Effect of glucose administration on cholesterol and bile acid metabolism in rats

Glucose administered to fasted rats caused a marked stimulation in hepatic cholesterogenesis and cholesterol 7 alpha-hydroxylation, and an increase in biliary excretion of cholesterol and total bile acids. The excretion of cholic acid was not incluenced during the first few hr after glucose administration, but was significantly increased after 5 hr. Chenodeoxycholic acid showed a similar change, but the increase was only ca. one tenth of that of cholic acid. The excretion of deoxycholic acid was markedly increased by 1 hr, but gradually decreased thereafter. Pretreatment with neomycin abolished the increase in deoxycholic acid by fasting and glucose administration. Other bile acid components showed no significant change. It thus was presumed that cholesterol endogenously synthesized in the liver was metabolized mainly to cholic acid. In contrast, exogenous cholesterol was metabolized mainly to chenodeoxycholic acid. During the period of the acute enhancement of cholic acid formation from the endogenous cholesterol, biliary excretion of deoxycholic acid was increased. This probably occurred through the depression of 7 alpha-rehydroxylation of deoxycholic acid, or through the enhancement of microbial formation of deoxycholic acid in the lumen, and through the increase of intestinal absorption.

Effect of modification of thyroid function on cholesterol 7 alpha-hydroxylation in rat liver

Hepatic activities of cholesterol synthesis and cholesterol 7 alpha-hydroxylation were determined in hyper- and hypo-thyroid rats after oral administration of glucose or cholesterol. Increases in activities of both cholesterol synthesis and cholesterol 7 alpha-hydroxylation induced by glucose administration were enhanced by pretreatment with thyroid powder but suppressed by pretreatment with thiouracil. The enhancement of 7 alpha-hydroxylation was produced by a relatively small amount of thyroid powder, but high doses were required to increase cholesterol synthesis. On the other hand, the suppression of 7 alpha-hydroxylation was brought about by a low dose of thiouracil, but high doses were required to decrease cholesterol synthesis. Although cholesterol synthesis increased similarly in both hypo- and hyper-thyroid rats after glucose administration, hydroxylase activity in hypothyroid rats began to increase more slowly and was always lower than that in hyperthyroid rats. Thus it is concluded that cholesterol 7 alpha-hydroxylase activity is more sensitive to thyroid function than are activities of cholesterol-synthetic enzymes. When exogenous cholesterol was given, hypothyroid rats showed a larger increase in serum cholesterol concentration than hyperthyroid rats, and there was an inverse relationship between serum cholesterol concentrations and hepatic cholesterol 7 alpha-hydroxylase activities.

Variations in the permeability properties of the inhibitory post-synaptic membrane of the crayfish neuromuscular junction when activated by different concentrations of GABA

1. The membrane conductance of the crayfish muscle was measured with intracellular micro-electrodes. Increase in the membrane conductance induced by various concentrations of gamma-amino butyric acid (GABA) was measured in the Br(-), NO(3) (-), I(-) and CNS(-) solutions and compared with that in Cl(-) solution. When the membrane was activated by lower concentrations of GABA, the resulting membrane conductance in NO(3) (-) and I(-) solutions was larger than in Cl(-) solution, while the membrane conductance activated by higher concentrations in NO(3) (-) and I(-) solutions were smaller than in Cl(-) solution. The membrane conductance activated by GABA was larger in Br(-) solution and smaller in CNS(-) solution than the membrane conductance activated in Cl(-) solution, throughout the concentration range of GABA examined.2. The reversal potential of inhibitory junctional potentials (i.j.p.s) was measured shortly after replacing Cl(-) by foreign anions: Br(-), NO(3) (-), I(-) and CNS(-). In I(-) solution the i.j.p.s produced by lower stimulation frequencies showed a reversal potential at a more hyperpolarized level than those produced by higher stimulation rates.3. Shortly after replacing Cl(-) by I(-), the GABA potential induced by iontophoretic application showed a triphasic potential change at the ;reversal' potential. The reversal level of the GABA potential produced by smaller doses was at a more hyperpolarized level than that produced by larger doses.4. Possibilities of a concentration effect of GABA are discussed. It is suggested that the relative permeability of the crayfish inhibitory membrane to anions changes depending on the concentration of GABA.

