Growth hormone (GH) deficiency is associated with increased sensitivity to insulin, but the molecular mechanisms involved in this association are poorly understood. In the current work, we have examined the consequences of the absence of the biological effects of GH on the first steps of the insulin signaling system in vivo in liver of mice with targeted disruption of the GH receptor/GH binding protein gene (GHR-KO mice). In these animals, circulating insulin concentrations are less than 4 microIU/ml, and glucose concentrations are low, concordant with a state of insulin hypersensitivity. The abundance and tyrosine phosphorylation state of the insulin receptor (IR), the IR substrate-1 (IRS-1), and Shc, the association between IRS-1 and the p85 subunit of phosphatidylinositol (PI) 3-kinase, the IRS-1- and the phosphotyrosine-associated PI 3-kinase in liver were examined. We found that, in liver of GHR-KO mice, the lack of GHR and GH eff! ects is associated with: (1) increased IR abundance, (2) increased insulin-stimulated IR tyrosine phosphorylation, (3) normal efficiency of IRS-1 and Shc tyrosine phosphorylation and (4) normal activation of PI 3-kinase by insulin. These alterations could represent an adaptation to the low insulin concentrations displayed by these animals, and may account for their increased insulin sensitivity.

The study of environmental factors affecting vertebrate reproduction has long interested both developmental and evolutionary biologists. Although photoperiod has been considered to be an important environmental parameter for vertebrates such as birds, temperature is probably a primary external factor responsible for reproductive cyclicity in reptiles. In spite of the progress made in the understanding of reptilian reproductive strategies and adaptations, much remains to be learned about the interplay between endocrine physiological factors, such as hormones, and environmental parameters. In this report, we have examined the effects of in vivo administered FSH on oocyte recruitment during the most significant periods of the reproductive cycle of the lizard, Podarcis sicula. The results show that when FSH is administered in proximity to the reproductive period, it stimulates oocyte growth and ovulation; when the hormone is administered at the beginning of the winter stasis it affects ovarian activity without inducing ovulation. Ovarian adenylate cyclase activity is moderately sensitive to in vitro FSH stimulation during the pre- and post-reproductive periods. The sensitivity to hormone stimulation increases significantly during the reproductive period and winter stasis. We have also tested the hypothesis that environmental temperature affects the responsiveness of ovarian adenylate cyclase to FSH stimulation. For such a purpose, we exposed animals to 28 degrees C or 4 degrees C in different periods of the ovarian cycle. The results show that, whenever the temperature applied mimics the thermal regime of the coming season, adenylate cyclase sensitivity to FSH shifts towards levels that anticipate the natural responsiveness.

One of the most remarkable but neglected aspects of osteoclast function is its unique adaptation that allows the cell to function despite its resorbing surface being exposed to extremely high levels of ambient Ca2+. Recently our studies have provided evidence of continuous transcellular Ca2+ disposal, suggesting that osteoclasts are able to prevent Ca2+ accumulation within the resorptive hemivacuole. It has also been shown that matrix protein degradation products that accumulate within the osteoclast resorptive vacuole are also undergoing transcellular transport by transcytosis. However, both experimental evidence and theoretical considerations suggest that transcellular transport of Ca2+ and matrix protein is likely to occur via distinct routes. In light of these considerations, we are able to provide convincing explanations for the apparent anomalies of osteoclast intracellular [Ca2+] responses to a variety of endocrine stimuli. The understanding of the mechanisms involved in Ca2+ handling by osteoclasts indicates the lack of a simple link between osteoclast function and changes in overall cytosolic [Ca2+].

SUMMARY

The persistent high calorie intake and rapid somatic growth in young rats, which was initiated by a super-abundant milk supply in the first weeks of life, was accompanied by an accelerated growth in the size and number of cells in the liver. The adult concentration of serum protein was also attained earlier.

An increased food intake in the adult which was produced by hypothalamic lesions, did not significantly affect growth of the skeleton, but it had the same effect on the liver as very rapid somatic growth. The increase in weight of the liver could not be accounted for merely by increased consumption of protein. If part of the liver was removed from an adult rat, both the rate of regeneration and the ultimate size and composition of the remainder were also determined by the calorie intake. The increased size of the liver of hyperphagic rats was associated with a greater blood volume, and consequently an increase in total serum protein, although the concentration of serum protein remained normal.

Although hyperphagia after hypothalamic lesions may continue for many weeks, with a progressive increase in obesity, the maximum weight of the liver was often achieved within 1 or 2 days. The chemical composition of the liver, however, changed progressively, and the effects closely resembled those brought about by insulin in the liver of diabetic rats. The similarity suggested that insulin might regulate the metabolic adaptation of the liver to a very high intake of food.

