Time-resolved contrast-enhanced MR angiography of spinal vascular malformations

BACKGROUND AND PURPOSE

The diagnosis of spinal vascular malformations may be challenging on conventional MR imaging because neither the location of the signal abnormality in the spinal cord nor the level of the abnormal flow voids correlates with the level of the fistula. We conducted a retrospective evaluation of the utility of using a time-resolved imaging of contrast kinetics sequence in the diagnosis, characterization, and localization of spinal vascular malformations, comparing it with the criterion standard of spinal DSA.

MATERIALS AND METHODS

Fifty-five consecutive patients with a suspected diagnosis of spinal vascular malformation underwent time-resolved imaging of contrast kinetics followed by spinal DSA. All scans were performed on a 1.5T scanner by using a standard 8-channel spine coil and were reported by a neuroradiologist before the DSA was performed.

RESULTS

Forty-seven lesions were confirmed on time-resolved imaging of contrast kinetics and classified as spinal dural arteriovenous fistulas (n = 33, with 1 patient having a type Ib fistula), perimedullary spinal cord arteriovenous fistulas (n = 10), and intramedullary arteriovenous malformations (n = 3). One patient had an extradural spinal vascular malformation. Time-resolved imaging of contrast kinetics identified the location of the arterial feeder to within 1 vertebral level in 27/33 patients (81.8%) with spinal dural arteriovenous fistulas and correctly predicted the side in 22/33 (66.6%) patients. Perimedullary spinal cord arteriovenous fistulas were erroneously considered to represent spinal dural arteriovenous fistulas before spinal DSA. The anatomy of the arterial supply to intramedullary arteriovenous malformations was also poorly characterized on time-resolved contrast-enhanced MR angiography.

CONCLUSIONS

It has been our experience that time-resolved imaging of contrast kinetics is a useful confirmatory tool when a spinal vascular malformation is suspected on the basis of clinical and conventional MR imaging findings. As experience with the technique grows and sequences are refined, it may be possible to rely on time-resolved imaging of contrast kinetics as a screening tool for the diagnosis of spinal vascular malformations.

Endothelium-dependent nitroxyl-mediated relaxation is resistant to superoxide anion scavenging and preserved in diabetic rat aorta

The aim of the study was to investigate whether diabetes-induced oxidant stress affects the contribution of nitroxyl (HNO) to endothelium-dependent relaxation in the rat aorta. Organ bath techniques were employed to determine vascular function of rat aorta. Pharmacological tools (3mM l-cysteine, 5mM 4-aminopyridine (4-AP), 200μM carboxy-PTIO and 100μM hydroxocobalamin, HXC) were used to distinguish between NO and HNO-mediated relaxation. Superoxide anion levels were determined by lucigenin-enhanced chemiluminescence. In the diabetic aorta, where there is increased superoxide anion production, responses to the endothelium-dependent relaxant ACh were not affected when the contribution of NO to relaxation was abolished by either HXC or carboxy-PTIO, indicating a preserved HNO-mediated relaxation. Conversely, when the contribution of HNO was inhibited with l-cysteine or 4-AP, the sensitivity and maximum relaxation to ACh was significantly decreased, suggesting that the contribution of NO was impaired by diabetes. Furthermore, whereas HNO appears to be derived from eNOS in normal aorta, in the diabetic aorta it may also arise from an eNOS-independent source, perhaps derived from nitrosothiol stores. Similarly, exposure to the superoxide anion generator, pyrogallol (100μM) significantly reduced the sensitivity to the NO donor, DEANONOate and ACh-induced NO-mediated relaxation but had no effect on responses to the HNO donor, Angeli's salt and ACh-induced HNO-mediated relaxation in the rat aorta. These findings demonstrate that NO-mediated relaxation is impaired during oxidative stress but the HNO component of relaxation is preserved under those conditions.