Anion interaction at the inhibitory post-synaptic membrane of the crayfish neuromuscular junction

1. The membrane potential and the membrane conductance of the crayfish muscle fibre in solutions containing various anions were measured with intracellular micro-electrodes.2. When Cl(-) in the solution was replaced by Br(-), NO(3) (-), I(-) or CNS(-), the addition of gamma-amino butyric acid (GABA) produced a transient hyperpolarization of the membrane.3. The reversal potential of the inhibitory junctional potentials (i.j.p.s) was at a slightly depolarized level relative to the resting potential in the normal Cl(-) solution. When Cl(-) in the bathing solution was replaced by foreign anions, the reversal potential shifted towards the hyperpolarized level. The hyperpolarization was in the order CNS(-) > I(-) > NO(3) (-) > Br(-).4. When a part of Cl(-) in the bathing solution was substituted by Br(-), the inhibitory membrane conductance activated by GABA was increased as the concentration of Br(-) increased. The inhibitory membrane conductance decreased when one quarter or a half of Cl(-) was replaced by CNS(-), NO(3) (-) or I(-), but it increased again in higher concentrations of these anions.5. Ca(2+), Mg(2+) and Ba(2+) showed no appreciable effect on the inhibitory membrane conductance activated by GABA, while they decreased the conductance of the resting muscle membrane.6. It was suggested that, at the activated inhibitory membrane, there is an interaction between anions and the permeability of Cl(-) is decreased by the presence of foreign anions, such as CNS(-), I(-) and NO(3) (-). The present results support the idea that the activated inhibitory membrane is charged positively and anions penetrate the membrane interacting with the charge sites.

A study of the action of picrotoxin on the inhibitory neuromuscular junction of the crayfish

1. The effect of picrotoxin on the neuromuscular junction of the crayfish (Cambarus clarkii) was investigated. The potential changes were recorded intracellularly and extracellularly with micro-electrodes. The membrane conductance of the muscle fibre was also measured.2. Picrotoxin depressed the amplitudes of the inhibitory junctional potentials and the potential changes produced by iontophoretically applied gamma-aminobutyric acid (GABA), but had no appreciable effect on the excitatory junctional potentials and the potential changes produced by L-glutamate.3. The presynaptic action of GABA and the neural transmitter was depressed by picrotoxin. The presynaptic action of beta-guanidinopropionic acid was also depressed by picrotoxin.4. The increase in the membrane conductance produced by the addition of GABA in the bath fluid was depressed by picrotoxin. The dose-response relation showed that picrotoxin depressed the conductance increase produced by GABA in a non-competitive manner. The action of picrotoxin on the conductance increase produced by GABA was more effective in low Cl- solution.5. The analysis of the dose-response curves showed that the action of picrotoxin was well expressed by the Michaelis-Menten equation, but the slope of the dose-response curve of GABA was steeper than this relation. It is proposed that the conductance of the junctional membrane was increased by the combination of two molecules of GABA with a receptor, and the attachment of one molecule of picrotoxin to a specific site depressed the conductance increase.

Anion permeability of the inhibitory post-synaptic membrane of the crayfish neuromuscular junction

1. The membrane conductance of the crayfish muscle fibre was calculated and the increase in the membrane conductance brought about by the application of gamma-aminobutyric acid (GABA) was investigated under various conditions. The permeability of the activated inhibitory post-synaptic membrane was estimated from the conductance change.2. When the concentration of Cl(-) was changed by substituting Cl(-) with methylsulphate, a linear relation was obtained between the membrane conductance change produced by GABA and the concentration of Cl(-). Cations have no appreciable effect on the GABA induced conductance change (less than 0.4%).3. The conductance change produced by GABA on the inhibitory post-synaptic membrane was measured in various solutions in which the Cl(-) was replaced by various anions. The degree of permeability depended on the anion species in the following order:Br(-) > Cl(-) > SCN(-) > I(-) > NO(3) (-) > HCOO(-) > ClO(3) (-) > ClO(4) (-) > BrO(3) (-)4. At high pH the conductance change produced by GABA was decreased, while at low pH it increased.5. These results show that the activated inhibitory post-synaptic membrane of crayfish muscle fibres has the characteristics of an anion selective membrane, suggesting the fixed positive charges in the inhibitory membrane.

A study of the inhibitory action of gamma-amino-butyric acid on neuromuscular transmission in the crayfish

1. The effect of gamma-aminobutyric acid (GABA) on the neuromuscular junction of the crayfish (Cambarus clarkii) was investigated. The drug was applied iontophoretically to a junction and the potential changes were recorded intracellularly as well as extracellularly with micro-electrodes.2. The amplitude of the potential produced by iontophoretically applied L-glutamate was decreased by simultaneous application of GABA to the same junctional area. When the membrane potential was clamped at the resting level the clamping current for the L-glutamate injection was not changed by GABA application, indicating that there is no appreciable competition between L-glutamate and GABA for the glutamate-sensitive receptor.3. The excitatory nerve was stimulated repetitively and the excitatory junctional potentials (e.j.p.s) were recorded extracellularly with a micro-electrode placed on the surface of the muscle. The average size of the extracellular e.j.p.s was reduced by the application of GABA.4. The number of quanta released per impulse calculated from the extracellular e.j.p.s was decreased during the application of GABA, while the quantum size remained unchanged.5. The size of the excitatory presynaptic nerve spike was decreased by the application of GABA. However, no appreciable change was observed in the inhibitory nerve spike.6. These results indicate that in addition to its post-synaptic action GABA acts on the excitatory presynaptic nerve terminal and reduces the release of the transmitter.