Additive effects of pentobarbital and halothane to inhibit synthesis of lung proteins

Propofol anesthesia does not alter regional rates of cerebral protein synthesis measured with L-[1-(11)C]leucine and PET in healthy male subjects

We report regional rates of cerebral protein synthesis (rCPS) in 10 healthy young males, each studied under two conditions: awake and anesthetized with propofol. We used the quantitative L-[1-(11)C]leucine positron emission tomography (PET) method to measure rCPS. The method accounts for the fraction (lambda) of unlabeled leucine in the precursor pool for protein synthesis that is derived from arterial plasma; the remainder comes from proteolysis of tissue proteins. Across 18 regions and whole brain, mean differences in rCPS between studies ranged from -5% to 5% and were within the variability of rCPS in awake studies (coefficient of variation range: 7% to 14%). Similarly, differences in lambda (range: 1% to 4%) were typically within the variability of lambda (coefficient of variation range: 3% to 6%). Intersubject variances and patterns of regional variation were also similar under both conditions. In propofol-anesthetized subjects, rCPS varied regionally from 0.98+/-0.12 to 2.39+/-0.23 nmol g(-1) min(-1) in the corona radiata and in the cerebellum, respectively. Our data indicate that the values, variances, and patterns of regional variation in rCPS and lambda measured by the L-[1-(11)C]leucine PET method are not significantly altered by anesthesia with propofol.

Inhibition of mammalian translation initiation by volatile anesthetics

Volatile anesthetics are essential for modern medical practice, but sites and mechanisms of action for any of their numerous cellular effects remain largely unknown. Previous studies with yeast showed that volatile anesthetics induce nutrient-dependent inhibition of growth through mechanisms involving inhibition of mRNA translation. Studies herein show that the volatile anesthetic halothane inhibits protein synthesis in perfused rat liver at doses ranging from 2 to 6%. A marked disaggregation of polysomes occurs, indicating that inhibition of translation initiation plays a key role. Dose- and time-dependent alterations that decrease the function of a variety of translation initiation processes are observed. At 6% halothane, a rapid and persistent increase in phosphorylation of the alpha-subunit of eukaryotic translation initiation factor (eIF)2 occurs. This is accompanied by inhibition of activity of the guanine nucleotide exchange factor eIF2B that is responsible for GDP-GTP exchange on eIF2. At lower doses, neither eIF2alpha phosphorylation nor eIF2B activity is altered. After extended exposure to 6% halothane, alterations in two separate responses regulated by the target of rapamycin pathway occur: 1) redistribution of eIF4E from its translation-stimulatory association with eIF4G to its translation-inactive complex with eIF4E-binding protein-1; and 2) decreased phosphorylation of ribosomal protein S6 (rpS6) with a corresponding decrease in active forms of a kinase that phosphorylates rpS6 (p70(S6K1)). Changes in the association of eIF4E and eIF4G are observed only after extended exposure to low anesthetic doses. Thus dose- and time-dependent alterations in multiple processes permit liver cells to adapt translation to variable degrees and duration of stress imposed by anesthetic exposure.

Inhibition of translation initiation by volatile anesthetics involves nutrient-sensitive GCN-independent and -dependent processes in yeast

Volatile anesthetics including isoflurane affect all cells examined, but their mechanisms of action remain unknown. To investigate the cellular basis of anesthetic action, we are studying Saccharomyces cerevisiae mutants altered in their response to anesthetics. The zzz3-1 mutation renders yeast isoflurane resistant and is an allele of GCN3. Gcn3p functions in the evolutionarily conserved general amino acid control (GCN) pathway that regulates protein synthesis and gene expression in response to nutrient availability through phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha). Hyperphosphorylation of eIF2alpha inhibits translation initiation during amino acid starvation. Isoflurane rapidly (in <15 min) inhibits yeast cell division and amino acid uptake. Unexpectedly, phosphorylation of eIF2alpha decreased dramatically upon initial exposure although hyperphosphorylation occurred later. Translation initiation was inhibited by isoflurane even when eIF2alpha phosphorylation decreased and this inhibition was GCN-independent. Maintenance of inhibition required GCN-dependent hyperphosphorylation of eIF2alpha. Thus, two nutrient-sensitive stages displaying unique features promote isoflurane-induced inhibition of translation initiation. The rapid phase is GCN-independent and apparently has not been recognized previously. The maintenance phase is GCN-dependent and requires inhibition of general translation imparted by enhanced eIF2alpha phosphorylation. Surprisingly, as shown here, the transcription activator Gcn4p does not affect anesthetic response.