Impairment of both nitric oxide-mediated and EDHF-type relaxation in small mesenteric arteries from rats with streptozotocin-induced diabetes

BACKGROUND AND PURPOSE

To investigate whether diabetes affects either or both nitric oxide (NO)-mediated and endothelium-derived hyperpolarizing factor (EDHF)-type relaxation in endothelium-dependent relaxation of mesenteric arteries from streptozotocin-induced diabetic rats.

EXPERIMENTAL APPROACH

Wire myography was employed to examine endothelial function of mesenteric arteries. Superoxide levels were measured by L-012 and lucigenin-enhanced chemiluminescence. Western blotting was used to quantify protein expression levels.

KEY RESULTS

Superoxide levels were significantly increased in diabetic mesenteric arteries compared with normal arteries. Diabetes significantly reduced the sensitivity to the endothelium-dependent relaxant, acetylcholine (ACh) in mesenteric arteries. When the contribution of NO to relaxation was abolished by N-nitro-L-arginine (L-NNA) + a soluble guanylate cyclase inhibitor (ODQ), the sensitivity to ACh was significantly decreased in the diabetic arteries compared with normal arteries, indicating an impaired EDHF-type relaxation despite increased expression of intermediate- and small-conductance calcium-activated potassium channels. Conversely, when the contribution of EDHF was inhibited with TRAM-34 + apamin + iberiotoxin, maximum relaxations to ACh were significantly decreased in diabetic compared with normal arteries, suggesting that the contribution of NO was also impaired by diabetes. Basal levels of NO release, indicated by contraction to L-NNA, were also significantly decreased in diabetic arteries. Western blot analysis demonstrated that diabetic arteries had an increased expression of Nox2, decreased pSer⁴⁷³ Akt and a reduced proportion of endothelial NO synthase (eNOS) expressed as a dimer, indicating uncoupling.

CONCLUSION AND IMPLICATIONS

The contribution of both NO and EDHF-type relaxations was impaired in diabetes and was caused by increased oxidative stress, decreased pSer⁴⁷³ Akt and/or eNOS uncoupling.

Immediate and sustained benefits of a "total" implementation of speech recognition reporting

Speech recognition reporting was introduced in our institution to address the significant delay between report dictation and the appearance of a typed report on the Picture Archiving and Communication System (PACS). We report our experience of a "total" implementation of a speech recognition reporting (SRR) system, which became the sole means of radiology reporting from day 1 of introduction. Prospectively gathered Radiology Information System (RIS) data were examined to determine the monthly mean reporting times and completion times for all studies from January 2004 to February 2006 (11 months before introduction of SRR and 15 months after introduction). Studies were grouped for analysis according to referral source (casualty, general practice, inpatient or outpatient). A large, sustained reduction in time to completion was noted in all referral groups at both hospital sites within our institution (6.79 +/- 0.92 days pre-SRR and 2.20 +/- 0.78 days post-SRR, independent two-sample Student's t-test, p<10(-11)). Workflow was maintained following the introduction of SRR: numbers of reports per month and mean times to report were unchanged. SRR eliminated the delays associated with report transcription and subsequent authorisation, dramatically reducing report turnaround times. Resistance to change has perhaps led to suboptimal implementation of SRR in some other institutions, such that benefits have not been fully realised. Our experience demonstrates the dramatic impact that a well-planned, organisation-wide implementation of SRR can have on radiology service delivery.

Factors contributing to differences in the regulation of cGMP in isolated porcine pulmonary vessels

