Inhibition of GABA metabolism in rat brain synaptosomes by midazolam (RO-21-3981)

Benzodiazepines are known to potentiate GABA (gamma aminobutyric acid) action in the brain. The effects of midazolam, a water-soluble benzodiazepine, on GABA disposal (14CO2 from [1-14C]GABA) and on the individual processes of GABA uptake, GABA release, and GABA-transaminase in the rat brain synaptosomal model system were studied. A 10 per cent inhibition of action was defined as ID10. Midazolam inhibited overall GABA disposal at ID10 = 13 micro M. The ID10 values for the three contributing process in the overall GABA disposal process are 580 micro M for GABA-transaminase activity, 96 micro M for GABA release, and 13 micro M for GABA uptake. The value for GABA release is probably not valid since it fell outside of the linear part of the regression line which was used for calculation. Therefore, GABA uptake inhibition appears to be responsible for the overall inhibition of GABA disposal. This value is consistent with the proposed hypothesis that anesthesia involves excess GABA in the synaptic area.

Inhibition of GABA metabolism in rat brain slices by halothane

Based on studies with rat cerebral cortex slices, it was previously hypothesized that halothane anesthesia may result from increased GABA (gamma-aminobutyric acid) content in the synapses. Since GABA is an inhibitory neurotransmitter, such increases may cause a reduction in synaptic activity. The increase in GABA content could arise from several possible causes which are examined in this study using rat cerebral cortex slices as a model. The effects of halothane on uptake, release, and catabolism of GABA were determined. Uptake was studied by the amounts of radioactive GABA accumulated by the slices, and release studied by that discharged into the medium from slices preloaded with radioactive GABA. Catabolism was assessed by preloading the slices with radioactive GABA and then followed by measuring the amount of radioactivity found in unmetabolized GABA or in pooled GABA metabolites. Since CO2 was established as a major metabolite, it was subsequently used alone to measure the inhibition of GABA catabolism in the presence of varying amounts of halothane. Halothane (3 per cent) did not affect the high-affinity uptake or the release of GABA but did inhibit the catabolism of GABA. Using 14CO2 production as an index of catabolism, the inhibition of GABA catabolism by halothane was dose-related (8.79 per cent inhibition/per cent halothane). Such results support the hypothesis that halothane anesthesia may result at least in part from an inhibition of GABA catabolism which, in turn, causes increased GABA level in the synapse with resultant synaptic inhibition.

Antecedent renal disease and the outcome of pregnancy

We assessed the effect of "healed" childhood renal disease on subsequent pregnancies by following-up a cohort of 224 children initially hospitalized with kidney disease. The pregnancy experience in this cohort was compared to two "control" cohorts comprising 81 female siblings and 191 age-matched female patients hospitalized contemporaneously for respiratory infection. The incidence of spontaneous abortion, stillbirth, and pregnancy-associated hypertension was not different among the cohorts; however, the incidence of infants with low birth weights was significantly greater in the renal and respiratory disease groups. Childhood kidney disease followed by impaired renal function (serum creatinine greater than 1.5 mg/dL) was associated with greater maternal and fetal morbidity. Kidney disease in childhood followed by apparent healing and no functional renal impairment does not have an adverse effect on maternal welfare, although the incidence of infants with low birth weight is apparently increased.

Metabolism of sodium nitroprusside in dogs awake and anesthetized with halothane

Sodium nitroprusside (SNP) is rapidly metabolized to cyanide (CN) and thiocyanate (SCN). The authors determined the rates of CN and SCN production during SNP infusion sufficient to maintain blood pressure at 80 per cent of baseline in the dog awake and during halothane anesthesia. Each dog served as its own control. The endogenous whole-blood CN concentration was significantly lower in anesthetized dogs (0.6 nmol/ml) than awake dogs (1.8 nmol/ml). CN concentration increased similarly during SNP infusion in awake (4.5 nmol/ml) and anesthetized dogs (2.3 nmol/ml). In another group of dogs, whole-blood CN concentration decreased significantly due to halothane anesthesia. The regression coefficient was -0.21 nmol CN/ml/hr. There was no significant difference in plasma SCN concentration following infusion of SNP in both awake (33 nmol/ml) and anesthetized dogs (26 nmol/ml). The cause of this decreased blood CN concentration with or without SNP infusion during halothane anesthesia is not known.

Effects of spironolactone, canrenone and canrenoate-K on cytochrome P450, and 11beta- and 18-hydroxylation in bovine and human adrenal cortical mitochondria

Effects of spironolactone, canrenone and canrenoate-K on adrenal cytochrome P450 (P450) and corticosteroid biosynthesis were examined by studying difference spectra, P450 reduction and corticoid hydroxylation in mitochondrial preparations isolated from zona fasciculata and zona glomerulosa of bovine adrenals and from adrenal adenoma and hyperplastic adrenal cortex removed from patients with hyperaldosteronism. All three agents bound to P450 producing type I difference spectra and underwent hydroxylation. They all inhibited 11beta-hydroxylation in bovine adrenal at 30 muM and higher concentrations. Canrenone, the most potent inhibitor, blocked enzyme activity by 60% at a concentration of 60 muM. Spironolactone stimulated P450 reduction. The order of potency of inhibition was found to correlate with the order of affinity of these agents for P450. 11beta-Hydroxylase in human adrenal appeared to be less sensitive to canrenone. All three agents or their hydroxylated metabolites blocked 18-hydroxylation in bovine adrenal at lower concentrations. Canrenoate-K, being the most effective, inhibited 52% at 20 muM. Low concentrations of canrenone (2.5-5.0 muM) were without effect on 11beta-hydroxylase but markedly inhibited 18-hydroxylation (62-76%) in hyperplastic human adrenals. The inhibitors produced mixed type inhibition of 11beta-hydroxylation and competitive type inhibition of 18-hydroxylation. These findings indicate that at low concentrations spironolactone and its major metabolites, canrenone and canrenoate-K, or their hydroxylated metabolites, can directly interfere with the biosynthesis of aldosterone in bovine and certain human adrenal cortical tissue.

