Amino acid transport during muscle contraction and its relevance to exercise.

The functional significance of amino acid transport in skeletal muscle has been explored by the use of a variety of techniques including work in isolated perfused organs, isolated incubated organs and tissue culture of muscle cells. The results suggest that although there is a wide variety of amino acid transport systems of different characteristics and with different responses to ionic, hormonal and nervous modulation, the amino acid glutamine (transported by system Nm) demonstrates some unusual properties not observed with amino acids transported by other systems. Glutamine is transported at very high rates in skeletal muscle and heart and both the glutamate and glutamine transporter appear to be adaptively regulated by the availability of glutamine. Glutamine appears to be involved in the regulation of a number of important metabolic processes in heart and skeletal muscle (e.g., regulation of the glutathione reduced/oxidised ratio and regulation of protein and glycogen synthesis). Furthermore, glutamine transport appears to interact with systems for regulation of volume control and many of the metabolic features attributable to changes in glutamine concentration appear to be modulated via alteration in cytoskeletal status.

Impaired endothelium-dependent relaxation of human hepatic arteries after preservation with the University of Wisconsin solution.

OBJECTIVE: To evaluate the effect of University of Wisconsin solution on endothelium-dependent relaxation and contraction of human hepatic arteries in vitro. DESIGN: Human hepatic arteries were harvested from 24 patients with hepatocellular carcinoma who had undergone hepatectomy. SETTING: A tertiary care center. INTERVENTIONS: Human hepatic arteries (n = 6 in each group) were harvested during resection for hepatocellular carcinoma. The arteries in group 1 (i.e., the control group) were immediately studied without preservation. The arteries in group 2 were preserved in cold (4 degrees C) physiological solution for 1 hour, while the arteries in groups 3 and 4 were preserved in University of Wisconsin solution for 1 and 16 hours, respectively. Segments of control and preserved hepatic arteries with or without endothelium were then suspended in organ chambers to measure the isometric force. RESULTS: The relaxation of segments of the hepatic arteries with endothelium in response to acetylcholine and adenosine diphosphate was significantly (P < .05) greater than that of segments without endothelium. The maximal relaxation of hepatic arterial segments with endothelium in groups 3 and 4 in response to acetylcholine was notably different from that of segments in groups 1 and 2. The maximal relaxation of hepatic arterial segments with endothelium in groups 3 and 4 in response to adenosine diphosphate was notably different from that of segments in groups 1 and 2. Perfusate hypoxia (mean +/- SD PO2, 30 +/- 5 mm Hg) caused the endothelium-dependent contraction of the arteries (the median initial tension in groups 1, 2, 3, and 4 was 251%, 233%, 276%, and 260%, respectively; P > .05). CONCLUSIONS: The endothelium-dependent relaxation of human hepatic arteries in response to acetylcholine and adenosine diphosphate was notably attenuated by University of Wisconsin solution. The impaired endothelium-dependent relaxation by University of Wisconsin solution and the prominent endothelium-dependent contraction of human hepatic arteries would favor vasospasm and thrombosis after hepatic transplantation.

Electrogenic L-histidine transport in neutral and basic amino acid transporter (NBAT)-expressing Xenopus laevis oocytes. Evidence for two functionally distinct transport mechanisms induced by NBAT expression.

We have investigated the neutral and basic amino acid transporter (NBAT)-induced transport of L-histidine in Xenopus laevis oocytes. Transport of L-histidine (pH 7.5) was electrogenic and Na+-dependent with a 14-fold increase in L-histidine- (1 mM) evoked current (I(His) = -14.7 +/- 1.5 nA) in NBAT-expressing oocytes compared with native (water-injected or uninjected) oocytes (-1.0 +/- 0.2 nA); the Na+-dependent histidine transport showed a stoichiometry of 1:1 (histidine:sodium). I(His) was stereospecific at pH 7.5 and saturable in both NaCl and tetramethylammonium chloride media. L-Histidine (1 mM) at pH 8.5, at which histidine is uncharged, evoked an Na+-independent outward current (11 +/- 1.2 nA) in NBAT-expressing oocytes. The total inward 0.1 mM I(His) increased from -9 +/- 0.8 nA at pH 7.5 to -19 +/- 2.6 nA at pH 6.5, at which histidine is predominantly cationic. The increase in I(His) from pH 7.5 to 6.5 was found to be almost entirely due to the Na+-independent component. At pH 7.5, L-histidine weakly inhibited the Na+-independent L-arginine uptake; however, this inhibition was much stronger (>90%) at pH 6.5. L-Histidine transport, at pH 7.5, is stimulated by NBAT expression, but unlike L-phenylalanine or L-arginine transport, L-histidine transport is Na+-dependent and stereoselective. The induction of Na+-dependent L-histidine transport in NBAT-expressing oocytes provides new evidence that NBAT stimulates functionally distinct amino acid transporters including Na+-dependent L-histidine and Na+-independent L-arginine and L-phenylalanine transporters. The parallel induction of two different mechanisms argues that NBAT is not an amino acid transporter itself but, instead, is a transport-activating protein for a range of amino acid translocases.