Effects of thiopental anesthesia on local rates of cerebral protein synthesis in rats

We have examined the effects of a surgical level of thiopental anesthesia in adult male rats on local rates of cerebral protein synthesis with the quantitative autoradiographic L-[1-14C]leucine method. The relative contribution of leucine derived from protein breakdown to the intracellular precursor amino acid pool for protein synthesis was found to be statistically significantly decreased in the anesthetized rats compared with controls. In the brain as a whole and in 30 of the 35 brain regions examined, rates of protein synthesis were decreased (1-11%) in the anesthetized rats. Decreases were statistically significant (P < or = 0.05) in the brain as a whole and in six of the regions, and they approached statistical significance in an additional 13 regions, indicating a tendency for a generalized but small effect.

Barbiturates in severe head injuries?

Possible mechanisms for the therapeutic effects of barbituric acid derivatives in severe head injuries have been discussed for half a century. In the following, a survey of the literature, and a discussion of three controlled clinical studies available until now is presented. A proven effect in terms of a beneficial long-term outcome for all injured patients has not been established. On the other hand there might be a subgroup of patients with an intact CO2 reactivity of the brain vessels who may profit from barbiturates administered after head trauma.

Metabolic effects of isoflurane on rat lungs perfused in situ

1. The current experiments studied the effects of the inhalation anesthetic, isoflurane, on 5-hydroxytryptamine (5-HT) metabolism, protein synthesis, and angiotensin-converting enzyme activity in perfused rat lungs. 2. Under first order reaction conditions, isoflurane decreased the accumulation of tissue 5-hydroxyindoleacetic acid, the principle metabolite of 5-HT in a concentration-related, competitive, and reversible manner, indicating inhibition of endothelial 5-HT uptake. 3. In apparent contrast, isoflurane appeared to stimulate uptake of 5-HT by an imipramine-sensitive process, into a cell type unable to metabolize the parent amine. 4. Isoflurane increased absolute angiotensin-converting enzyme activity only at an inspired concentration of 5%. The anesthetic did not affect lung protein synthesis.

Effects of ketamine and fentanyl on lung metabolism in perfused rat lungs

The effects of ketamine and fentanyl on serotonin (5-hydroxytryptamine; 5-HT) metabolism, angiotensin-converting enzyme (ACE), and protein synthesis (PS) were investigated in an isolated lung model. Rat lungs were perfused in situ with a blood-free physiological salt solution. The pulmonary vasculature was exposed to ketamine (0.005-2.1 mM) or fentanyl (1.8-4.5 microM) for up to 2 h. After 1 h, accumulation of 5-[14C]hydroxyindoleacetic acid (5-HIAA) by the lung was monitored as an index of 5-HT metabolism. ACE activity was estimated from hydrolysis of [3H]benzoylphenylalanyl-alanyl-proline, a synthetic substrate for the enzyme. [3H]phenylalanine was added to the perfusate after 1 h, and its incorporation into acid-precipitable lung protein was measured over the subsequent hour. Ketamine inhibited 5-HT uptake in a concentration-related manner. The inhibition was characterized as competitive and reversible. Fentanyl had no effect on lung 5-HIAA accumulation. Neither drug altered ACE activity or protein synthesis over the concentration ranges tested. The results indicate an action by ketamine that inhibits the 5-HT membrane-transport process. The different effects observed by ketamine and fentanyl on this process could contribute to the diverse pharmacological properties of these two drugs.

Reversible inhibition of protein synthesis in lung by halothane

Alterations in the synthesis and degradation of proteins were investigated in intact lungs exposed to the volatile anaesthetic halothane. In rat lungs perfused in situ with Krebs-Henseleit bicarbonate buffer containing 4.5% (w/v) bovine serum albumin, 5.6 mM-glucose, plasma concentrations of 19 amino acids and 690 microM-[U-14C]-phenylalanine and equilibrated with O2/N2/CO2 (4:15:1), protein synthesis, calculated based on the specific radioactivity of aminoacyl-tRNA, was inhibited by halothane. The anaesthetic did not affect degradation of lung proteins. The inhibition of protein synthesis was rapid in onset, dose-dependent, and quickly reversible. It did not appear to be associated with overall energy depletion, with non-specific changes in cellular permeability, or with decreased availability of amino acids as substrates for protein synthesis.