The possibility that growth hormone (GH) has effects on long bone growth independent of insulin-like growth factor-I (IGF-I) has long been debated. If this is true, then long bone growth should be more profoundly affected by the absence of GH (since both GH and GH-stimulated IGF-I effects are absent) than by the absence of IGF-I alone (since GH is still present and actually elevated). To test this hypothesis, we compared long bone growth in mice with targeted deletions of Igf1 vs growth hormone receptor (Ghr). Tibial linear growth rate was reduced by approximately 35% in Igf1 null mice and by about 65% in Ghr null mice between postnatal days 20 and 40, a time of peak GH effect during normal longitudinal growth. The Igf1 null mouse growth plate demonstrated significant enlargement of the germinal zone; chondrocyte proliferation and numbers were normal but chondrocyte hypertrophy was significantly reduced. In contrast, the Ghr null mouse germinal zone was hypoplastic, chondrocyte proliferation and numbers were significantly reduced, and chondrocyte hypertrophy was also reduced. We have previously demonstrated that IGF-II is highly expressed in growth plate germinal and proliferative zones, so we considered the possibility that GH-stimulated IGF-II production might promote germinal zone expansion and maintain normal proliferation in the Igf1 null mouse growth plate. Supporting this view, IGF-II mRNA was increased in the Igf1 null mouse and decreased in the Ghr null mouse growth plate.Thus, in the complete absence of IGF-I but in the presence of elevated GH in the Igf1 null mouse, reduction in chondrocyte hypertrophy appears to be the major defect in longitudinal bone growth. In the complete absence of a GH effect in the Ghr null mouse, however, both chondrocyte generation and hypertrophy are compromised, leading to a compound deficit in long bone growth. These observations support dual roles for GH in promoting longitudinal bone growth: an IGF-I-independent role in growth plate chondrocyte generation and an IGF-I-dependent role in promoting chondrocyte hypertrophy. The question of whether GH has direct effects on chondrocyte generation is still not settled, however, since it now appears that IGF-II may medicate some of these effects on the growth plate.

Abstract

The renal actions of vasopressin were studied in the conscious female rat. Vasopressin caused a dose-dependent increase in sodium excretion when administered at 40–160 pmol/min. The highest dose, which increased sodium excretion from 10·4 ± 0·3 μmol/min (n=32) to 18·3 ± 0·8 μmol/min (n=8, P<0·001), also caused a significant rise in glomerular filtration rate (GFR). The antidiuretic and natriuretic responses to vasopressin varied significantly over the 4 days of the oestrous cycle. Both responses were greatest on pro-oestrus, being −57 ± 3 and 52 ± 3% above the control values with 80 pmol vasopressin/min. Responses of similar magnitude were also seen on dioestrus day 1. On these two cycle days the effects on urine flow and sodium excretion were accompanied by a significant increase in GFR. Smaller antidiuretic and natriuretic responses were seen on oestrus and dioestrus day 2, without concomitant changes in GFR. As the plasma vasopressin concentrations produced by hormone infusion were similar on each day of the cycle, the renal responsiveness to vasopressin appears to vary over the 4 days of the oestrous cycle. This may be important in terms of alteration and possible disturbances of fluid balance which may occur during reproductive cycles and pregnancy.

Journal of Endocrinology (1996) 148, 457–464

The mechanism by which chronic treatment with HIV (human immunodeficiency virus)-1 protease inhibitors leads to a deterioration of glucose metabolism appears to involve insulin resistance, and may also involve impaired insulin secretion. Here we investigated the long-term effects of HIV-1 protease inhibitors on glucose-stimulated insulin secretion from beta cells and explored whether altered insulin secretion might be related to altered insulin signaling. INS-1 cells were incubated for 48 h with different concentrations of amprenavir, indinavir, nelfinavir, ritonavir or saquinavir, stimulated with 20 mM d-glucose, and insulin determined in the supernatant. To evaluate insulin signaling, cells were stimulated with 100 nM insulin for 2 min, and insulin-receptor substrate (IRS)-1, -2 and Akt phosphorylation determined. Incubation for 48 h with ritonavir, nelfinavir and saquinavir resulted in impaired glucose-induced insulin secretion at 2.5, 5 and 5 μM respectively, whereas amprenavir or indinavir had no effects even at 20 and 100 μM respectively. The impaired insulin secretion by ritonavir, nelfinavir and saquinavir was associated with decreased insulin-stimulated IRS-2 phosphorylation, and, for nelfinavir and saquinavir, with decreased insulin-stimulated IRS-1 and Thr³⁰⁸-Akt phosphorylation. No such effects on signaling were observed with amprenavir or indinavir. In conclusion, certain HIV-1 protease inhibitors, such as ritonavir, nelfinavir and saquinavir, not only induce peripheral insulin resistance, but also impair glucose-stimulated insulin secretion from beta cells. With respect to the long-term effect on beta-cell function there appear to be differences between the protease inhibitors that may be clinically relevant. Finally, these effects on insulin secretion after a 48 h incubation with protease inhibitor were associated with a reduction of the insulin-stimulated phosphorylation of insulin signaling parameters, particularly IRS-2, suggesting that protease inhibitor-induced alterations in the insulin signaling pathway may contribute to the impaired beta-cell function.