The endothelium-dependent response of human hepatic artery after preservation with the UW solution.

The University of Wisconsin's (UW) solution has been used commonly for current liver transplantation. However, its effect on the vascular endothelium remains unclear. Experiments were designed to study the effects. Human hepatic arteries harvested from patients with hepatocellular carcinoma undergoing liver resection were preserved in 4 degree C physiological solution (group 1, the content showed on the text) and UW solution (group 2) for 1 hr. Segments of preserved and control (group 3) hepatic arteries were suspended in organ chamber to measure the isometric force. The relaxations to acetylcholine (ACH) and adenosine diphosphate in segments of hepatic artery with endothelium were significantly greater than those segments without endothelium. The maximal relaxation to ACH in arterial segments with endothelium of group 2 was significantly different from those of group 1 and 3 (group 1 to group 3, 82 +/- 2%, 57 +/- 6%, and 83 +/- 4% of the initial tension contracted by neoepinephrine (3 X 10-7 mole/l, P < 0.05). The maximal relaxation to adenosine diphosphate was similar to the response to ACH. Perfusate hypoxia (oxygen tension 30 +/- 5 mmHG) caused endothelium-dependent contraction of the arterial segments (group 1 to group 3, 233 +/- 32%, 276 +/- 35%, and 251 +/- 40% of the initial tension, P < 0.05). Endothelium-independent relaxation and contraction of human hepatic artery to sodium nitroprusside and norepinephrine were not altered by UW solution. In summary, the impaired endothelium-dependent relaxation by UW solution and prominent endothelium-dependent contraction to hypoxia of human hepatic artery would favor vasospasm and thrombus formation after liver transplantation.

Endothelium-dependent contraction of canine coronary artery is enhanced by crystalloid cardioplegic solution.

Experiments were designed to determine whether hyperkalemic crystalloid cardioplegic solution enhances endothelium-dependent contraction of coronary arteries. Segments of canine coronary arteries (n = 8 in each group) were preserved in cold (4 degrees C) crystalloid cardioplegic solution (group 1) and physiologic solution (group 2) for 60 minutes. Segments of preserved and control (group 3) coronary arteries with or without endothelium were suspended in organ chambers to measure isometric force. Perfusate hypoxia (oxygen tension 35 +/- 5 mm Hg) caused endothelium-dependent contraction in the arteries of all three groups. However, vascular segments with endothelium of group 1 exhibited hypoxic contraction (68.5% +/- 15.3% of the initial tension contracted by prostaglandin F2 alpha 2 x 10(-6) mol/L, p < 0.05) that was significantly greater than contraction of the group 2 and group 3 segments with endothelium (26.6% +/- 5.6% and 20.6 +/- 4.4%). The hypoxic contraction in arteries of group 1 could be attenuated by NG-monomethyl-L-arginine, the blocker of endothelial cell synthesis of the nitric oxide from L-arginine. The action of NG-monomethyl-L-arginine could be reversed by L-arginine but not D-arginine. Thus after preservation with cardioplegic solution, augmented endothelium-dependent contraction, occurs by L-arginine-dependent pathway, would favor coronary vasospasm after cardiac operation.

Enhancement of endothelium-dependent contraction of the canine coronary artery by UW solution.

University of Wisconsin (UW) solution has been used almost routinely in the preservation of the hepatic, pancreatic, renal, and cardiac allografts. However, its effect on vascular endothelium is unknown. Experiments were designed to evaluate its effect on canine coronary endothelium. Canine coronary arteries (n = 8 in each group) were preserved in cold (4 degrees C) UW solution (group 1) and physiological solution (group 2) for 6 hr immediately after harvesting. Segments of preserved and control (group 3) coronary arteries with or without endothelium were then suspended in organ chambers to measure isometric force. Perfusate hypoxia (pO2 30 +/- 5 mmHg) caused endothelium-dependent contraction in the arteries of all 3 groups. However, vascular segments with endothelium of group 1 exhibited hypoxic contractions (107 +/- 26% of the initial tension contracted by prostaglandin F2 alpha 2 x 10(-6) mol/L, P < 0.05) that were significantly greater than those of the group 2 and group 3 segments with endothelium (25 +/- 5% and 20 +/- 4%). The hypoxic contraction in arteries of group 1 could be attenuated by NG-monomethyl-L-arginine (L-NMMA), the blocker of endothelial cell synthesis of the nitric oxide from L-arginine. The action of L-NMMA could be reversed by L-arginine but not D-arginine. Endothelium-dependent relaxation of coronary endothelium to acetylcholine and adenosine diphosphate and endothelium-independent relaxation and contraction of coronary smooth muscle were not altered by the UW solution. After preservation with the UW solution, endothelium-dependent contraction of the canine coronary arteries, occurs by L-arginine-dependent pathway, is enhanced. This augmentation by the UW solution would favor vasospasm after transplantation.