Evidence for vitamin D-independent active calcium absorption in newborn piglets

Summary The role of 1,25-dihydroxycholecalciferol (calcitriol) for intestinal calcium (Ca²⁺) absorption was studied in newborn (<1 week old) and weaned piglets (>6 weeks old). In both groups, normal piglets and piglets suffering from inherited pseudo vitamin D-deficiency rickets, type I (PVDRI) were used. In this inherited disorder, renal production of calcitriol is absent. Plasma samples were assayed for calcitriol and total Ca, and dissociation constants (K_d) and maximum binding capacities (B_max) of intestinal calcitriol receptors were determined under equilibrium conditions at 4°C. Unidirectional Ca²⁺-flux rates were measured across stripped duodenal mucosae in Ussing chambers in the absence of electrochemical gradients. The plasma calcitriol concentrations of neonatal (26.5±7.1 pg/ml, n=11; ± SEM) and weaned PVDRI piglets (18.8±5.7 pg/ml, n=8)were unphysiologically low and differed significantly from control animals (83.6±14.8 pg/ml, n=8, and 86.9±9.6 pg/ml, n=11, respectively). However, newborn PVDRI piglets had normal plasma Ca levels at least during the first days of life. They became hypocalcemic and developed clinical symptoms of rickets during the following weeks. In newborn PVDRI and control piglets, B_max was significantly lower (84±28 fmol/mg protein and 127±55 fmol/mg protein, n=9, respectively) than in weaned piglets (741±82 fmol/mg protein, n=9, and 778±121 fmol/mg protein, n=8, respectively). Significant net Ca²⁺-fluxes were found in both newborn PVDRI and control piglets (88.8±25.1 nmol · cm^-2 · h^-1, n=6, and 86.5±10.5 nmol · cm⁻² · h⁻¹,n=9, respectively). However, active net Ca²⁺ absorption was completely absent in weaned PVDRI piglets. These results indicate the presence of vitamin D-independent mechanisms for active intestinal Ca²⁺ absorption during early postnatal life in pigs.     

Lymphocyte stimulation by acetylcholine receptor in polymyositis

Summary Lymphocytes of twenty-seven patients with polymyositis were incubated in vitro with cholinergic receptor rich membranes obtained from the electric organs of Torpedo Marmorata.Lymphocytes of polymyositic patients were slightly stimulated; positive responses were present mainly in patients affected from more than a year. Sensitization against the nicotinic cholinergic receptor may explain the occurrence of the myasthenic syndrome with polymyositis.Fellow of the A. Villa Rusconi foundation     

A soluble androgen receptor in the cytoplasm of the male mastomys prostate

Summary A steroid receptor protein was isolated from the cytoplasmic fraction of Mastomys prostate. Following in vivo and in vitro labelling of the tissue with tritiated testosterone or dihydrotestosterone, samples were analysed by gel exclusion chromatography or sucrose density gradient centrifugation. A steroid receptor complex was isolated on Sephadex G-200. Analysis of the steroids associated with this complex showed that the major part of the bound radioactivity was 5 -dihydrotestosterone. The binding was inhibited by unlabelled testosterone and could not be demonstrated in the liver cytosol. Using sucrose desity gradient centrifugation, the dihydrotestosterone receptor complex sedimented at 5.6 s together with heavier aggregates. In the presence of 0.4 M KCl a single complex was sedimented at 4. 6 s. The results demonstrate a receptor protein in the cytosol of the Mastomys prostate which binds to dihydrotestosterone and is comparable to that of rat prostate.     

Solubilization of a mammalian β-adrenergic receptor

Summary Binding sites for iodohydroxybenzylpindolol with characteristics of a ₂-adrenergic receptor have been identified in a crude membrane fraction from guinea-pig lung. The binding sites could be solubilized by treatment of the membrane fraction with digitonin. Upon solubilization receptors retain their ₂-adrenergic type as indicated by the rank order of potencies of agonists in binding-inhibition experiments. The solubilized receptors demonstrate a marked increase in affinity for agonists compared with particulate receptors whereas antagonist affinity remains unchanged. Solubilized receptors are insensitive to divalent cations (Mg²⁺, Mn²⁺, Ca²⁺) which increase the potency of agonists for particulate receptors. The effects of Mg²⁺ can be reversed by Gpp(NH)p in particulate preparations; Gpp(NH)p is ineffective for the solubilized preparation. These experiments establish that -adrenergic receptors can be solubilized even from crude mammalian membrane preparations. They also show that the mammalian -adrenergic receptor in situ is under constraints with respect to agonist affinity and is modulated by divalent cations and guanyl nucleotides in the intact membrane.with technical assistance of L. Braun and C. KonradThis report is part of a dissertation to be presented by J. K. to the Fachbereich Humanmedizin, Justus Liebig-Universität Giessen, in partial fulfillment of the requirements for a Doctor of Medical Science degree     

3H-Apomorphine interactions with dopamine receptors in calf brain

3H-apomorphine binds to membranes from areas of the corpus striatum and limbic system of calf brain saturable and with a drug specificity indicating that it labels dopamine receptors. In terms of drug specificity, log-logit displacement curve slopes and number of binding sites, 3H-apomorphine interacts with receptors in a manner more like 3H-dopamine than 3H-haloperidol. These properties of 3H-apomorphine binding are those of an apparently "pure" agonist in contrast to the partial agonist effects of apomorphine upon the dopamine-sensitive adenylate cyclase.     