Guanosine 3',5'-cyclic monophosphate (cGMP) is an important second messenger in many biological systems including vascular smooth muscle where it mediates relaxation. Cellular levels of cGMP are regulated primarily by three enzymes; nitric oxide (NO) synthase, soluble guanylate cyclase, and cGMP-phosphodiesterase. Basal cGMP levels of isolated endothelium intact porcine pulmonary vein are five fold higher than in pulmonary artery. The objective of this study was to investigate possible reasons for this difference. Therefore, we compared NO synthase activity of pulmonary vein with artery and used pharmacologic approaches to compare soluble guanylate cyclase and phosphodiesterase activities in these vessels. NO synthase activities of pulmonary vein and artery were measured by monitoring the conversion of exogenous L-[14C]arginine to L-[14C]citrulline and by quantifying NO formation from endogenous L-arginine. Rates (pM/min per mg protein) of basal L-citrulline and NO formation from endothelium intact pulmonary vein (29.0 +/- 4.8 and 44 +/- 7.1, respectively) were significantly higher than from artery (8.3 +/- 2.2 and 17.1 +/- 3.3). Western blot analysis indicated higher constitutive NO synthase protein in the vein than in artery. N-nitro-L-arginine (0-100 microM), a potent inhibitor of NO synthase, induced contractions of the pulmonary vein which were significantly higher than those of the artery. N-nitro-L-arginine (5 and 20 microM) in the presence of phosphodiesterase inhibitors, decreased basal cGMP levels of endothelium intact blood vessels. In endothelium denuded pulmonary vein and artery, basal cGMP levels were not different from each other, but increased significantly following stimulation of soluble guanylate cyclase with exogenous NO. In the presence of both non-specific and specific cGMP phosphodiesterase inhibitors, exogenous NO-induced cGMP levels of endothelium denuded tissues were not significantly different from each other. However, in the absence of the phosphodiesterase inhibitors, exogenous NO-induced cGMP was significantly less in the artery than in the vein. These results suggest that (I) the intact porcine pulmonary vein contains higher levels of NO synthase activity than pulmonary artery, and that (II) the soluble guanylate cyclase activities in pulmonary vein and artery are equally responsive to NO, and finally (III) pulmonary artery expresses greater phosphodiesterase activity than vein. Higher NO synthase and lower phosphodiesterase activity may explain the greater accumulation of cGMP in the pulmonary vein compared to the artery.

Degranulation enhances release of a stable contractile factor from rabbit polymorphonuclear leukocytes

We investigated the release of a stable contractile factor(s) from rabbit isolated polymorphonuclear leukocytes (PMNs; 10(8) cells/ml) incubated in Tyrode buffer at 37 degrees C. PMNs were untreated, stimulated with N-formylmethionyl-leucyl-phenylalanine (FMLP; 0.1 microM), of degranulated with cytochalasin B (1 microM) in combination with FMLP (0.1 microM). Products from unstimulated PMNs incubated for 60 min caused significantly greater contraction of rabbit isolated aorta (0.56 +/- 0.12 g, n = 8) than did products released from PMNs during a 5-min incubation (0.32 +/- 0.07 g, n = 11, P < 0.05). Stimulation alone did not affect contractile factor release; however, products released from degranulated PMNs caused significantly greater aortic contraction (0.48 +/- 0.08 g, n = 5) than products from nondegranulated PMNs (0.24 +/- 0.04 g, n = 5, P < 0.05) after a 5-min incubation. The contractile activity of PMN-derived products was virtually abolished by heat (90 degrees C, 10 min) or protease (trypsin; 166 U/ml, 5 h) treatment. These findings suggest a PMN-derived protein vasoconstrictor(s) is spontaneously released at a slow rate in vitro and that degranulation can enhance this rate of release. Because PMN degranulation in vivo is associated with inflammation, these results support suggestions that PMN-derived contractile factors may contribute to the impaired blood flow observed during postischemic reperfusion.

Halothane attenuates nitric oxide relaxation of rat aortas by competition for the nitric oxide receptor site on soluble guanylyl cyclase

