Sustained sensitivity modifications induced by brief length perturbations in the crayfish slowly adapting stretch receptor

These experiments in the slowly adapting stretch receptor of crayfish test the effects of brief length perturbations (i.e., pulses) when presented in isolation at different constant elongations or superimposed on trapezoidal stretches of different amplitudes. Within "in vivo" lengths, during static responses, perturbations reduced firing rates to below control values and, in extreme cases, could silence the receptor. This effect, or "down-step," was sustained, occurred above a threshold pulse amplitude and background stretch, and increased with both stimulus characteristics, but was not present during dynamic responses. Beyond "in vivo" lengths, and in a few cases within those limits but close to the extremes, the receptor was silent but perturbations could restore activity. Lengthening pulses were more effective than shortening ones in generating after-effects. Perturbations change, during indefinitively long periods, the receptor's length or static sensitivity acting as a negative feedback which tends to maintain the discharge rate within fixed values. Perturbations disclose marked nonlinearities, which suggest that the classical view of a proportional control in the reflex loop in which the receptor participates may not operate in natural conditions.     

Effects of hyperthermia on DNA interstrand crosslinking after treatment with BCNU in 9L rat brain tumor cells

The effects of hyperthermia (42 degrees C) on 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU)-mediated DNA interstrand crosslink formation were investigated in 9L rat brain tumor cells using the technique of alkaline elution. When cells were treated with 60 microM BCNU for 1 hr at 37 degrees C and incubated for 6 hr in drug-free medium at 42 degrees C, there was a 50% increase in crosslinking; and when cells were treated at 42 degrees C and incubated at 37 degrees C, there was a 45% increase in crosslinking compared with the results for cells treated and incubated at 37 degrees C. When cells were treated and incubated at 42 degrees C, there was a 129% increase in DNA crosslinking. The same relative order of results was found for cell survival. These results suggest that hyperthermia can increase DNA interstrand crosslink formation and the consequent cell death through two independent mechanisms: an increase in the amount of initial alkylation because of the increased rate of hydrolysis of BCNU at higher temperatures, and the effect of heat on DNA structure that leads to an increase in the number of crosslinks formed.     

Action of DCCD on the H+/O stoichiometry of mitoplast cytochrome c oxidase

The mechanistic H+/O ejection stoichiometry of the cytochrome c oxidase reaction in rat liver mitoplasts is close to 4 at level flow when the reduced oxidase is pulsed with O2. Dicyclohexylcarbodiimide (DCCD) up to 30 nmol/mg protein fails to influence the rate of electron flow through the mitoplast oxidase, but inhibits H+ ejection. The inhibition of H+ ejection appears to be biphasic; ejection of 2-3 H+ per O is completely inhibited by very low DCCD, whereas inhibition of the remaining H+ ejection requires very much higher concentrations of DCCD. This effect suggests the occurrence of two types of H+ pumps in the native cytochrome oxidase of mitoplasts.     

Further characterization of [3H]fulunitrazepam binding sites on cultured mouse astroglia

Astroglial cells in primary cultures bind [³H]flunitrazepam with a high affinity on a single type of site and on a number of binding sites which increased during astroglial growth and differentiation. These binding sites show a particular pharmacological spectrum characterized by an inhibition of high affinity by RO-5-4864 (4-chlorodiazepam), an anticonvulsant of the benzodiazepine family and by an inhibition of binding of lower affinities by diazepam clonazepam and clobazam. RO-5-4864 and clonazepam compete for the same binding site in astroglia. The heat stability and the hormonal modulation by thyroxine are similar for astroglia and neuronal-cells. Benxodiazepines modulate the astroglial 5-HT receptor. Such an effect could be a possible physiological response to benzodiazepines for astroglial cells in primary cultures.     

New enzymatic changes of L-cystathionine catalyzed by bovine tissue extracts

L-Cystathionine was used as substrate for enzyme systems prepared by heating bovine tissue extracts in the presence of pyruvate at 60 degrees C for 10 min. Analysis of the products indicated that the systems converted L-cystathionine into the cyclic ketimine form which was detected by its spectral properties and by chromatography on the amino acid analyzer. Alanine, alpha-aminobutyrate and cystine were also produced. Pyruvate and alpha-ketobutyrate enhance the production of the ketimine by liver, kidney and heart extracts, and are necessary for the brain extracts: alpha-Ketoglutarate is much less effective and its presence favors the production of homocystine by all the extracts. Homocystine was found in the brain incubates when any of the ketoacids assayed were added. The overall reaction is explained by the action of heat stable cystathionine gamma-lyase and beta-synthase which produce alpha-ketobutyrate and pyruvate used for the transamination of the remaining cystathionine to the monoketoacid. This last compound cyclizes spontaneously into the ketimine form thus avoiding the removal of the second amino group. This represents a new nontransulfurative path leading to the production of a seven membered etherocyclic product whose biochemical implications are yet unexplored.     