The effect of therapy on the concentration and occupancy of androgen receptors in human prostatic cytosol

With a protamine sulphate precipitation method, total and free cytosol AR was assayed in BPH and prostatic carcinoma tissue, in order to investigate possible differences in AR concentration that might relate to the histo-pathology of the tissue or to endocrine manipulation of the patients. Similar ranges of total cytosol AR concentrations were observed in BPH and untreated prostatic carcinoma, but the latter tended to have a higher proportion of apparently free sites. Moreover, the proportion of "free" sites in the untreated carcinoma tissue appeared to be related to the proportion of poorly differentiated carcinoma in the specimen. In patients whose endogenous androgen levels had been lowered by treatment, the proportion of free sites tended to be higher, but a considerable proportion of sites appeared to be still occupied. Carcinoma tissue from some estrogen-treated patients had high cytosol AR concentrations. It is suggested that, in some treated patients, androgens of adrenal origin may occupy some AR sites and that some carcinomas may contain a considerable concentration of nonfunctional AR.     

Adenosine receptor activation by adenine nucleotides requires conversion of the nucleotides to adenosine

Summary Adenine nucleotides cause adenosine receptor-mediated increases in cyclic AMP in the VA13 human fibroblast line. Levels of adenosine accumulated in the medium are insufficient to account for the responses to adenine nucleotides. Since rapid conversion of the nucleotides to adenosine by 5-nucleotidase in the vicinity of the receptor might account for the responses, six experimental methods were developed to distinguish between local conversion and direct action of the nucleotides. Results of all six methods favored local conversion. (1) 5-Nucleotidase inhibitors blocked the accumulations of cyclic AMP elicited by AMP, ADP, and ATP, but did not affect the response to adenosine. The most potent inhibitor of both conversion of AMP and response to AMP was ,-methylene-ADP (APCP). (2) Adenosine deaminase blocked the responses to AMP, ADP, ATP, and adenosine-containing coenzymes. (3) Theophylline, a specific competitive adenosine antagonist, was an insurmountable inhibitor of the increases in cyclic AMP caused by AMP, ADP, and ATP. The insurmountability was presumably due to substrate sataration of the converting enzyme 5-nucleotidase. (4) Although ADP and ATP had partial agonist-like dose-response curves, they did not inhibit the response to adenosine. (5) Nine cell lines which responded to adenosine were tested for response to AMP. Cell lines with high levels of 5-nucleotidase had large responses to AMP, those with intermediate levels of 5-nucleotidase had large or intermediate responses to AMP, and those with low 5-nucleotidase levels did not respond to AMP. (6) Inhibition of the uptake of labelled adenosine was used as an indicator of unlabelled adenosine concentrations near the cell membrane. Unlabelled AMP inhibited uptake nearly as effectively as unlabelled adenosine. APCP reversed the inhibition by AMP but not the inhibition by adenosine.The adenosine receptor is concluded to be an enity distinct from adenine nucleotide receptors.Submitted in partial fulfillment of the requirements for the degree Doctor of Philosophy in Neurosciences, University of California, San Diego. Supported by NIMH DA-00265 and PHS RR 05665. The author has been a NSF Graduate Fellow. An abstract of this material has been published (Bruns 1977)     

Quality control of estrogen receptor assays in the Netherlands

Summary Lyophilized receptor-positive tissue powders and cytosols, prepared from calf uterus and human breast tumor tissue, are used to assess the validity of routine dextran-coated charcoal estrogen receptor assays. Since 1978 lyophilized reference preparations have been analyzed twice yearly by 18 laboratories in the Netherlands. During 8 consecutive trials 20 different lyophilized samples were studied. The inter-laboratory variability of estrogen receptor results decreased with time. Most laboratories found receptor values around the median value of all groups together, though some participants consistently reported estrogen receptor values that were higher or lower than the median. The variability of estrogen receptor results between labs seemed to be associated with cytosol dilution, determination of non-specific binding, concentration and volume of dextrancoated charcoal, and the use of single dose assays or Scatchard analysis. The agreement on the presence or absence of estrogen receptors was more than 98% for lyophilized reference samples with high receptor content. For samples with low receptor content 85% agreement was observed, while 12% of the assays performed on receptor-negative material were reported to be estrogen receptor-positive. The use of the same protein determination (Coomassie Brilliant Blue) and human serum albumin standard has decreased the interlaboratory variation coefficient of the protein results to 7.5%. Address for reprnts: A. Koenders, Dept. of Experimental and Chemical Endocrinology, St. Radboud Hospital, Geert Grooteplein Z 8, 6500 HB Nijmegen, the Netherlands List of participating laboratories and institutions: Hospital de Lichtenberg, Amersfoort; Antoni van Leeuwenhoek Hospital, Amsterdam; Foundation Medical Laboratories, Breda; Foundation of Cooperative Hospitals Delft (SSDZ), Delft; Catharina Hospital, Eindhoven; Hospital de Stadsmaten and Ziekenzorg, Enschede; Academic Hospital, Groningen; The Wever Hospital, Heerlen; Laboratory of Public Health, Leeuwarden; Department of Pathological Chemistry, Academic Hospital, Leiden; Department of Experimental and Chemical Endocrinology, Sint Radboud Hospital, Nijmegen; Scientific Development Group Organon International B.V., Oss; Department of Biochemistry II, Erasmus University Rotterdam, Rotterdam; Rotterdam Radio-Therapeutic Institute/ Dr Daniel den Hoed Clinic, Rotterdam; Institute of Oncology Dr Bernard Verbeeten, Tilburg; Department of Endocrinology, Academic Hospital, Utrecht; Laboratory for Nuclear Medicine Voorburg, Vught; Sophia Hospital, Zwolle.     

