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SUMMARY
The intestinal absorption of phosphate has been studied in conscious pigs, each prepared with a Thirty–Vella loop of jejunum. The feeding of diets low in either calcium or phosphorus caused a significant increase in the efficiency of absorption of phosphate from the solution used to perfuse the jejunal loop in both intact and parathyroidectomized (PTX) pigs. An intravenous infusion of parathyroid hormone (0·22 u. kg−1 h−1) into a PTX pig also enhanced the absorption of phosphate. The increase in the absorption of phosphate when the low phosphorus diet was fed was not caused by an increase in the concentration gradient of phosphate ions between the jejunal lumen and blood.
It is concluded that the intestinal absorption of phosphate shows similar changes to those of calcium when diets low in calcium or phosphorus are fed and that parathyroid hormone, although capable of stimulating the absorption of phosphate, is not essential for this adaptation. These effects are probably brought about by changes in the renal production and mucosal uptake of 1,25-dihydroxycholecalciferol, the active metabolite of vitamin D3.
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Stewart (1971) showed that mice of the Peru strain produce [8-lysine]-vasopressin rather than [8-arginine]-vasopressin which is found in other strains of mice and is the usual form of the neurohypophysial hormone in mammals other than the Suina (Ferguson & Heller, 1965). Most mammals are more sensitive to [8-arginine]-vasopressin than [8-lysine]-vasopressin as an antidiuretic hormone (Sawyer, Chan & van Dyke, 1962); but in pigs [8-lysine]-vasopressin is nearly as potent as [8-arginine]-vasopressin (Ferguson & Heller, 1965), indicating some co-adaptation of the renal receptors and the chemical structure of the hormone. It was therefore decided to assess the relative sensitivity of Peru and CBA/FaCam mice to [8-arginine]-vasopressin and [8-lysine]-vasopressin.
Although stomach-loading of conscious Peru mice will induce a rapid water diuresis (Stewart, 1968), this is inhibited by anaesthesia and operational stress. Two bioassay methods using conscious animals have therefore been developed and used on CBA/FaCam and Peru adult male mice. In one method,
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The activity of the thyroid gland of rats was observed by measuring the uptake of 131I in the living animal over a period of several days. The method is described in detail, and the importance of iodine contained in the diet or applied to the skin, in affecting the results, is stressed.
Reduction of the atmospheric pressure to 250 and 380 mm Hg decreases the uptake of 131I. The lower the pressure, the less iodine is concentrated in the thyroid.
Under natural conditions a small decrease of thyroid activity at an altitude of 3450 m (490 mm Hg) can be noticed, but not at 2010 m altitude (592 mm Hg).
The depression of the thyroid is temporary; after some days 131I is taken up again with normal velocity.
At low atmospheric pressure (below 480 mm Hg) the body temperature decreases, but this, too, is restored to normal in 3–4 days.
The adaptation of thyroid activity to low atmospheric (oxygen) pressure may play an important part during acclimatization to high altitudes. The mechanism underlying the alteration in thyroid function and, in particular, the relation between the thyroid and adrenocortical activity is discussed. It is suggested that the increased adrenocortical activity and the decrease of thyroid activity observed at low oxygen or atmospheric pressure may be inter-connected.
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M.R.C. Blood Pressure Unit, Western Infirmary, Glasgow, G11 6NT
(Received 27 August 1975)
The rat has been widely used in the investigation of the physiology and pathology of the renin—angiotensin system. This paper describes an adaptation of the radioimmunoassay of angiotensin II in human plasma (Dusterdieck & McElwee, 1971) for use in the rat.
Antibodies against [Asn1, Val5]-angiotensin II (Hypertensin, Ciba) were raised in rabbits using a minor modification of the method described by Goodfriend, Fasman, Kemp & Levine (1966). The antiserum used cross-reacted 100% with [Asp1, He5]-angiotensin II (Schwartz-Mann), 2–8 heptapeptide (Ile4), and 3–8 hexapeptide (Ile3) fragments but only 0·6% with [Asp1,Ile5]-angiotensin I and 0·008% with rat renin substrate obtained from dialysed nephrectomized rat plasma.
