Hepatic portal vein denervation impairs oral glucose tolerance but not exenatide's effect on glycemia.

The hepatoportal area is an important glucohomeostatic metabolic sensor, sensing hypoglycemia, hyperglycemia, and hormones such as glucagon-like peptide-1 (GLP-1). We have reported previously that activation of hepatoportal sensors by intraportal infusion of glucose and GLP-1 or by subcutaneous administration of GLP-1 receptor activator exenatide and of intraportal glucose improved glycemia independent of corresponding changes in pancreatic hormones. It is not clear whether this effect is mediated via the portal vein (PV) or by direct action on the liver itself. To test whether receptors in the PV mediate exenatide's beneficial effect on glucose tolerance, we performed 1) paired oral glucose tolerance tests (OGTT) with and without exenatide and 2) intravenous glucose tolerance tests before and after PV denervation in canines. Denervation of the portal vein affected oral glucose tolerance; post-denervation (POST-DEN) OGTT glucose and insulin AUC were 50% higher than before denervation (P = 0.01). However, portal denervation did not impair exenatide's effect to improve oral glucose tolerance (exenatide effect: 48 ± 12 mmol·l⁻¹·min before vs. 64 ± 26 mmol·l⁻¹·min after, P = 0.67). There were no changes in insulin sensitivity or secretion during IVGTTs. Portal vein sensing might play a role in controlling oral glucose tolerance during physiological conditions but not in pharmacological activation of GLP-1 receptors by exenatide.

The effect of portacaval anastomosis on the expression of glutamine synthetase and ornithine aminotransferase in perivenous hepatocytes.

There is functional zonation of metabolism across the liver acinus, with glutamine synthetase restricted to a narrow band of cells around the terminal hepatic venules. Portacaval anastomosis, where there is a major rerouting of portal blood flow from the portal vein directly to the vena cava bypassing the liver, has been reported to result in a marked decrease in the activity of glutamine synthetase. It is not known whether this represents a loss of perivenous hepatocytes or whether there is a specific loss of glutamine synthetase. To answer this question, we have determined the activity of glutamine synthetase and another enzyme from the perivenous compartment, ornithine aminotransferase, as well as the immunochemical localization of both glutamine synthetase and ornithine aminotransferase in rats with a portacaval shunt. The portacaval shunt caused a marked decrease in glutamine synthetase activity and an increase in ornithine aminotransferase activity. Immunohistochemical analysis showed that the glutamine synthetase and ornithine aminotransferase proteins maintained their location in the perivenous cells. These results indicate that there is no generalized loss of perivenous hepatocytes, but rather, there is a significant alteration in the expression of these proteins and hence metabolism in this cell population.

iNOS expression in vascular resident macrophages contributes to circulatory dysfunction of splanchnic vascular smooth muscle contractions in portal hypertensive rats.

Portal hypertension, a major complication of cirrhosis, is caused by both increased portal blood flow due to arterial vasodilation and augmented intrahepatic vascular resistance due to sinusoidal constriction. In this study, we examined the possible involvement of resident macrophages in the tone regulation of splanchnic blood vessels using bile duct ligated (BDL) portal hypertensive rats and an in vitro organ culture method. In BDL cirrhosis, the number of ED2-positive resident macrophages increased by two- to fourfold in the vascular walls of the mesenteric artery and extrahepatic portal vein compared with those in sham-operated rats. Many ED1-positive monocytes were also recruited into this area. The expression of inducible nitric oxide (NO) synthase (iNOS) mRNA was increased in the vascular tissues isolated from BDL rats, and accordingly, nitrate/nitrite production was increased. Immunohistochemistry revealed that iNOS was largely expressed in ED1-positive and ED2-positive cells. We further analyzed the effect of iNOS expression on vascular smooth muscle contraction using an in vitro organ culture system. iNOS mRNA expression and nitrate production significantly increased in vascular tissues (without endothelium) incubated with 1 µg/ml lipopolysaccharide (LPS) for 6 h. Immunohistochemistry indicated that iNOS was largely expressed in ED2-positive resident macrophages. α-Adrenergic-stimulated contractility of the mesenteric artery was greatly suppressed by LPS treatment and was restored by N(G)-nitro-L-arginine methyl ester (NO synthase inhibitor); in contrast, portal vein contractility was largely unaffected by LPS. Sodium nitroprusside (NO donor) and 8-bromo-cGMP showed greater contractile inhibition in the mesenteric artery than in the portal vein with decreasing myosin light chain phosphorylation. In the presence of an α-adrenergic agonist, the mesenteric artery cytosolic Ca(2+) level was greatly reduced by sodium nitroprusside; however, the portal vein Ca(2+) level was largely unaffected. These results suggest that the induction of iNOS in monocytes/macrophages contributes to a hypercirculatory state in the cirrhosis model rat in which the imbalance of the responsiveness of visceral vascular walls to NO (mesenteric artery >> portal vein) may account for the increased portal venous flow in portal hypertension.

