmed by lowering of plasma GH concentration to values within the reference range within 2 h after complete mammectomy in two dogs with progestin-induced elevations of plasma GH levels. In one of these dogs the arterial and elevations of plasma GH levels. In one of these dogs the arterial and venous GH concentrations across the mammary gland were measured and an arterio-venous GH gradient was demonstrated. Displacement studies in the RIA and analysis by reversed-phase HPLC revealed that mammary-derived GH is highly similar to pituitary-derived GH. Immuno-histochemical staining revealed that GH immunoreactivity was localized in focal areas of hyperplastic ductular epithelium. In mammary tissue of healthy untreated female dogs no GH immunoreactivity was found. It is concluded that treatment of dogs with synthetic progestins can induce the overproduction of GH in the mammary gland. This GH is biologically active, highly similar to pituitary derived GH, and originates from foci of hyperplastic ductular epithelium of the mammary gland.