Since the octapeptide native to the rat is thought to be in the Ile5 form (Powell-Jackson, Brown, Lever, Macgregor, Macadam, Titterington, Robertson & Waite, 1972), [Asp1, Ile5]-angiotensin II was iodinated with Na125I as described
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Search for other papers by E. A. SELLERS in
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SUMMARY
Thyroxine-binding antithyroid antibodies produced by hetero- or isoimmunization were examined in animals in different metabolic conditions. Guinea-pigs and rats were treated with thyroxine (T4) or methimazole or kept at 4 °C. During treatment they were immunized with heterologous or isologous thyroid extracts. Control animals were given liver extracts or saline. Rabbits were hetero- or iso-immunized but otherwise were untreated.
All thyroid-immunized animals had a significantly higher level of tracer thyroxine binding in the globulin fraction of serum than had controls, regardless of metabolic status. Only thyroid hetero-immunized animals showed anti-thyroid antibodies as measured by routine immunological tests.
In serum from animals with high levels of T4, less tracer was bound in the globulin fraction. This change did not represent a decrease in the amounts of T4 bound by the antibodies, but rather a redistribution of the hormone in the serum proteins.
Methimazole decreased not only the specific thyroxine binding by the antithyroid antibodies but also the non-specific binding in the globulin fraction of control serum. Adaptation to cold apparently did not affect the thyroxine binding to the antibodies.
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Although the inhibitory effect of glucocorticoids on the intestinal absorption of calcium is well recognized, their effect on the absorption of phosphate is less well documented. We studied the effect of the oral administration of betamethasone (BM; 25 μg/kg per day) on the duodenal absorption of phosphate in chicks fed normal calcium, normal phosphorus (NCaNP), normal calcium, low phosphorus (NCaLP) or low calcium, normal phosphorus (LCaNP) diets using the ligated loop technique in vivo. The daily oral administration of BM for 8 days significantly reduced the absorption of phosphate in chicks fed the NCaNP diet (21% decrease) but had less effect in chicks fed the NCaLP (14% decrease) or LCaNP (9% decrease) diets in which birds the absorption of phosphate was significantly raised (49 and 87% respectively). In one group of chicks, BM was administered for 9 days before the birds were transferred to the NCaLP or LCaNP diets. Adaptation was again unaffected by the treatment. Thirty-four per cent of the absorbed phosphate was retained in the duodenal tissue. Treatment with BM reduced the amount retained but this may have been caused by the lower weight of the duodenal segment in these chicks as BM administration markedly reduced growth rate. We have concluded that the duodenal absorption of phosphate in the chick can be inhibited by treatment with BM, although this may be secondary to the reduced rate of growth, but the increase in the absorption of phosphate caused by feeding NCaLP or LCaNP diets was unaffected by the steroid.
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SUMMARY
By use of the 'eel tube', it was shown in the freshwater eel that there was a regular net influx (gain) of sodium and a net outflux (loss) of potassium in quiescent animals with steady and normal respiration. Adaptation of eels to distilled water for periods of up to 6 weeks resulted in an increased net extrarenal (gills) uptake of sodium and an increased net loss of potassium when the head of the animal was placed in tap water made up to 600 μmoles Na+.
Stress, injection of cortisol, metyrapone, metyrapone plus betamethasone and aldactone reduced or abolished net sodium gain and had variable effects on net potassium loss. The injection of aldosterone or a potassium chloride solution, but not that of a sodium chloride solution, produced an increased net gain of sodium.
Removal of the corpuscles of Stannius was without demonstrable effect on net sodium and potassium fluxes; they remained within the normal range. Hypophysectomy reduced net sodium gain. Adrenalectomy markedly diminished or abolished net sodium gain with increased potassium loss. Net sodium gain was restored to normal values by injection of aldosterone.