Obligatory role for phosphatidylinositol 4,5-bisphosphate in activation of native TRPC1 store-operated channels in vascular myocytes.

In the present study the effect of phosphatidylinositol 4,5-bisphosphate (PIP(2)) was studied on a native TRPC1 store-operated channel (SOC) in freshly dispersed rabbit portal vein myocytes. Application of diC8-PIP(2), a water soluble form of PIP(2), to quiescent inside-out patches evoked single channel currents with a unitary conductance of 1.9 pS. DiC8-PIP(2)-evoked channel currents were inhibited by anti-TRPC1 antibodies and these characteristics are identical to SOCs evoked by cyclopiazonic acid (CPA) and BAPTA-AM. SOCs stimulated by CPA, BAPTA-AM and the phorbol ester phorbol 12,13-dibutyrate (PDBu) were reduced by anti-PIP(2) antibodies and by depletion of tissue PIP(2) levels by pre-treatment of preparations with wortmannin and LY294002. However, these reagents did not alter the ability of PIP(2) to activate SOCs in inside-out patches. Co-immunoprecipitation techniques demonstrated association between TRPC1 and PIP(2) at rest, which was greatly decreased by wortmannin and LY294002. Pre-treatment of cells with PDBu, which activates protein kinase C (PKC), augmented SOC activation by PIP(2) whereas the PKC inhibitor chelerythrine decreased SOC stimulation by PIP(2). Co-immunoprecipitation experiments provide evidence that PKC-dependent phosphorylation of TRPC1 occurs constitutively and was increased by CPA and PDBu but decreased by chelerythrine. These novel results show that PIP(2) can activate TRPC1 SOCs in native vascular myocytes and plays an important role in SOC activation by CPA, BAPTA-AM and PDBu. Moreover, the permissive role of PIP(2) in SOC activation requires PKC-dependent phosphorylation of TRPC1.

Gene expression profiles of vascular smooth muscle show differential expression of mitogen-activated protein kinase pathways during captopril therapy of heart failure.

Congestive heart failure (CHF) is characterized by increased vascular tone and an impairment in nitric-oxide-mediated vasodilatation. We have demonstrated that the blunted response to nitric oxide is due, in part, to a reduction in the leucine-zipper-positive isoform of the myosin-targeting subunit (MYPT1) of myosin light-chain phosphatase. Additionally, we have shown that angiotensin-converting enzyme inhibition, but not afterload reduction with prazosin, preserves leucine-zipper-positive MYPT1 isoform expression in vascular smooth muscle cells and normalizes the sensitivity to cGMP-mediated vasodilatation. We therefore hypothesized that in CHF, growth regulators and cytokines downstream of the angiotensin II receptor are involved in modulating gene expression in vascular tissue. Rats were divided into control and captopril-treated groups following left coronary artery ligation. Gene expression profiles in the aorta and portal vein at baseline and 2 and 4 weeks after myocardial infarction (MI) were analyzed using microarray technology and quantitative real-time PCR. After MI, microarray analysis revealed differential mRNA expression of 21 genes in the aorta of captopril-treated rats 2 and 4 weeks after surgery when compared to gene expression profiles at baseline and without captopril therapy. Real-time PCR demonstrated that captopril suppressed the expression of protein kinases in the angiotensin-II-mediated mitogen-activated protein kinase signaling pathway, including Taok1 and Raf1. These data suggest that in CHF, captopril therapy modulates gene expression in vascular smooth muscle, and some of the beneficial effects of ACE inhibition may be due to differential gene expression in the vasculature.

Biochemical selectivity of oral versus intravenous aspirin in rats. Inhibition by oral aspirin of cyclooxygenase activity in platelets and presystemic but not systemic vessels.