Glucocorticoids are steroid hormones, secreted by the adrenals to regulate a range of metabolic, immunologic, and homeostatic functions. Due to their potent anti-inflammatory effects, synthetic glucocorticoids are widely used to treat inflammatory disorders. However, their use especially at high doses and over the long-term is associated with several unwanted side effects that compromises their intended use (e.g. glucocorticoid-induced osteoporosis and/or diabetes, myopathy, and skin atrophy). Both endogenous and synthetic glucocorticoids exert their effects through the glucocorticoid receptor, a transcription factor present in nearly all nucleated cells. Glucocorticoid receptor knockout mouse models have proved to be valuable tools in understanding how glucocorticoids contribute to skeletal health and disease. These models, described in this review, have helped to establish that the effects of glucocorticoids on the skeleton are multifaceted, cell specific and concentration dependent. Intriguingly, while endogenous glucocorticoids are essential for bone formation, high-dose exogenous glucocorticoids may induce bone loss. Additionally, the actions of endogenous glucocorticoids vary greatly depending on the disease microenvironment. For example, endogenous glucocorticoids have predominately beneficial anti-inflammatory effects in rheumatoid arthritis, but detrimental actions in osteoarthritis by driving cartilage loss and abnormal bone formation. Studies in tissue-specific knockout models provide important insights that will aid the development of new glucocorticoid therapeutics that can specifically target certain cell types to minimise unwanted effects from current glucocorticoid therapy.

Pituitary follicle-stimulating hormone (FSH) synthesis is regulated by transforming growth factorβsuperfamily ligands, most notably the activins and inhibins. Bone morphogenetic proteins (BMPs) also regulate FSHβ subunit (Fshb) expression in immortalized murine gonadotrope-like LβT2 cells and in primary murine or ovine primary pituitary cultures. BMP2 signals preferentially via the BMP type I receptor, BMPR1A, to stimulate murine Fshb transcription in vitro. Here, we used a Cre–lox approach to assess BMPR1A’s role in FSH synthesis in mice in vivo. Gonadotrope-specific Bmpr1a knockout animals developed normally and had reproductive organ weights comparable with those of controls. Knockouts were fertile, with normal serum gonadotropins and pituitary gonadotropin subunit mRNA expression. Cre-mediated recombination of the floxed Bmpr1a allele was efficient and specific, as indicated by PCR analysis of diverse tissues and isolated gonadotrope cells. Furthermore, BMP2 stimulation of inhibitor of DNA binding 3 expression was impaired in gonadotropes isolated from Bmpr1a knockout mice, confirming the loss of functional receptor protein in these cells. Treatment of purified gonadotropes with small-molecule inhibitors of BMPR1A (and the related receptors BMPR1B and ACVR1) suppressed Fshb mRNA expression, suggesting that an autocrine BMP-like molecule might regulate FSH synthesis. However, deletion of Bmpr1a and Acvr1 in cultured pituitary cells did not alter Fshb expression, indicating that the inhibitors had off-target effects. In sum, BMPs or related ligands acting via BMPR1A or ACVR1 are unlikely to play direct physiological roles in FSH synthesis by murine gonadotrope cells.

Glucocorticoids (GCs) are routinely believed to work solely through genomic mechanisms. Recent evidence indicates that GCs can act at the membrane to exert rapid nongenomic effects on various tissues and cells. To ascertain whether nongenomic effects of GCs exist on the allergic asthma reaction, Hartley guinea pigs were sensitized with ovalbumin and challenged with the same antigen given by aerosol. Some animals received inhaled budesonide (3 mg/ml suspended in Hydroxypropyl methylcellulose vehicle) for 5 minutes before ovalbumin challenge; Other animals received saline or blank vehicle as control. We measured the changes of lung resistance and dynamic lung compliance, the pulmonary function used to evaluate allergic asthma severity. Inhaled budesonide inhibited allergic reaction within 10 minutes, which would preclude genomic-mediated responses that normally takes several hours to occur. This study infers for the first time that rapid nongenomic effect of GCs exists on allergic asthma reaction, and provides a new way to investigate nongenomic mechanism of GCs. Further study would raise the possibility of new therapeutic strategies for allergic disease including asthma.

