The effect of cholinesterase inhibitors and corticosteroids on rat nerve-muscle preparations treated with hemicholinium-3

Dexamethasone affects radioactive choline uptake in rat diaphragm nerve endings and not in muscle fibres

The initial rate of radioactive choline (Ch) uptake in the endplate-rich area (EPA) of both stimulated and unstimulated hemidiaphragms is significantly increased by 0.2 microM dexamethasone (Dex) in the presence of 10 microM Ch. In autoradiographs, the mean grain densities above the muscle fibres are not altered by Dex. The mean grain densities above the nerve endings are significantly increased in the presence of Dex in stimulated tissue, and slightly but not significantly increased in unstimulated tissue. There is a positive correlation between the initial rate of Ch uptake in the EPA and the amount of isotope in the nerve terminals, in the absence and presence of Dex. Without correcting for the large amount of diffusion which occurs, the ratio of the grain densities above the nerve terminals to that above the muscle fibres in the presence of Dex is 2.12 in stimulated tissue, and 1.40 in unstimulated tissue. The ratio in the stimulated tissue is significantly greater than the control ratio in the absence of Dex (1.66). Therefore, Dex affects radioactive Ch uptake in nerve endings and not in muscle fibres in the rat diaphragm. The stimulation-induced increase in the uptake of isotope into the nerve endings is abolished in a Na+-depleted medium, and in the absence of Ca2+. Dex has no effect on this abolition. We conclude that relatively low concentrations of Dex affect Ch transport in rat diaphragm nerve endings by a mechanism as yet to be defined.

Depletion of total acetylcholine by hemicholinium-3 in isolated rat diaphragm is less in the presence of dexamethasone

Low concentrations of dexamethasone (Dex) stimulate the initial rate of radioactive choline (Ch) accumulation in the endplate-rich area (EPA) of indirectly stimulated hemidiaphragms, while higher concentrations (greater than 0.6 microM) inhibit. This biphasic concentration-effect curve is found even in the presence of 26 microM hemicholinium-3 (HC-3), an inhibitor of Ch accumulation. In incubations (3 min) where the total hemidiaphragm acetylcholine (ACh) content is not altered by 26 microM HC-3, the inhibition by HC-3 of both the Ch accumulation rate and the incorporation of radioactive Ch into ACh in the EPA of stimulated tissues is less in the presence of 0.2 microM Dex. In 120 min incubations with 15 microM HC-3 and without added Ch, the tissue ACh content is depleted in both stimulated and unstimulated hemidiaphragms. In both cases the depletion of ACh is significantly less in the presence of 0.2 microM Dex. In stimulated tissues a comparable depletion of ACh due to 15 microM HC-3 is also found with 1 and 10 microM Ch added to the medium. It is significantly less when 0.2 microM Dex and 1 microM Ch are added to the medium. In 120 min incubations with stimulated tissue, the amount of "bound' ACh is increased by addition of 30 microM Ch to the medium, decreased in the presence of 0.2 microM Dex, and greatly decreased in the presence of 15 microM HC-3. In the presence of Dex plus HC-3, the decrease in the amount of "bound' ACh due to either Dex or HC-3 alone, is abolished provided that 30 microM Ch is also present.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in high affinity choline accumulation rate in rat cerebral cortex during anaesthesia with ketamine and pentobarbital

After i.p. injection of ketamine (75 mg/kg) or pentobarbital-Na (40 mg/kg) to rats, there was a rapid, then a steady decrease of the sodium-dependent high affinity choline (HAC) accumulation rate to a minimum. This minimum was followed by a rapid increase (ketamine) or a gradual rise (pentobarbital-Na). Immediately after the rats came out of anaesthesia, the accumulation rate had not yet completely recovered. We suggest the ketamine or pentobarbital-Na induce alterations in central cholinergic systems, i.e. changes in choline uptake and incorporation into acetylcholine. It is conceivable that interactions of cholinesterase inhibitors or corticosteroids with the anaesthetics are based upon some modification of these changes.

Interactions of cholinesterase inhibitors and glucocorticoids with ketamine and pentobarbitone-induced general anaesthesia in the rat: possible effects on central cholinergic activity

Doses of 100, 150 and 200 micrograms kg-1 of the cholinesterase inhibitor neostigmine reverse the anaesthetic action of ketamine. The antagonistic effect is increased as the dose is increased. The duration of anaesthesia induced by pentobarbitone is reversed by the cholinesterase inhibitor in doses of 150, 200 and 250 micrograms kg-1. Choline, in a dose of 50 mg kg-1, significantly antagonizes the action of the two anaesthetics, whereas hemicholinium-3, an inhibitor of the uptake of choline and the synthesis of acetylcholine, markedly potentiates their action. Dexamethasone induces a significant reduction of the duration of anaesthesia produced by ketamine and pentobarbitone. The potentiation of the anaesthetic effect caused by hemicholinium-3 is also reversed by dexamethasone. The acetylcholine content in rat cerebral cortex is increased after treatment with ketamine and pentobarbitone. Measurements of the course of the plasma level of pentobarbitone do not reveal alterations in the pharmacokinetic profile by either neostigmine or dexamethasone. These results indicate that central cholinergic systems may somehow be involved in the anaesthesia induced by ketamine and pentobarbitone and that the interactions described in this paper may be the result of modification by neostigmine and dexamethasone of the alterations in cholinergic activity caused by the two anaesthetics.

Effects of corticosteroids on neuromuscular blocking actions of d-tubocurarine

Dexamethasone (50 microgram/kg) significantly increased the LD50 of d-tubocurarine (d-TC) when administered i.p. simultaneously with d-TC. Choline (50 and 100 mg/kg) gave some protection against the lethal effects of d-TC and the cholinesterase inhibitors neostigmine (250 microgram/kg) and physostigmine (1000 microgram/kg) provided full protection against doses of d-TC twice the LD50. The blocking effect of d-TC (75 microgram/kg) on the sciatic nerve-tibialis anterior muscle preparation was antagonized by dexamethasone. Prednisolone delayed the occurrence of a complete neuromuscular block caused by d-TC in the phrenic nerve-diaphragm preparation, and antagonized the effect of d-TC on short tetanic contractions. d-TC (5 mumol/l) inhibited the [14C]choline uptake in the endplate-rich region of the rat diaphragm during stimulation. This inhibition was antagonized by dexamethasone as well as by physostigmine. The incorporation of radioactive choline into acetylcholine was inhibited in the presence of d-TC (15 mumol/l), and both dexamethasone and physostigmine counteracted this inhibition. It is concluded from these experiments that d-TC very probably has an effect on the choline carrier system. These experimental results support the hypothesis that glucocorticoids may improve reduced muscle performance by direct presynaptic effects at the neuromuscular junction.