In rats intravenous aspirin was only slightly more effective an inhibitor of platelet thromboxane B2 (TxB2) than of aorta 6-keto-prostaglandin (PGF)1 alpha generation (1.9 versus 2.1 mg/kg). In contrast, oral aspirin was about five times more effective on platelet than on aorta cyclooxygenase activity. The "biochemical selectivity" of aspirin as an inhibitor of platelet and vascular cyclooxygenase thus was not apparent after intravenous administration of the drug. However, this could be achieved by relatively low doses of oral (or intraduodenal) aspirin, on account of "presystemic" acetylation of platelet cyclooxygenase. Even in this condition, though, aspirin selectivity was relative to "systemic" peripheral vessels but not to the vessels of the enterohepatic circulation. Indeed after an oral or intraduodenal dose of 5 mg/kg aspirin, generation of portal vein 6-keto-PGF1 alpha was inhibited to much the same extent as platelet TxB2, while inferior vena cava 6-keto-PGF1 alpha formation was spared.

Bradykinin B1 receptor expression induced by tissue damage in the rat portal vein: a critical role for mitogen-activated protein kinase and nuclear factor-kappaB signaling pathways.

The bradykinin B1 receptor (B1R) is normally absent under physiological conditions, but is highly inducible during inflammatory conditions or following tissue damage. The present study attempted to determine some of the mechanisms underlying B1R upregulation following tissue injury in rat portal vein. Damage induced by tissue isolation and in vitro incubation caused a significant and time-dependent increase in des-Arg9-bradykinin (des-Arg9-BK) responsiveness that paralleled the B1R mRNA expression, as confirmed by real-time quantitative PCR. In vitro incubation of rat portal vein also induced the activation of some members of the mitogen activated protein kinase (MAPK) family, namely, extracellular signal-regulated kinase (ERK), c-Jun NH2-terminal kinase (JNK), and p38 MAPK, an effect accompanied by degradation of the inhibitory protein IkappaBalpha and translocation of nuclear transcription factor-kappaB (NF-kappaB) to the nucleus. The blockade of p38 MAPK, JNK or NF-kappaB, but not ERK pathways with selective inhibitors, resulted in a significant reduction of the upregulated contractile response caused by the selective B1R agonist des-Arg9-BK, and largely prevented the induction of B1R mRNA expression in the rat portal vein. Together, these results demonstrate that in vitro tissue damage induces activation of several intracellular signaling pathways that have a key role in the control of B1R expression. B1R could exert a pivotal role in the development of the cardiovascular response associated with vascular damage.

Dopamine-sensitive adenylate cyclase in rabbit portal vein.

The effect of dopamine (DA) on 3',5'-cyclic adenosine monophosphate (cAMP) generation in the rabbit portal vein was investigated. Dopamine added to homogenates of rabbit portal vein increased the concentration of cAMP. DA-elicited cAMP increase was exclusively inhibited by DA antagonists fluphenazine and haloperidol suggesting the existence of DA receptors (of the DA1 subtype) in the portal vein.

The activity of diagnostic enzymes and the concentration of lipids in the blood vessels of cattle.

Blood vessel walls are shown to contain creatine phosphokinase, lactate dehydrogenase, gamma glutamyl transpeptidase and aspartate transaminase activity. The activity of these enzymes in the serum may be enhanced by leakage from damaged blood vessels. The activity of the enzymes alanine transaminase and alkaline phosphatase as well as the content of triglycerides, cholesterol and lipoproteins are very low in the vascular tissue and are unlikely to be of diagnostic value in vascular tissue injury.

[Differences of nitric oxide synthase expression and activity between splanchnic arterial and venous vessels of cirrhotic rats].

OBJECTIVE: To understand the implication of differences of nitric oxide synthase (NOS) expression and activity between splanchnic arterial and venous vessels in the pathogenesis of portal hypertension. METHODS: Cirrhosis was induced in 100 Wistar rats by subcutaneously administration of carbon tetrachloride. NOS localization, activity and gene expression in the mesenteric artery and the portal vein vessels of both cirrhotic and normal rats were investigated by immunohistochemistry, chemoluminescence and reverse transcription-polymerase chain reaction, respectively. RESULTS: There was inducible NOS enzyme isoform in al1 layers of splanchnic vessels of cirrhotic rats, whereas endothelial NOS isoform largely in vascular endothelia. NOS activity and its mRNA expression all were significantly increased in cirrhotic rats when compared with normal rats (P<0.05 or 0.01).Moreover, the activities of general and constitutive NOS and the expression of endothelial NOS mRNA in cirrhotic rats were significantly higher in the mesenteric artery than in the portal vein (P<0.01). CONCLUSIONS: Enhanced expression and activity of endothelial NOS enzyme isoform may be mainly responsible for increased NO production of splanchnic vessels in cirrhotic rats, and the differences of NOS expression and activity between the mesenteric artery and the portal vein vessels may be one of the pathogeneses of portal hypertension in which NO might be involved.

