Agonist-induced functional desensitization of the mu-opioid receptor is mediated by loss of membrane receptors rather than uncoupling from G protein

The effects of acute exposure of the opioid peptide [D-Ala2,N-MePhe4, Gly-ol5]enkephalin (DAMGO) on the mu-opioid receptor were examined in Chinese hamster ovary (CHO) K-1 and baby hamster kidney stable transfectants. In the CHO cell line, acute 1-hr treatment with DAMGO decreased the density of receptors without affecting the affinity or proportion of agonist-detected sites and attenuated the ability of the agonist to inhibit forskolin-stimulated cAMP accumulation. In contrast, similar 1-hr treatment of baby hamster kidney cells did not affect receptor density or agonist ability to inhibit cAMP accumulation, but longer duration of agonist exposure resulted in a reduction in membrane receptor, identical to the CHO cells. These results suggested that for the mu-opioid receptor, alteration in receptor density was the major determinant for the observed agonist-induced desensitization. Consistent with this notion, the ratio of the DAMGO concentration yielding half-maximal occupation of the mu receptor to that yielding half-maximal functional response was < 1. This suggests the necessity for a high mu receptor occupancy rate for maximal functional response, so that any loss of cell surface opioid-binding sites was a critical determinant in reducing the maximal response. This hypothesis was further supported by the observation that irreversible inactivation of fixed proportions of opioid-binding sites with beta-chlorn-altrexamine demonstrated that there were few spare receptors, which is in contrast to what has been reported for other G protein-coupled receptors, including the delta-opioid receptor. Taken together, these data suggest that the opioid agonist DAMGO has a high affinity for the mu receptor but must occupy > 70% of the available receptors to generate the maximal second messenger-linked response.

Effects of growth hormone on rat renal epidermal growth factor expression

The kidney is a site of synthesis for several polypeptide growth factors including epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I). Interactions between growth hormone (GH) and growth factors have been described that regulate renal growth factor expression. For example, GH and EGF each enhances the expression of IGF-I in kidney. To further define interrelationships in this renal GH-growth factor axis, we characterized the effect of GH on renal EGF expression in hypophysectomized, pituitary-intact (normal) rats, and hypersomatotropic rats. Levels of extractable immunoreactive mature EGF, levels of a 142-kDa EGF-precursor present in renal membrane fractions, and levels of EGF mRNA were significantly reduced in kidneys from hypophysectomized rats compared with levels in normal rats. Each was increased significantly after the administration of GH to hypophysectomized rats. In contrast, induction of hypersomatotropism in normal rats by injection of GH for 17 days did not affect levels of extractable mature EGF or EGF mRNA measured in kidneys. We conclude that GH enhances the renal synthesis of EGF in hypopituitary, but not in hypersomatotropic states.

Time course of postoperative hypoxaemia

OBJECTIVE

To study the time course of nocturnal episodic and constant hypoxaemia during the first five nights after a major abdominal operation in patients not given supplementary oxygen.

DESIGN

Open study.

SUBJECTS

17 patients undergoing major elective operations and with no recognised risk factors.

MAIN OUTCOME MEASURES

Arterial oxygen saturation measured by pulse oximetry.

RESULTS

The level of constant hypoxaemia was lowest during night 2 (p < 0.01) as was the time spent below 90% saturation (p < 0.01) compared with the other four postoperative nights. Episodic hypoxaemia was most common during night 3 (p < 0.05). It was not possible to predict the postoperative occurrence of hypoxaemia from the preoperative spirometric measurements. Preoperative awake arterial oxygen saturation correlated significantly with mean constant hypoxaemia on postoperative nights 1, 3, 4, and 5, but not with episodes of sudden desaturation after operation. Preoperative overnight oximetry correlated significantly with constant hypoxaemia on all five postoperative nights and with episodic hypoxaemia on nights 3 and 4.

CONCLUSION

In the light of these results which show the natural history of postoperative hypoxaemia without supplementary oxygen, and because postoperative hypoxaemia may be associated with dysfunction of organ systems after major operations, controlled studies of supplementary oxygen in the late postoperative period are warranted.

