Effects of halothane on transport of 5-hydroxytryptamine by platelet membranes

Effects of volatile and intravenous anesthetics on the uptake of GABA, glutamate and dopamine by their transporters heterologously expressed in COS cells and in rat brain synaptosomes

Although the neurotransmitter uptake system is considered a possible target for the presynaptic action of anesthetic agents, observations are inconsistent concerning effects on the transporter and their clinical relevance. The present study examined the effects of volatile and intravenous anesthetics on the uptake of GABA, glutamate and dopamine in COS cells heterologously expressing the transporters for these neurotransmitters and in the rat brain synaptosomes. Halothane and isoflurane, but not thiamylal or thiopental, significantly inhibited uptake by COS cell systems of GABA, dopamine and glutamic acid in a concentration-dependent manner within clinically relevant ranges for anesthesia induced by these agents. Similarly, in synaptosomes halothane and isoflurane but not thiopental significantly suppressed the uptake of GABA and glutamic acid, respectively. These results do not support the hypothesis that volatile and intravenous anesthetics exert their action via specific inhibition of GABA uptake to enhance inhibitory GABAergic neuronal activity. Rather, they suggest that presynaptic uptake systems for various neurotransmitters including GABA may be the molecular targets for volatile anesthetic agents.

Molecular cloning, expression and characterization of a bovine serotonin transporter

The serotonin transporter (SERT) is a member of a highly homologous family of sodium/chloride dependent neurotransmitter transporters responsible for reuptake of biogenic amines from the extracellular fluid. SERT constitutes the pharmacological target of several clinically important antidepressants. Here we report the molecular cloning of SERT from the bovine species. Translation of the nucleotide sequence revealed 44 amino acid differences compared to human SERT. When transiently expressed in HeLa cells and compared with rat and human SERTs the K(m) value for uptake was increased 2-fold. V(max) and B(max) were both increased about 4-fold indicating the turnover number is conserved. The pharmacological profile revealed a decreased sensitivity towards imipramine, desipramine, citalopram, fluoxetine and paroxetine compared with human SERT, while the sensitivity towards 3, 4-methylenedioxymethamphetamine (MDMA) was mainly unchanged. RT-PCR amplification of RNA from different tissues demonstrated expression of SERT in placenta, brain stem, bone marrow, kidney, lung, heart, adrenal gland, liver, parathyroid gland, thyroid gland, small intestine and pancreas.

Serotonin transport in reconstituted endothelial cell plasma membrane proteoliposomes: effect of hypoxia

To determine whether changes in the lipid dynamics of the plasma membrane bilayer are responsible for hypoxic stimulation of serotonin (5-hydroxytryptamine [5-HT]) transport in pulmonary artery endothelial cells, we solubilized and isolated phospholipid and protein fractions from plasma membrane vesicles derived from endothelial cells exposed to 20% O2 (normoxia) or 0% O2 (hypoxia) for 24 h. Four different combinations of proteoliposomes were prepared by reconstituting (1) normoxic protein and normoxic phospholipid, (2) normoxic protein and hypoxic phospholipid, (3) hypoxic protein and normoxic phospholipid, and (4) hypoxic protein and hypoxic phospholipid. Fluorescence anisotropy of diphenylhexatriene (DPH), a measure of fluidity, and 5-HT transport were evaluated in each of the four groups of reconstituted proteoliposomes. 5-HT transport by the reconstituted proteoliposomes was saturable, linear with protein (5 to 25 micrograms) and time (15 to 60 s), and optimal with a phospholipid-to-protein ratio of 3:1. There were no significant differences in intravesicular volume, phospholipid-to-protein ratio, and size distribution among the four different groups of proteoliposomes. 5-HT transport was significantly higher and fluorescence anisotropy of DPH was significantly lower in proteoliposomes made from hypoxic phospholipids irrespective of the source of protein. Hypoxia also had a direct effect on the 5-HT transporter since uptake was increased slightly in proteoliposomes from group 3. These results indicate that changes in the plasma membrane phospholipids, and to a much lesser extent changes in the 5-HT transporter, are responsible for increases in the transmembrane transport of 5-HT by hypoxic endothelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Anesthetic effects on 5-hydroxytryptamine uptake by rat brain synaptosomes

As a neurotransmitter involved in modulating central nervous system nociception and awareness, 5-hydroxytryptamine (5-HT) may play an important role in the clinical sequelae of certain anesthetic compounds. Anesthetic agents are known to affect peripheral, non-neuronal 5-HT uptake but little is known about their effects on 5-HT metabolism in the central nervous system. The effects of several anesthetic compounds on 5-HT uptake were examined in synaptosomes isolated from rat brain cortex. Inhibition of this uptake process was observed by exposure to clinically relevant concentrations of the volatile anesthetics halothane, isoflurane, and enflurane. The non-volatile agent, ketamine also inhibited uptake while the narcotic fentanyl had an effect only at the highest concentrations tested. Non-volatile agents which had neither a consistent nor significant effect on synaptosomal 5-HT uptake included pentobarbital, sufentanil, and etomidate. These alterations of 5-HT metabolism could represent a mechanism that contributes to anesthetic action.