Aberrant expression and distribution of enzymes of the urea cycle and other ammonia metabolizing pathways in dogs with congenital portosystemic shunts.

The detoxification of ammonia occurs mainly through conversion of ammonia to urea in the liver via the urea cycle and glutamine synthesis. Congenital portosystemic shunts (CPSS) in dogs cause hyperammonemia eventually leading to hepatic encephalopathy. In this study, the gene expression of urea cycle enzymes (carbamoylphosphate synthetase (CPS1), ornithine carbamoyltransferase (OTC), argininosuccinate synthetase (ASS1), argininosuccinate lyase (ASL), and arginase (ARG1)), N-acetylglutamate synthase (NAGS), Glutamate dehydrogenase (GLUD1), and glutamate-ammonia ligase (GLUL) was evaluated in dogs with CPSS before and after surgical closure of the shunt. Additionally, immunohistochemistry was performed on urea cycle enzymes and GLUL on liver samples of healthy dogs and dogs with CPSS to investigate a possible zonal distribution of these enzymes within the liver lobule and to investigate possible differences in distribution in dogs with CPSS compared to healthy dogs. Furthermore, the effect of increasing ammonia concentrations on the expression of the urea cycle enzymes was investigated in primary hepatocytes in vitro. Gene-expression of CPS1, OTC, ASL, GLUD1 and NAGS was down regulated in dogs with CPSS and did not normalize after surgical closure of the shunt. In all dogs GLUL distribution was localized pericentrally. CPS1, OTC and ASS1 were localized periportally in healthy dogs, whereas in CPSS dogs, these enzymes lacked a clear zonal distribution. In primary hepatocytes higher ammonia concentrations induced mRNA levels of CPS1. We hypothesize that the reduction in expression of urea cycle enzymes, NAGS and GLUD1 as well as the alterations in zonal distribution in dogs with CPSS may be caused by a developmental arrest of these enzymes during the embryonic or early postnatal phase.

Superoxide dismutase activity in portal vein endothelium after partial liver resection.

PURPOSE: To investigate superoxide dismutase (SOD) activity in the portal vein endothelium and malondialdehyde acid (MDA) production in liver tissue of rats submitted to 70% hepatectomy. METHODS: Twelve rats were distributed in two groups (hepatectomy and sham). Animals were sacrificed on post operative day 1 and portal vein, liver tissue and blood samples were collected. Portal vein SOD production was measured using lucigenin-amplified chemiluminescence assays. MDA measurement was used as an index of oxidative stress through the formation of TBARS (Thiobarbituric Acid Reactive Species). RESULTS: There was no difference in post operative bilirubin, AST, ALT levels between groups. DHL level was higher in the hepatectomy group (p=0.01). MDA production in the remnant liver tissue and endothelial portal vein SOD activity were also significantly (p<0.05) elevated in the hepatectomy group when compared to control group. There was no correlation between MDA and SOD activity. SOD activity, on the other hand, showed a positive correlation with LDH level (p=0.038) and MDA levels showed a positive correlation with AST and ALT levels (p<0.001). CONCLUSION: There is an increased production of malondialdehyde acid in liver tissue after partial hepatectomy and increased activity of superoxide dismutase in portal vein endothelium as well.

Superoxid dismutase activity in portal vein endothelium after partial liver resection

PURPOSE: To investigate superoxide dismutase (SOD) activity in the portal vein endothelium and malondialdehyde acid (MDA) production in liver tissue of rats submitted to 70% hepatectomy. METHODS:Twelve rats were distributed in two groups (hepatectomy and sham). Animals were sacrificed on post operative day 1 and portal vein, liver tissue and blood samples were collected. Portal vein SOD production was measured using lucigenin-amplified chemiluminescence assays. MDA measurement was used as an index of oxidative stress through the formation of TBARS (Thiobarbituric Acid Reactive Species). RESULTS: There was no difference in post operative bilirrubin, AST, ALT levels between groups. DHL level was higher in the hepatectomy group (p=0.01). MDA production in the remnant liver tissue and endothelial portal vein SOD activity were also significantly (p<0.05) elevated in the hepatectomy group when compared to control group. There was no correlation between MDA and SOD activity. SOD activity, on the other hand, showed a positive correlation with LDH level (p=0.038) and MDA levels showed a positive correlation with AST and ALT levels (p<0.001). CONCLUSION: There is an increased production of malondialdehyde acid in liver tissue after partial hepatectomy and increased activity of superoxide dismutase in portal vein endothelium as well.