Method for determination of intracellular sodium in perfused cancer cells by 23Na nuclear magnetic resonance spectroscopy

Changes of intracellular sodium concentrations are often an indication of disease or malfunction. In this work, shift reagent-aided 23Na NMR spectroscopic determination of intracellular sodium was adapted to measurements with perfused cells embedded in agarose gel threads. Ehrlich ascites tumor cells (EHR2) and their multidrug-resistant counterparts (EHR2/DNR+) were immobilized and perfused until the metabolic steady state had been reached as shown by 31P NMR spectroscopy. Subsequent addition of 5 mM dysprosium(III) bis(tripolyphosphate) to the perfusion medium caused a separation of extracellular and intracellular 23Na NMR signals, making quantification of the intracellular sodium possible. The dysprosium shift reagent was apparently nontoxic to the cells, as shown by the unchanged level of ATP and other intracellular phosphates. NMR visibility of the intracellular sodium was determined in suspensions of EHR2 and EHR2/DNR+ cells by treatment with digitonin; the increase of intensity of the extracellular sodium resonance observed after the digitonin treatment corresponded well (97 +/- 3%) to the sum of intracellular and extracellular sodium observed with intact cells prior to the digitonin treatment. The resistant EHR2/DNR+ cells contained a moderately higher intracellular sodium level than the wild-type EHR2 cells, 1.02 +/- 0.10 and 0.77 +/- 0.07 mumol Na/mg protein, respectively. Closely similar levels of intracellular sodium were found by flame photometry. Thus, 23Na NMR offers a reliable method for noninvasive quantification of intracellular sodium in perfused cancer cells.

Recovery of pulmonary diffusing capacity after maximal exercise

Pulmonary diffusing capacity (DICO), together with spirometric variables, arterial oxygen tension (paO2) and cardiac output were determined before and at intervals after maximal arm cranking, treadmill running and ergometer rowing. Independent of the type of exercise, D1CO increased immediately post-exercise from a median 13.6 (range 7.3-16.3) to 15.1 (9.3-19.6) mmol min-1 kPa-1 (P < 0.01). However, it decreased to 11.6 (6.9-15.5) mmol min-1 kPa-1 (P < 0.01) after 24 h with cardiac output and paO2 at resting values, and D1CO normalized after 20 h. Thoracic electrical impedance at 2.5 and 100 kHz increased slightly post-exercise, indicating a decrease in thoracic fluid balance, and there were no echocardiographic signs of left ventricular failure at the time of the decrease in D1CO. Also, active muscle (limb) circumference and volume, and an increase in haematocrit from 43.8 (38.0-47.0) to 47.1 (42.7-49.8) (P < 0.01), had normalized at the time of the decrease in D1CO. Vital capacity, forced vital capacity, forced expiratory volume in 1 s, peak and peak mid-expiratory flows did not change. However, total lung capacity increased from 6.8 (5.0-7.6) to 7.0 (5.1-7.8) litres (P < 0.05) immediately after exercise and remained elevated at 6.9 (5.1-8.7) litres (P < 0.05) when a decrease in D1CO was noted. The results demonstrate that independent of the type of maximal exercise, an approximate 15% reduction in D1CO takes place 2-3 h post-exercise, which normalizes during the following day of recovery.

Proline-specific endopeptidases from microbial sources: isolation of an enzyme from a Xanthomonas sp