These results are discussed with regard to the homeostatic mechanisms in the euryhaline teleosts.
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SUMMARY
(1) The interaction between thyroid hormone and thyroxine-binding globulin (TBG) has been studied in 39 pigs from eight litters, aged from 12 hr. to 11 days. In addition, single observations were made on six batches of 42 pigs from 1 day to 6 months old and in five newborn sheep.
(2) A rapid increase in the level of the unsaturated TBG capacity was found to occur from the 1st day after birth.
(3) Continuous changes in the unsaturated TBG capacity resulted in a decline in the plasma level of free thyroxine and in a decrease in the uptake of [131I]triiodothyronine by erythrocytes. Equilibrium between the hormonal iodine metabolism and its blood carrier is reached at the end of the 1st week of life.
(4) These findings suggest that in discussing thyroid metabolism in neonatal animals it is important to distinguish between the early neonatal adaptive period and the remainder of the period of thyroid hyperactivity. The physiological implications of the higher level of free thyroxine in the blood after birth for the adaptation to post-uterine life is discussed.
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Mitochondrial heat shock protein 70 (mtHsp70), an important mitochondrial chaperone, is increased in cardiac muscle mitochondria of hyperthyroid rats. To determine the mechanism(s) underlying this increase, we used variations in thyroid status. In Series I, rats were made hyperthyroid by injecting them with 3,3', 5-triiodo-l-thyronine (T(3)) for 5 days, or by treating them with vehicle. In Series II, animals were given 6-n-propyl-2-thiouracil in their drinking water (0.05% w/v) for a period of 32-42 days to make them hypothyroid. During the last 5 days of treatment these animals received injections of either T(3) or vehicle. T(3) treatment resulted in parallel increases in mtHsp70 protein and mRNA levels in a variety of tissues, suggesting transcriptional regulation. However, evidence of tissue-specific post-transcriptional regulation was also apparent. In isolated heart mitochondria, T(3) treatment resulted in a 1.8-fold increase in mtHsp70. This was due to the 1. 6-fold greater import of mtHsp70 into mitochondria in T(3), compared with hypothyroid animals, and it could not be attributed to an altered rate of intramitochondrial mtHsp70 degradation. The rate of processing of mtHsp70 to its mature form, reflecting mitochondrial processing peptidase activity, was unaffected by T(3), but was more rapid than mtHsp70 import. These data indicate a novel mechanism by which T(3) modifies the mitochondrial phenotype via the adaptations in the protein import pathway.
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The trafficking of retinoids in the retina represents a model to study soluble hormone-binding proteins in a complex system subject to profound evolutionary adaptations. Although a remarkable illustration of convergent evolution, all visual systems detect light in the same way, that is through the photoisomerization of an 11-cis retinoid to a corresponding trans isomer. What is strikingly different between the systems, is the mechanism by which the 11-cis chromophore is reformed and visual pigment regenerated in a process known as the visual cycle. The variations of the cycle address a problem inherent to retinoids themselves. That is, the properties that make these molecules suited for light detection also account for their susceptibility to oxidative and isomeric degradation, and cellular toxicity. The cycle therefore provides an opportunity to examine the role of soluble hormone-binding proteins within an integrative and evolutionary context. The present review focuses on interphotoreceptor retinoid-binding protein (IRBP), a controversial glycolipoprotein that recruits a protein fold common to Cterminal-processing proteases and the crotonase family. This unorthodox retinoid-binding protein is entrapped in the subretinal compartment of those eyes that translocate visual cycle retinoids between the photoreceptors and the retinal pigment epithelium. Recent studies suggest that we should look beyond a strictly carrier function if we are to appreciate the role of IRBP in the visual cycle. Here we draw lessons from other soluble hormone-binding proteins to anticipate avenues of future research likely to provide insight into the structure and function of IRBP in vision.