Differential dependence of stretch and shear stress signaling on caveolin-1 in the vascular wall.

The role of caveolae in stretch- versus flow-induced vascular responses was investigated using caveolin 1-deficient [knockout (KO)] mice. Portal veins were stretched longitudinally for 5 min (acute) or 72 h (organ culture). Basal ERK1/2 and Akt phosphorylation were increased in organ-cultured KO veins, as were protein synthesis and vessel wall cross sections. Stretch stimulated acute phosphorylation of ERK1/2 and long-term phosphorylation of focal adhesion kinase (FAK) and cofilin but did not affect Akt phosphorylation. Protein synthesis, and particularly synthesis of smooth muscle differentiation markers, was increased by stretch. These effects did not differ in portal veins from KO and control mice, which also showed the same contractile response to membrane depolarization and inhibition by the Rho kinase inhibitor Y-27632. KO carotid arteries had increased wall cross sections and responded to pressurization (120 mmHg) for 1 h with increased ERK1/2 but not Akt phosphorylation, similar to control arteries. Shear stress by flow for 15 min, on the other hand, increased phosphorylation of Akt in carotids from control but not KO mice. In conclusion, caveolin 1 contributes to low basal ERK1/2 and Akt activity and is required for Akt-dependent signals in response to shear stress (flow) but is not essential for trophic effects of stretch (pressure) in the vascular wall.

Intrahepatic flow disturbance: possibility of a hidden cause of drug toxicity.

The liver has an intricate microvascular system that allows homogenous perfusion throughout the organ. However, the regulatory mechanisms of intrahepatic circulation are still unclear, and the effects of drugs on this system have rarely been reported. Oxethazaine, a topical anesthetic, was incidentally found to induce a consistent increase in portal pressure in the isolated perfused rat liver, which led us to characterize this phenomenon. For this, a vital staining method was developed to detect microcirculatory alterations in the isolated liver. Using this method, not only vasoconstrictors like endothelin-1, but the drugs oxethazaine and clomipramine, a tricyclic antidepressant, were found to induce flow redistribution to the deeper and hilar portions of the liver with minimal perfusion at the periphery, which was due to a short-circuit flow at the center owing to the constriction of the intrahepatic portal vein branches. Hepatic nerve stimulation also produced a similar flow disturbance. Since the portal pressure increases by these compounds were inhibited by the Rho-kinase inhibitors Y27632 and HA1077, portal vein branches may employ a Rho-kinase-dependent pathway for sustained contraction. However, oxethazaine, clomipramine, and endothelin-1 may activate this pathway differently. The intrahepatic flow disturbance could play a hidden role in drug toxicity of certain drugs.

The eNOS cofactor tetrahydrobiopterin improves endothelial dysfunction in livers of rats with CCl4 cirrhosis.

In cirrhosis, intrahepatic endothelial dysfunction is one of the mechanisms involved in the increased resistance to portal blood flow and therefore in the development of portal hypertension. Endothelial nitric oxide synthase (eNOS) uncoupling due to deficiency of tetrahydrobiopterin (BH4) results in decreased production of NO and plays a major role in endothelial dysfunction in other conditions. We examined whether eNOS uncoupling is involved in the pathogenesis of endothelial dysfunction of livers with cirrhosis. Basal levels of tetrahydrobiopterin and guanosine triphosphate (GTP)-cyclohydrolase (BH4 rate-limiting enzyme) expression and activity were determined in liver homogenates of control and rats with CCl4 cirrhosis. Thereafter, rats were treated with tetrahydrobiopterin, and eNOS activity, NO bioavailability, assessed with a functional assay, and the vasodilator response to acetylcholine (endothelial function) were evaluated. Livers with cirrhosis showed reduced BH4 levels and decreased GTP-cyclohydrolase activity and expression, which were associated with impaired vasorelaxation to acetylcholine. Tetrahydrobiopterin supplementation increased BH4 hepatic levels and eNOS activity and significantly improved the vasodilator response to acetylcholine in rats with cirrhosis. In conclusion, the impaired response to acetylcholine of livers with cirrhosis is modulated by a reduced availability of the eNOS cofactor, tetrahydrobiopterin. Tetrahydrobiopterin supplementation improved the endothelial dysfunction of cirrhotic livers.