An extensive screening among microorganisms for the presence of post-proline-specific endopeptidase activity was performed. This activity was found among ordinary bacteria from soil samples but not among fungi and actinomycetes. This result is in contrast to the previous notion that this activity is confined to the genus Flavobacterium. A proline endopeptidase was isolated from a Xanthomonas sp. and characterized with respect to physicochemical and enzymatic properties. The enzyme is composed of a single peptide chain with a molecular weight of 75,000. The isoelectric point is 6.2. It is inhibited by diisopropylfluorophosphate and may therefore be classified as a serine endopeptidase. The activity profile is bell shaped with an optimum at pH 7.5. By using synthetic peptide substrates and intramolecular fluorescence quenching it was possible to study the influence of substrate structure on the rate of hydrolysis. The enzyme specifically hydrolyzed Pro-X peptide bonds. With Glu at position X, low rates of hydrolysis were observed; otherwise the enzyme exhibited little preference for particular amino acid residues at position X. A similar substrate preference was observed with respect to the amino acid residue preceding the prolyl residue in the substrate. The enzyme required a minimum of two amino acid residues toward the N terminus from the scissile bond, but further elongation of the peptide chain by up to six amino acid residues caused only a threefold increase in the rate of hydrolysis. Attempts to cleave at the prolyl residues in oxidized RNase failed, indicating that the enzyme does not hydrolyze long peptides, a peculiar property it shares with other proline-specific endopeptidases.

Use of the signal peptide of Pisum vicilin to translocate beta-glucuronidase in Nicotiana tabacum

A hybrid protein system was used for the study of protein transport in plant cells. A nucleotide sequence (vic) encoding a putative signal peptide of 15 amino acid residues, derived from the published aa sequence of one Pisum vicilin, was synthesized and fused in frame to the gus gene encoding a bacterial cytosolic beta-glucuronidase (GUS). When the hybrid vic::gus gene was expressed in tobacco cells using the cauliflower mosaic virus 35S promoter, the hybrid GUS protein was targeted to, and glycosylated inside the rough endoplasmic reticulum. Glycosylation could be blocked with the antibiotic tunicamycin. The study of transient expression in protoplasts showed that extracellular secretion efficiency was low, which may be due to the nature of the GUS protein.

Iododeoxyuridine incorporation and radiosensitization in three human tumor cell lines

Iododeoxyuridine is a halogenated pyrimidine and non-hypoxic cell radiosensitizer currently being used in clinical trials. The amount of radiosensitization by IdUrd is related to the amount of incorporation of the drug into a cell's DNA. These experiments were carried out in three human tumor cell lines (lung, glioma, and melanoma) in monolayer culture exposed to concentrations of IdUrd from 0.1-10 microM for one and three cell cycles before irradiation to determine incorporation and sensitization as a function of drug exposure. Except for the lung cell line, which required greater than 1 microM IdUrd, these cells demonstrate radiosensitization when exposed to 0.1 microM or greater of IdUrd. Maximum sensitization occurred at 10 microM IdUrd for all the cell lines at three cell cycles. The percent thymidine replacement by IdUrd increased with increasing concentrations, but was cell line dependent. Maximum percent replacement occurred at 10 microM at three cell cycles for all the cell lines: lung = 22.4%, glioma = 32.0%, and melanoma = 39.1%. The relationships between percent thymidine replacement and sensitization are not identical across these human tumor cell lines. If IdUrd is going to be a successful radiosensitizer in clinical trials, sustained plasma levels of 10 microM or greater for at least three cell cycles should be achieved during irradiation. This may be best accomplished with repeated short exposures to IdUrd (three cell cycles or approximately 4 days in these cell lines) every 1-2 weeks during radiation. Measurements of thymidine replacement in a tumor biopsy should be attempted prior to radiation to develop a predictive assay for radiosensitization.

Prevalence of antibiotic-resistant Escherichia coli in Danish pigs and cattle

The present paper reports on the antibiotic resistance of E. coli isolated from Danish piglets and calves in 1987-1988, and compares the results with similar investigations performed during the periods 1971-1972 and 1977-1978. Rectal swabs from 52 piglets and from 78 calves were examined. All the animals studied harboured resistant E. coli. This is a significant increase compared to the previously conducted investigations. The number of strains having three or more resistance markers did not differ significantly from the previous findings. The spectrum of resistance markers among Danish piglets and calves had increased through all three investigations and resistance to chloramphenicol was still found to be considerable 10 years after the withdrawal of chloramphenicol as a therapeutic drug for farm animals in Denmark in 1978. Certain resistance patterns (sulfonamide + streptomycin, sulfonamide + streptomycin + tetracycline, sulfonamide + streptomycin + tetracycline + ampicillin) were found to be shared by numerous strains, suggesting a genetic linkage of the resistance markers.