The thermodynamics of complex formation of cyclic tetra-aza-tetracetic acids

Enthalpy changes for the complexation of alkaline-earth and transition metals with three cyclic tetra-aza-tetracetic acids (cDOTA, cTRITA and cTETA) were obtained by continuous titration calorimetry. From these values and free energy data, the entropy changes for the same reactions were derived. The results show that these complexes are stabilised by both favourable enthalpy and entropy changes, except those of Mg²⁺ and those of Sr²⁺ and Ba²⁺ with cTETA. Generally, the entropy changes for the reactions of the alkaline-earth metals are higher than for the reactions of the non cyclic polyaminocarboxylic acids, but for the reactions of the transition metals the entropy changes are comparable for the cyclic and non cyclic ligands. These results are discussed in terms of a model of ‘cage’ coordination of the metals.The enthalpy changes decrease with the increase in size of the tetra-aza ring (except in the case of Cu²⁺) but no specific cavity size effect is noticeable. Consideration of the temperature-dependent and temperature-independent contributions to ΔH supports the idea that the number of coordinated nitrogen atoms and carboxylate groups vary along the series.     

Chronic administration of an organophosphorus insecticide to rats alters cholinergic muscarinic receptors in the pancreas

Male rats were treated for 10 days with the organophosphorus insecticide, acetylcholinesterase inhibitor, O,O-diethyl S-[2-(ethylthio)ethyl]phosphorodithioate (disulfoton, 2 mg/kg/day by gavage). At the end of the treatment, binding of [³H]quinuclidinyl benzilate ([³H]QNB) to cholinergic muscarinic receptors and cholinesterase (ChE) activity were assayed in the pancreas. Functional activity of pancreatic muscarinic receptor was investigated by determining carbachol-stimulated secretion of α-amylase in vitro. ChE activity and [³H]QNB binding were significantly decreased in the pancreas from disulfoton-treated rats. The alteration of [³H]QNB binding was due to a decrease in muscarinic receptor density with no change in the affinity. Basal secretion of amylase from pancreas in vitro was not altered, but carbachol-stimulated secretion was decreased. The effect appeared to be specific since pancreozymin was able to induce the same amylase release from pancreases of control and treated rats. The results suggest that repeated exposures to sublethal doses of an organophosphorus insecticide lead to a biochemical and functional alteration of cholinergic muscarinic receptors in the pancreas.     

Age-related and diurnal changes in Met5-Enk-Arg6-Phe7 and Met5-enkephalin contents of pituitary and rat brain structures

The beta-endorphin, met5-enkephalin-arg6-phe7 (MEAP) and met5-enkephalin (ME) changes related to age and diurnal rhythms were studied in various regions of rat brain and in the pituitary by specific radioimmunoassays. The contents of MEAP, met5-enkephalin and beta-endorphin were higher in the pituitary of old rats (18 months old) than that of young rats (23 days old) while the content of these opioid peptides was higher in the hypothalamus of young rats than in that of old rats. Beta-endorphin was also higher in the striatum of 23 days old rats, but no age-associated changes were observed in the hippocampus, brain stem or cortex. In the diurnal rhythm study, it was found that in the hypothalamus and striatum of the adult rat (2-3 months old), both MEAP and ME contents were higher at mid-dark than at mid-light and that in the intermediate posterior lobe of the pituitary, the ME content was also higher at mid-dark.     

Toxicity oft-butylhydroperoxide in hepatocyte monolayers exposed to hypoxia and reoxygenation

Summary Inasmuch as it is known that the toxicity of anesthetic agents is potentiated by hypoxia and that the reductive metabolism of these agents results in the formation of lipid hydroperoxides, we investigated the toxicity of hydroperoxides under low-oxygen concentrations. We found that hypoxia exacerbates the toxicity oft-butyl hydroperoxide, shifting the dose-response curve oft-butyl hydroperoxide vs. lysis of hepatocytes approximately an order of magnitude to the left. Furthermore, although at the end of a 4-h exposure to 0.5% O₂ hepatocyte monolayers seemed normal by three indices (release of⁵¹Cr and serum glutamate transaminase or exclusion of trypan blue), they were completely lysed after an additional 20 h reoxygenation at 20%. O₂. In contrast, monolayers exposed to 2% O₂ for 4 h seemed normal after 20 h reoxygenation. However, cells exposed to both a subtoxic dose of hydroperoxide and 4 h of 2% O₂, although seeming healthy at the end of the hypoxic period, were completely lysed within 20 h after reoxygenation. The study was supported by grant OH 00978 from the National Institutes for Occupational Safety and Health, Atlanta, Georgia.