Endothelial cells play an important role in the regulation of vascular activity through the release of endothelium derived relaxing factor (EDRF) now believed to be nitric oxide (NO). NO and the NO donor drug nitroglycerin relax vascular smooth muscle by stimulating soluble guanylyl cyclase leading to elevation of intracellular levels of cyclic guanosine 3',5'-monophosphate (cGMP). Halothane has been shown to inhibit the action of NO on blood vessels. This study was designed to further investigate the mechanisms by which halothane attenuates NO-induced vascular relaxations. This was done by examining the effects of halothane on nitroglycerin and NO-induced relaxations in the presence and absence of the inhibitors of soluble guanylyl cyclase, methylene blue and 6-anilino-5,8-quinolinedione (LY 83583). Thoracic aortas from anesthetized male Sprague-Dawley rats were excised and cut into rings and the endothelium was removed. The aortic rings were suspended in organ baths containing Krebs solution and equilibrated at their optimal passive tension. When a stable plateau of contraction was produced by EC60 concentrations of norepinephrine, increasing concentrations of nitroglycerin or NO were added to the baths to relax the rings. This contraction-relaxation procedure was repeated three or four times. In some baths halothane was administered by a calibrated vaporizer 10 min before beginning the second procedure. Either methylene blue or LY 83583 was added to the baths 20 min before the third procedure. The combination of halothane, methylene blue or LY 83583 was added before the fourth procedure. Halothane, methylene blue or LY 83583 significantly inhibited nitroglycerin-induced relaxation individually. Halothane and LY 83583 also significantly inhibited NO-induced relaxations (5 x 10(-9)-3 x 10(-8) M and 5 x 10(-9)-3 x 10(-5) M, respectively) individually. The combination of halothane and methylene blue or halothane and LY 83583 significantly inhibited nitroglycerin-induced relaxation, also, the combination of halothane and LY 83583 significantly inhibited NO-induced relaxations. Halothane, methylene blue and LY 83583 treatment led to rightward shift in the concentration-effect curves. Halothane, in combination with methylene blue or LY 83583, produced inhibition equivalent to the sum of their individual effects. The present study demonstrates that the halothane, methylene blue and LY 83583 attenuate nitroglycerin and NO-induced relaxations of endothelium-denuded rat aortic rings. This suggests that halothane, methylene blue and LY 83583 may act through competitive antagonism at a common site of action on soluble guanylyl cyclase in the EDRF/NO relaxation pathway.

Prevention of ischaemia-induced coronary vascular dysfunction

Myocardial ischaemia and reperfusion cause dysfunction of the coronary vasculature leading to a sustained reduction in coronary blood flow and an impairment of responses to both endothelium-dependent and endothelium-independent vasodilators. In contrast, when previously ischaemic arteries are removed from the myocardium and vascular function is examined in vitro, it is evident that while endothelial function is impaired, smooth muscle reactivity remains intact. Therefore, other changes must be responsible for the general reduction in vasodilator reserve. Examination of the vasculature in the ischaemic myocardium by electron microscopy reveals adhesion of leukocytes and plugging of capillaries. There also is evidence that polymorphonuclear leukocytes (PMNs) release a factor that constricts coronary arterioles, and that release of this factor is increased by atherosclerosis. The identity of this factor remains uncertain, but the calcium antagonist amlodipine prevents the coronary vasoconstriction. Amlodipine is also able to prevent the impaired perfusion and the reduction in vasodilator reserve that occurs after myocardial ischaemia and reperfusion in the dog. In addition, amlodipine prevents the endothelial dysfunction observed in isolated arteries after ischaemia and reperfusion. The interaction between the endothelium and activated PMNs may be a suitable target for pharmacological intervention to improve postischaemic vascular function.

Actions of diaspirin cross-linked hemoglobin on isolated rat and dog vessels

The objectives of these studies were to investigate the responses of isolated blood vessels from rats and dogs to the administration of diaspirin cross-linked hemoglobin (DCLHb) and to determine the mechanisms of these responses. Isolated vascular rings (3 to 5 mm) were suspended at optimal passive tension in Krebs-filled (37 degrees C) tissue baths and bubbled with 95% O2-5% CO2, and isometric tension was recorded. With the vessels under basal conditions increasing concentrations of DCLHb (10(-8)-3 x 10(-6) mol/L) were added. DCLHb addition was repeated during a submaximal contraction with norepinephrine and again during acetylcholine relaxation. The effects of the nitric oxide synthase inhibitor L-nitro arginine (10(-5) mol/L) on the responses to DCLHb were also determined. Dog vessels developed very little tension (1% to 5% of norepinephrine maximum), whereas rat arteries contracted between 9% and 15% when exposed to DCLHb under basal conditions. However, both the dog and rat vessels developed significant tension to DCLHb when they were precontracted (5% to 54%) and also when they were relaxed with acetylcholine (21% to 93%). L-nitro arginine eliminated the contractile responses to DCLHb but did not cause contraction of any of the vessels under basal conditions. We conclude that in this model the mechanism of DCLHb-induced contractions of in vitro dog and rat vessels is dependent on interference with nitric oxide. This is similar to the mechanism of DCLHb action in isolated pig vessels reported previously. Differences in responses of dog, rat, and pig vessels under basal conditions in vitro are the result of active generation of nitric oxide by pig but not by dog or rat vessels.