Apparent heterogeneity of cardiac A 1 adenosine receptors as revealed by radioligand binding experiments on N-ethylmaleimide-treated membranes

Summary While G protein-coupled receptors are often studied by analyzing antagonist radioligand: cold agonist inhibition curves using an independent site model, it is now clear that K_L and K_H values determined in these analyses are not reliable estimates of the affinities of the agonists for free and G protein-coupled forms of the receptor. Thus, such experiments cannot be used to contrast the characteristics of a given type of receptor in different tissues, i.e., to probe for the existence of receptor subtypes. Since treatment with N-ethylmaleimide treatment blocks receptor: G_i/G₀ protein interactions, such analyses on N-ethylmaleimide-pretreated membranes should allow direct assessment of the affinities of competing ligands for the free receptor or for multiple receptor subtypes.As A₁ adenosine receptors couple to G_i, and perhaps to G_o, we have performed A₁ adenosine receptor radio-ligand competition studies first on control, then on N-ethylmaleimide-pretreated bovine cardiac and cerebral cortical membranes. Results of experiments with the antagonist radioligand [³H]xanthine amine congener appeared to be confounded by ligand binding to A₂ adenosine receptors present in the cardiac membrane preparations. Further experiments utilized the A₁-specific radioligand [³H] 1,3-dipropyl-8-cyclopentylxanthine. These experiments confirmed once more that the K_L values determined by computer analysis of competition curves performed on control membranes are not reliable estimates of the affinities of the competing ligand for free receptors. Furthermore the results supported the hypothesis that similar analyses on NEM-treated membranes provide reliable estimates of the affinity(s) of competing ligands for free receptors. Lastly, the results suggest that cardiac membranes contain two subtypes of A₁ adenosine receptors that are differentiated by 5-modified but not N⁶-modified adenosine analogs. One of these receptor subtypes appears to be the same as the A₁ receptor detected in cortical membranes.Abbreviations [¹²⁵I]ABA [1251](N⁶-p-aminobenzyl)adenosine - [³H]CPX [³H]8-cyclopentyl-1,3-dipropylxanthine - [³H]R-PIA [³H]N⁶-R-phenylisopropyladenosine - [⁶H]XAC [⁶H]xanthine amine congener - CHA N⁶-cyclohexyladenosine - EDTA ethylenediaminetetraacetic acid - [¹²⁵I]BW-A844U [¹²⁵I]3-(4-amino)phenethyl-l-propyl-8-cyclopentylxanthine - NCCA 5-N-cyclopropylcarboxamide adenosine - NECA 5-N-ethylcarboxamide adenosine - PMSF phenylmethylsulphonyl fluoride - NEM N-ethylmaleimide Recipient of a Postdoctoral Fellowship from the Chicago Heart Association     

Endothelium-derived bradykinin is responsible for the increase in calcium produced by angiotensin-converting enzyme inhibitors in human endothelial cells

Summary The effects of angiotensin-converting enzyme (ACE) inhibitors on intracellular calcium concentration ([Ca²⁺]_i) were examined under resting conditions and after stimulation with bradykinin in cultured human umbilical vein endothelial cells. The ACE inhibitors ramiprilat and enalaprilat (0.3 M) enhanced the increase in [Ca²⁺]_i elicited by bradykinin (3 nM) and also caused an increase in resting [Ca²⁺]_i when given alone. This increase in resting [Ca²⁺]_i was long-lasting and accompanied by an increased formation of nitric oxide, as assessed by a N^G-nitro-l-arginine-sensitive cyclic GMP accumulation in the cells. Both increases in resting [Ca²⁺]_i and nitric oxide production by ACE inhibitors were inhibited by preincubation of the cells with the B₂-receptor antagonist Hoe 140. These data indicate that ACE inhibitors are able to unmask a release of bradykinin from cultured human endothelial cells. This endothelium-derived bradykinin can exert an autocrine function by stimulating endothelial B₂-receptors with a subsequent increase in [Ca²⁺]_i and nitric oxide formation. Send offprint requests to R. Busse at the above address