A neurochemical hypothesis for halothane anesthesia

We hypothesize that anesthetic agents exert their effects through an inhibition of oxidation of reduced nicotinamide adenine dinucleotide (NADH) in the synaptic regions of the brain. This inhibition could lead to gamma-amino-butyric acid (GABA) accumulation in the synaptic cleft and to hyperpolarization of the post-synaptic membrane. The resulting reduction in synaptic transmission then would manifest itself as the anesthetic state, reducing the work performed by the brain so that cerebral respiration is reduced and cerebral metabolite energy is conserved.

Effects of anesthesia on intermediary metabolism

Major inhalational anesthetics cause inhibition in the electron transport chain in the region of Complex I resulting in decreased oxygen utilization, inhibition of metabolism of NAD-linked substrates, but not of succinate, inhibition of mitochondrial calcium uptake, and depression of synaptic transmission because of postulated changes in ACh sensitivity or GABA inhibition. Many cellular metabolic effects in CNS and other tissues are secondary to the above. Many metabolic changes noted with anesthetics occur subsequent to activation of the sympathetic nervous system either directly by the anesthetic or by surgical stimulation in the presence of light anesthesia. Many important studies remain to be done.

Metabolism of (4-14C) cholesterol by human adrenal glands in vitro and its inhibition by metyrapone

The in vitro conversion of [4-14C] cholesterol1) to steroid hormones was studied in 1 normal, 1 adenomatous and 2 hyperplastic, surgically resected, human adrenals. The degree of overall conversion per gram of tissue was similar (4.4–5.6 %) in NADPH-supplemented homogenates from normal or moderately hyperactive adrenals; a 2-fold increase was found in a markedly hyperfunctioning gland. Only labelled cortisol, corticosterone, 11-deoxycortisol, 11-deoxycorticosterone and androstenedione were isolated from incubations with [4-14C] cholesterol, but cortisol accounted for slightly more than 50 % of overall substrate conversion in the normal adrenal, while 11-deoxycortisol and androstenedione predominated in the abnormal glands. "Apparent 11β-hydroxylase activity" was considerably lower in the abnormal glands than in normal tissue when assessed using [4-14C] cholesterol as substrate, but this deficit was not observed in comparable incubations with 14C-labelled pregnenolone, progesterone or 11-deoxycorticosterone. [4-14C] Cholesterol was a more efficient precursor of androstenedione than either [4-14C] pregnenolone or [4-14C] progesterone.

In parallel studies, metyrapone (1.0 mm) depressed the formation of both 11-oxy and 11-deoxymetabolies from [4-14C] cholesterol, overall inhibition ranging from 66 to 86%. The conversion of [4-14C] 11-deoxycorticosterone to corticosterone was inhibited by 93 % under identical conditions. Metyrapone did not, however, impair the overall transformation of either [4-14C]pregnenolone or [4-14C]progesterone, since the inhibition of cortisol and corticosterone biosynthesis was associated with an increment in 11-deoxycortisol (with [4-14C] pregnenolone) or 11-deoxycorticosterone (with [4-14C]progesterone). From these studies it appears likely that the additional site of metyrapone inhibition of steroid biosynthesis suggested by others on the basis of clinical observations involves the rate-limiting, ACTH-regulated conversion of cholesterol to pregnenolone.

Sites of metyrapone inhibition of steroid biosynthesis by rat adrenal mitochondria

Rat adrenal mitochondrial preparations supplemented with an NADPH-generating system were incubated with various labelled substrates in order to evaluate further the action of metyrapone on the utilization of cholesterol for steroid biosynthesis1). The formation of pregnenolone from [4-14C] cholesterol (0.5, 1.0 and 2.0 μCi) in 5, 10 and 15 min incubations was decreased by 72–82 % in the presence of metyrapone (0.5 mm). Similarly, the generation of labelled side chain fragments from [26-14C]-cholesterol was depressed 36–42 % by 0.2 mm metyrapone and 65–70 % by 1.0 mm inhibitor during 30, 60 and 90 min incubations. Metyrapone inhibition of the side chain cleavage was not observed, however, if cholesterol was replaced as substrate by its C-20 hydroxylated analog: The formation of pregnenolone from [7-3H]20α-hydroxycholesterol (0.5, 1.0 and 2.0 μCi), also an NADPH-mediated mitochondrial reaction, was not affected by similar concentrations of metyrapone, indicating that the inhibition observed with cholesterol as substrate is not related to non-specific toxic effects, to interference with NADPH generation or to impairment of NADPH function in the mitochondrial electron transport system. Parallel incubations with [4-14C] 11-deoxycorticosterone and with [4-14C] cholesterol over a wide range of inhibitor concentrations (0.01–1.0 mm) demonstrated that the effects of metyrapone on 11β-hydroxylation and on the side chain cleavage were dose-related; at low concentrations, however, metyrapone was a more potent inhibitor of 11β-hydroxylation than of cholesterol conversion to pregnenolone. These studies demonstrate clearly in the rat adrenal the dual inhibitory effect of metyrapone sug-gested by previous in vivo and in vitro observations in man.