Novel cypermethrin formulation for the control of sea lice on salmon (Salmo salar)

A novel formulation of cypermethrin was applied as a bath treatment to Atlantic salmon infested with sea lice on a commercial fish farm on the Isle of Skye, Scotland. Twenty 15 m x 15 m cages were treated with cypermethrin at a concentration of 5 micrograms/litre sea water. The numbers of sea lice of all stages were recorded on five fish per cage before the treatment and one, seven and 16 days after treatment. Statistically significant reductions in the numbers of chalimus III and IV pre-adults and adults were recorded over the whole period; the average percentage reductions at one and 16 days after treatment were 59 per cent and 90 per cent (chalimus III and IV), 98 per cent and 95 per cent (pre-adults), and 99 per cent (adults), respectively.

Hemoglobin-induced contraction of pig pulmonary veins

The effects of hemoglobin Ao (HbAo), alpha alpha cross-linked hemoglobin (alpha alphaHb), cyanomet alpha alpha cross-linked hemoglobin (cyanomet alpha alphaHb), and human serum albumin (HSA) were compared under basal conditions and during relaxation with acetylcholine (ACh), sodium nitroprusside (SNP), and papaverine (PAP) in porcine pulmonary veins. Isometric tension changes were recorded in isolated rings (3 to 4 mm) that were suspended in Krebs solution bubbled with 95% O2/5% CO2. Increasing concentrations of HbAo and alpha alphaHb (10(-9) - 3 x 10(-6) mol/L) caused concentration-dependent increases in tension that reached a maximum of 4.20 +/- 0.3 gm and 3.78 +/- 0.6 gm, respectively. Cyanomet alpha alphaHb and HSA (10(-9) - 3 x 10(-6) mol/L) did not cause significant increases in tension. The maximum responses to HbAo and alpha alphaHb were significantly increased during relaxation with ACh and SNP but not with PAP. In contrast, SNP (10(-4) mol/L) and PAP (10(-5) mol/L), but not ACh, reversed contractions induced by HbAo and alpha alphaHb. These studies support the concept that hemoglobin-induced vascular contraction is primarily mediated by inactivation of the vasodilator nitric oxide in vitro. We suggest that this mechanism is common to acellular hemoglobins in which the ligand binding site is unimpaired and in which the heme iron is in the ferrous (+2) state.

Effects of halothane and isoflurane on carbon monoxide-induced relaxations in the rat aorta

BACKGROUND

Halothane and isoflurane previously were reported to attenuate endothelium-derived relaxing factor/nitric oxide-mediated vasodilation and cyclic guanosine monophosphate (cGMP) formation in isolated rat aortic rings. Carbon monoxide has many chemical and physiologic similarities to nitric oxide. This study was designed to investigate the effects of halothane and isoflurane on carbon monoxide-induced relaxations and cGMP formation in the isolated rat aorta.

METHODS

Isometric tension was recorded continuously from endothelium denuded rat aortic rings suspended in Krebs-filled organ baths. Rings precontracted with submaximal concentrations of norepinephrine were exposed to cumulative concentrations of carbon monoxide (26-176 microM). This procedure was repeated three times, with anesthetics delivered 10 min before the second procedure. Carbon monoxide responses of rings contracted with the same concentration of norepinephrine (10(-6) M and 2 x 10(-6) M) used in the anesthetic-exposed preparations also were examined. The concentrations of cGMP were determined in denuded rings using radioimmunoassay. The rings were treated with carbon monoxide (176 microM, 30 s) alone, or carbon monoxide after a 10-min incubation with halothane (0.34 mM or 0.72 mM). To determine whether the sequence of anesthetic delivery influenced results, vascular rings pretreated with halothane were compared with nonpretreated rings.