It has been documented that stress or glucocorticoids have conflicting effects on memory under different conditions. However, it is not fully understood why stress can either impair or enhance memory. Here, we have examined the performance of six age groups of Wistar rats in a water maze spatial task to evaluate the effects of stress under different conditions. We found that the impairment or enhancement effect of an 'elevated platform' (EP) stress on memory was dependent on previous stress experience and on age. EP stress impaired memory retrieval in water maze naive animals, but enhanced rather than impaired memory retrieval in young water maze stress-experienced animals. Furthermore, exogenously applied corticosterone or foot shock stress before water maze training prevented the impairment of memory retrieval that should be induced by treatment with corticosterone or foot shock before the 'probe trial'. Again, memory retrieval was enhanced in young animals under these conditions, and this enhancement can be prevented by the glucocorticoid receptor antagonist RU 38486. Thus, glucocorticoid receptor activation not only induced impairment of memory but also increased the capacity of young animals to overcome a later stress. The present findings suggest that the effect of stress on memory can be switched from impairment to enhancement dependent on both stress experience and age.

ABSTRACT

Peptides containing residues 1–34 of parathyroid hormone-related protein (PTHrP) and of bovine parathyroid hormone (bPTH), and recombinant full-length PTHrP(1–141) were infused i.v. into anaesthetized thyroparathyroidectomized rats to compare their action and potency on the renal handling of calcium, phosphate and cyclic AMP (cAMP) in vivo. All three peptides decreased the excretion of calcium and increased the excretion of phosphate and cAMP in the urine, with PTHrP(1–34) and PTHrP(1–141) having virtually equipotent effects. Thus the essential requirements for the major physiological activity of PTHrP on the kidney are contained within the 34 amino-terminal amino acids. For all three peptides, the lowest infusion rate that increased phosphate and cAMP excretion was 0·01 nmol/kg per h, whereas the lowest infusion rate that decreased calcium excretion was 0·025 nmol/kg per h for the PTHrP peptides and 0·1 nmol/kg per h for bPTH(1–34). The response to the PTHrP peptides was maximal at an infusion rate of 01 nmol/kg per h for both calcium and phosphate. Since the kidney is either equally sensitive to PTHrP and bPTH(1–34), or more sensitive to PTHrP than to bPTH(1–34), the hypercalcaemia of humoral hypercalcaemia of malignancy may develop because uncontrolled secretion of PTHrP increases the renal reabsorption of calcium to such an extent that even a modest increase in the inflow of calcium into the blood raises plasma calcium concentration.

Journal of Endocrinology (1989) 122, 229–235

In human obesity as well as in rat obesity models a decrease in spontaneous and stimulated GH secretion has been a constant finding. The presence of a decreased pituitary GH synthesis in diet-induced obese male rats was investigated and its possible relationship with obesity-related changes in peripheral hormones was analyzed. Cafeteria-diet-overfed obese male Wistar rats with body fat percentage above 30% had a significantly decreased pituitary GH mRNA transcript level assessed by both Northern blot and in situ hybridization, and a lower pituitary GH protein level as demonstrated by immunocytochemistry. The GH transcript level correlated negatively with the serum leptin and positively with the IGF-I concentration. No differences in circulating tri-iodothyronine, non-fasting insulin and corticosterone levels were found between overfed and control rats. GH release by cultured pituitary cells from overfed rats was comparable to that by cells prepared from control rats. In contrast, incubation of normal pituitary cells with serum from overfed rats for 3 days gave a significantly lower GH release than after incubation with serum from non-obese rats. In conclusion, cafeteria-diet-induced obese male Wistar rats have a decreased pituitary GH gene expression and a modifiable GH release in in vitro experiments. A possible role for peripheral circulating factors, like leptin and IGF-I, in decreasing the pituitary GH synthesis and release in obese rats is discussed.

The progesterone receptor (PR, encoded by Pgr) plays essential roles in reproduction. Female mice lacking the PR are infertile, due to the loss of the protein’s functions in the brain, ovary, and uterus. PR is also expressed in pituitary gonadotrope cells, but its specific role therein has not been assessed in vivo. We therefore generated gonadotrope-specific Pgr conditional knockout mice (cKO) using the Cre-LoxP system. Overall, both female and male cKO mice appeared phenotypically normal. cKO females displayed regular estrous cycles (vaginal cytology) and normal fertility (litter size and frequency). Reproductive organ weights were comparable between wild-type and cKO mice of both sexes, as were production and secretion of the gonadotropins, LH and FSH, with one exception. On the afternoon of proestrus, the amplitude of the LH surge was blunted in cKO females relative to controls. Contrary to predictions of earlier models, this did not appear to derive from impaired GnRH self-priming. Collectively, these data indicate that PR function in gonadotropes may be limited to regulation of LH surge amplitude in female mice via a currently unknown mechanism.