Muscle mass effect on arterial desaturation after maximal exercise

We measured arterial oxygen saturation before and immediately after randomly allocated 6 min of "all-out" maximal arm cranking, treadmill running, and ergometer rowing in 10 men and women with a median maximal oxygen uptake of 4.47 (range 3.22-5.34) 1.min-1. Arterial saturation for oxygen was unaltered after arm cranking, but decreased 1.7 (-2.5-6.0) % (P less than 0.05) after running, and 2.2 (1.0-8.7) % (P less than 0.01) after rowing. Arterial saturation was inversely related to capillary blood lactate, which reached 11.8 (7.4-14.0), 12.6 (8.9-18 2), and 14.3 (12.0-19.3) mmol.l-1 (P less than 0.01), respectively, and arterial bicarbonate fell to 15.0 (13.0-23.6), 12.4 (7.2-20.4), and 10.8 (0.0-12.5) mmol.l-1 (P less than 0.01). Thus, pH decreased to 7.25 (7.22-7.40), 7.17 (6.95-7.35), and 7.09 (6.84-7.19) (P less than 0.01). When measured immediately post-exercise, arterial oxygen tension was unchanged or elevated from rest, eliminating the possibility that the arterial desaturation was caused by a pulmonary diffusion limitation. The results of this investigation show that arterial desaturation associated with maximal exercise takes place in proportion to the involved muscle mass, as do deviations in blood lactate, bicarbonate, and hydrogen concentrations.

Human error and the problem of causality in analysis of accidents

Present technology is characterized by complexity, rapid change and growing size of technical systems. This has caused increasing concern with the human involvement in system safety. Analyses of the major accidents during recent decades have concluded that human errors on part of operators, designers or managers have played a major role. There are, however, several basic problems in analysis of accidents and identification of human error. This paper addresses the nature of causal explanations and the ambiguity of the rules applied for identification of the events to include in analysis and for termination of the search for 'causes'. In addition, the concept of human error is analysed and its intimate relation with human adaptation and learning is discussed. It is concluded that identification of errors as a separate class of behaviour is becoming increasingly difficult in modern work environments. The influence of this change on the control of safety of large-scale industrial systems is discussed.

Effect of buffer systems and pHi on the measurement of [Ca2+]i with fura 2

The fluorescent probe, fura 2, is widely used to measure agonist-induced changes in intracellular calcium concentration ([Ca2+]i) in cultured cells. However, in many instances, the results obtained in the same cell type have differed from one study to the next. The possibility that such differences might be due to experimental conditions was examined by using fura 2 in four different cell types responding to appropriate agonists when the cells were incubated in either CO2/HCO3-- or HEPES-buffered media. Examined were: 1) the response of rat glomerular mesangial cells to arginine vasopressin, 2) the response of vascular smooth muscle cells to angiotensin II, 3) the response of adrenal glomerulosa cells to angiotensin II, and 4) the response of hypothalamic cells to insulin-like growth factor-1. In each cell type there was a significant difference in the pattern of agonist-induced change in [Ca2+]i when HEPES vs. CO2/HCO3- was used as the buffer system: in HEPES buffer, agonist addition led to a transient rise in [Ca2+]i followed by a fall to a sustained plateau 27 to 34 nM higher than the original basal value, whereas in CO2/HCO3- buffer, agonist addition led to an identical transient increase in [Ca2+]i followed by a fall to a value within 10 nM or less of the preagonist level. The plateau value of [Ca2+]i in the different buffers was examined in relationship to known differences in intracellular pH (pHi). It was found that measurements of [Ca2+]i with fura 2 were influenced by shifts in pHi that occur when cells are incubated in either HEPES-buffered or CO2/HCO3- media of differing pHo values. However, at any given value of pHi, the apparent [Ca2+]i measured in cells incubated in HEPES-buffered media was slightly higher than in cells incubated in CO2/HCO3- buffered media.