Cardiac impairment and nitric oxide synthase activity in the chronic portal vein-stenosed rat.

Decreased cardiac contractility and beta-adrenergic responses have been observed in the chronic portal vein-stenosed (PVS) rat. Because nitric oxide (NO) may be increased in PVS and has been recognized as a negative inotropic agent, we investigated the induction of NO synthase (NOS2) and/or changes in constitutive NOS (NOS3) as factors in the cardiac dysfunction of the PVS rat. Ten to twelve days after portal vein stenosis or sham operation, cardiac function was evaluated in paced left ventricular papillary muscles (LVPM) and right ventricular strips (RV). To determine if NO modulation of contractile function was altered in PVS, we examined the increase in developed tension produced by the effect of Nomega-nitro-L-arginine (L-NNA) on the myocardial force-frequency relationship. Cardiac tissue NOS2 and NOS3 activities were assayed, Western blot analyses of NOS2 and NOS3 expression were performed, and circulating nitrate-nitrite (NOX) levels (an indicator of in vivo NOS activity) were assayed. Basal LVPM and RV contractile indexes were significantly reduced in PVS (30-50%), without a change in the relaxation rate. No between-group differences in the cardiac NOS2 or NOS3 enzymatic activities of PVS and sham-operated (SO) rats were observed. Western blots revealed no cardiac NOS2 expression in either SO or PVS rats. In contrast, NOS3 was expressed in both SO and PVS rats, but there was no quantitative difference in expression between the two groups. Changes in the cardiac force-frequency relationship (staircase effect) after L-NNA were consistent with NOS3 modulation of contractile function in both SO and PVS rats, but there was no between-group difference in the modulation. Circulating NOX concentrations did not differ between SO and PVS rats. In conclusion, protein expression data, enzymatic assays, end-product assays, and functional data indicate that between-group differences in NOS2 and NOS3 activity are not responsible for the cardiac impairment that has been observed in the chronic PVS rat.

Periportal zonation of the cytosolic acetyl-CoA synthetase of male rat liver.

Several important metabolic functions of the mammalian liver have been shown to be located in zones with respect to the complex microcirculation of the organ. The zonal distribution of the cytosolic component of the acetyl-CoA synthetase activity has been investigated using the dual-digitonin-pulse-perfusion technique, which allows highly zone-selective sampling of cytosol from the periportal and perivenous zone of rat liver. Approximately 80% of the cytosolic enzymes are eluted from the hepatocytes in the periportal and perivenous sub-zones affected by digitonin, while less than 1% of the glutamate dehydrogenase activity (a marker enzyme of the mitochondrial compartment) is eluted. A twofold higher activity of the cytosolic form of acetyl-CoA synthetase is found in the periportal zone compared to the perivenous zone in fed male rats. Following a fasting/refeeding transition, this activity gradient is abolished in a manner similar to that observed for the enzyme acetyl-CoA carboxylase. Since the latter enzyme is utilizing the product of acetyl-CoA synthetase, acetyl-CoA, the similarity in the observed regulation suggests a functional coupling between cytosolic acetate activation and fatty-acid synthesis.

[Effect of He-Ne laser irradiation on the functional activity of smooth muscle cells in the portal vein]. / Vliianie izlucheniia He-Ne lazera na funktsional'nuiu aktivnost' gladkomyshechnykh kletok vorotnoi veny krys.

Irradiation of the rat portal vein's fragments with the He-Ne laser for 3, 5 and 10 minutes reduced the fragments tone by half. Frequency of phasic and tonic contractions did not change, their amplitude, however, increased by neatly 40% as compared with the initial level. The NO synthase blocker N-nitro-L-arginine administered prior to the irradiation had no effect on the above parameters. The data obtained suggest that the decrease of the vessel tone is due to production of the EDRF and cGMP. The increase in the amplitude of phasic and tonic contractions of the vein's smooth muscle cells is associated with an increased Ca++ entry in each contraction cycle.