A radioligand-binding assay was established for eel atrial natriuretic peptide (eANP), and ANP receptors were characterised in the eel gill. ANP binding to the gill membrane fraction was saturable with increasing ligand concentrations and was specific to ANP peptides, i.e. eANP, ventricular natriuretic peptide (VNP) and C-type natriuretic peptide (CNP). A Scatchard analysis revealed a single class of high-affinity receptors with a Kd of 59.2 pM and a Bmax of 67.9 fmol/mg protein. The Kd value is within the range of plasma ANP concentration of the eel. Kd and Bmax did not differ between freshwater (FW)- and seawater (SW)-adapted fish. The gill receptors exhibited similar affinity for eANP, eVNP and eCNP, and ANP binding was almost completely displaced by C-ANF, a specific ligand for guanylate cyclase-uncoupled receptors. The presence of this type of receptor (natriuretic peptide receptor (NPR)-C and NPR-D) was also indicated in eel gill by affinity labelling. cGMP production was stimulated by the addition of eCNP but not by eANP and eVNP, and this was observed only in FW eels. Thus, most ANP receptors in eel gill are NPR-C and NPR-D, but a small number of the NPR-B type are also present in FW eels. In addition to the gill, specific ANP receptors were detected in the red body of the swim bladder, the brain, digestive tracts, kidney, head kidney and urinary bladder, in most of which ANP action has been reported. The density of ANP receptors decreased in most tissues after adaptation to SW except the gill, brain, atrium and spleen.

We have previously shown that lambs delivered by caesarean section 1 week prematurely become hypothermic due to reduced brown adipose tissue function in conjunction with low plasma concentrations of cortisol and thyroid hormones. The present study therefore aimed to determine whether maternal dexamethasone (a synthetic corticosteroid) administration could improve thermoregulation in premature lambs to the extent that they become similar to term lambs. Lambs were either delivered by caesarean section into a warm (30 degrees C; WD) or cool (15 degrees C; CD) ambient temperature at 140 days of gestation, 2 days after maternal dexamethasone treatment, or at 146 days for controls. During the first 30 min of life the decline in colonic temperature was greater in dexamethasone treated lambs compared with controls delivered into the same ambient temperature. All lambs then restored colonic temperature although this adaptation took longest in dexamethasone treated lambs CD but these subsequently attained highest plateau colonic temperatures. Oxygen consumption, breathing frequency and plasma free fatty acid concentrations were highest in dexamethasone treated lambs CD. There were no differences in plasma thyroid hormones between groups, but cortisol concentrations were lower in dexamethasone treated lambs irrespective of delivery temperature. Analysis of brown adipose tissue samples at 6 h of life demonstrated that dexamethasone treated lambs WD had more uncoupling protein and, in both dexamethasone treated and control lambs, uncoupling protein content was higher in lambs CD compared with those WD. An effect of ambient temperature on thermogenic activity was only observed in the dexamethasone treated group. It is concluded that maternal dexamethasone treatment can significantly improve thermoregulation after birth following premature delivery by caesarean section. As a consequence, dexamethasone treated lambs delivered 1 week prematurely do not remain hypothermic and have higher or similar colonic temperatures compared with untreated lambs born 1-2 days before term.

ABSTRACT

It is accepted that renal phosphate wasting is the basis of hypophosphataemia in vitamin D-resistant hypophosphataemic rickets (VDRR). Abnormal renal adaptation to phosphate deprivation has also been reported in these patients. We studied sodium-dependent phosphate transport and its modulation by phosphate deprivation in skin fibroblasts cultured from healthy subjects and patients with VDRR. Control fibroblasts exhibited high-affinity sodium-dependent phosphate transport (77 ± 12 μmol/l) which resembled the ubiquitous transport of renal and non-renal cells. Phosphate deprivation (incubation in low phosphate medium) increased the maximal velocity (V _max) of the transport by 2·7-fold after 24 h, with no change in the affinity. The increase in V _max was dependent on gene transcription and protein synthesis. The sodium-dependent phosphate transport exhibited in fibroblasts from VDRR patients did not significantly differ from that of control subjects, except that the V _max of the phosphate transport was higher in cells from patients with VDRR under normal and phosphate-deprivation conditions, although the difference was significant only after 24 h of phosphate deprivation (V _max: 22·6 ± 2·4 pmol/mg protein per s in VDRR vs 16 ± 3·6 pmol/mg protein per s in controls, P < 0·05).

These data demonstrate that sodium-coupled phosphate transport in human skin fibroblasts has the properties of ubiquitous sodium-phosphate co-transport and show that this transport is not deficient in patients with VDRR. Indeed paradoxically the V _max was 40% higher in VDRR than in control subjects after 24 h of phosphate deprivation. The transport must be either different from that of kidney cells responsible for the phosphate leak, or differently modulated. Therefore, skin fibroblasts cannot be used to determine the molecular defect responsible for the renal phosphate leak in VDRR patients.

Journal of Endocrinology (1992) 133, 301–309

Abstract

Zinc depletion attenuates growth and decreases circulating IGF-I. To investigate the mechanisms responsible for the IGF-I decline, we determined the effects of dietary zinc (Zn) deficiency on body and organ growth, serum IGF-I, serum GH-binding protein (GHBP), liver GH receptors and liver expression of their corresponding gene. After 1 week of adaptation to a normal zinc diet, a zinc-deficient diet (ZD; Zn, 0 p.p.m.) or a zinc-normal diet (CTR; Zn, 75 p.p.m.) was available ad libitum to 4-week-old Wistar rats for 4 weeks. Pair-fed animals (PF) received the zinc-normal diet in the same absolute amount as that consumed the day before by the ZD group. The food intake of ZD and PF rats was reduced by 32% (P<0·001) compared with the CTR group. Zinc depletion specifically reduced body weight gain (−22%, P<0·05), serum IGF-I concentrations (−52%, P<0·001), hepatic GH receptors (−28%; P<0·05) and serum GHBP levels (−51%; P<0·05), compared with the PF group. GH concentrations were reduced in ZD animals compared with CTR rats (P<0·01). The caloric restriction of PF animals also decreased body weight gain (−50%, P<0·001), serum IGF-I concentrations (−21%, P<0·05), liver GH receptors (−38%, P<0·001) and serum GHBP levels (−38%, P<0·01), when compared with the CTR group. Both ZD and PF groups had reduced liver IGF-I and GH receptor/GHBP mRNA levels in comparison with the CTR group (P<0·01). However, only liver IGF-I mRNA levels were specifically reduced by zinc deficiency (ZD vs PF rats; P<0·05). Our observations suggest that beside the decline of GH secretion, decreased hepatic GH receptors and/or GHBP concentrations might be responsible for the decline of circulating IGF-I in ZD animals.

Journal of Endocrinology (1995) 144, 449–456

ABSTRACT

Radioimmunoassays were established for the measurement of total androgens and the specific measurement of testosterone and 5α-dihydrotestosterone in peripheral plasma of the brush-tail possum. Androgen concentrations were measured in blood collected from indwelling jugular cannulae (i) to determine whether the normal pattern of androgen secretion in this species was episodic and (ii) to attempt to relate total androgen and the pattern of testosterone secretion to the changes previously reported in prostatic, but not epididymal, weight in the breeding season. Blood was collected from restrained animals at varying time-intervals during daylight hours and darkness. Despite an apparent good adaptation to the sampling procedure there was generally a progressive decline in plasma androgen level during the collection period. This was true for animals bled during or out of the breeding season. There was no significant seasonal effect on the androgen concentration in the initial blood sample. When less restraint was used, two of three animals showed fluctuations in androgen levels over the 7-h sampling period. Testosterone levels in blood obtained by cardiac puncture were four- to nine-fold higher than those of 5α-dihydrotestosterone but levels of these androgens in samples obtained during the breeding season were not significantly different from those obtained out of season. The results do not argue for a pulsatile release of testosterone in the possum but do demonstrate a marked capacity for changes in the peripheral androgen concentration. There was a poor correlation between testosterone and 5α-dihydrotestosterone levels and prostatic weight.

J. Endocr. (1985) 105, 63–70

SUMMARY

Plasma corticosterone (fluorometric assay), pituitary ACTH (bioassay using isolated adrenal cells) and hypothalamic corticotrophin releasing factor (CRF) (bioassay using isolated pituitary cells) were measured singly in groups of six female rats which were killed at 11.00, 15.00, 19.00, 21.00, 23.00, 01.00, 03.00, 05.00, 07.00 and 11.00 h, after 5 weeks of adaptation to a photoperiod of 12 h light:12 h darkness. Locomotor activity was recorded continuously, using actographic cages, and the waking/sleep pattern was recorded by electroencephalography from chronically implanted control rats during the first hours of the light span.

The three hormones measured fluctuated with a 24 h rhythmicity, with extreme values ranging between 4·12 ± 1·42 and 31·78 ± 194 (s.e.m.) μg/100 ml for corticosterone, 4486 ± 269 and 16629 ± 882μu./mg pituitary for ACTH, and 439 ± 20 and 1270 ± 39μu. ACTH production/hypothalamus/105 pituitary cells.

The onset of the ascending phase of the rhythm started during the first 2 h of light for CRF, 2 h later for ACTH, and again 2 h later for corticosterone. Similarly, the estimated acrophase of the rhythms occurred respectively, 9·4 (CRF), 10·3 (ACTH) and 14·4 h (corticosterone) after onset of light. These phase relationships point to a central origin of the adrenal rhythm.

The diurnal activation of CRF at the very beginning of the light phase was concomitant with an almost immediate reduction of the locomotor activity and onset of sleep. These correlations favour the hypothesis of a common temporal control of both the adrenal and the sleep/waking rhythms.