RESULTS

Carbon monoxide (26-176 microM) caused a dose-dependent reduction of norepinephrine-induced tension, with a maximal relaxation of 1.51 +/- 0.07 g (85 +/- 7% of norepinephrine-induced contraction). Halothane (0.34 mM and 0.72 mM) significantly attenuated the carbon monoxide-induced relaxations, but only the highest concentration of isoflurane (0.53 mM) significantly attenuated the carbon monoxide-induced relaxations. Carbon monoxide (176 microM) significantly increased cGMP content (+88.1 +/- 7.1%) and preincubation of the aortic rings with halothane (0.34 mM and 0.72 mM) inhibited this increase (-70.7 +/- 6.8% and -108.1 +/- 10.6%, respectively). When aortic rings and carbon monoxide were added simultaneously to Krebs solution equilibrated with halothane (0.72 mM), no inhibition of cGMP formation occurred.

CONCLUSION

Carbon monoxide-induced endothelium-independent relaxations of rat aortic rings were decreased by clinically relevant concentrations of halothane and isoflurane. The carbon monoxide-induced elevations of cGMP were attenuated by halothane only when the anesthetic was incubated with aortic rings before carbon monoxide treatment. The possible clinical significance of the actions of the anesthetics on this endogenous vasodilator is yet to be determined.

Cholesterol feeding enhances vasoconstrictor effects of products from rabbit polymorphonuclear leukocytes

We have studied the vasoactive properties of products released from rabbit polymorphonuclear leukocytes (PMNs) before and after short-term (4 and 8 wk) dietary supplementation with 1% cholesterol. Plasma cholesterol levels were similar after 4 and 8 wk of cholesterol diet, whereas gross atherosclerotic lesions were present at 4 wk but significantly more extensive after 8 wk. PMN products from all rabbits caused endothelium-dependent contraction of isolated, control (nonatherosclerotic) rabbit aorta submaximally contracted with phenylephrine. However, both 4 and 8 wk of cholesterol feeding resulted in equivalent contractions by PMN products, which were significantly greater than contractions by control PMNs. Endothelium-dependent contraction (by PMN products) and relaxation (by acetylcholine) were attenuated by 8 wk of cholesterol feeding. PMN products attenuated acetylcholine-induced relaxation of aorta from cholesterol-fed rabbits and of control aorta treated with phenoxybenzamine to reduce muscarinic receptor reserve. We conclude that elevation of plasma cholesterol results in increased release of a PMN product(s) that causes endothelium-dependent constriction.

Effects of halogenated and non-halogenated anesthetics on diaspirin cross-linked hemoglobin induced contractions of porcine pulmonary veins

Diaspirin crosslinked hemoglobin (DCLHb) is a resuscitative fluid presently undergoing clinical trials. Administration of DCLHb is associated with an elevation of mean arterial pressure in vivo and contraction of isolated blood vessels in vitro. The mechanisms for the vascular actions are unknown but may be due to inhibition of nitric oxide (NO). Halothane has been reported to inhibit NO induced relaxation. We examined the effect of anesthetics on DCLHb induced contraction of blood vessels. Porcine pulmonary veins were excised, cut into rings and placed in organ chambers filled with 25 ml Krebs-Ringer solution (37 degrees C). Following equilibration at their optimal length the rings were exposed to increasing concentrations of serotonin(10(-8)M-10(-5)M). Endothelial activity was confirmed by relaxation greater than 80% with ACh (10(-6)M). DCLHb (1.5 x 10(-8)M to 6 x 10(-7)M) contracted porcine pulmonary veins (1.04 +/- 0.17g to 3.45 +/- 0.22g), and halothane (0.5% and 1%) significantly inhibited these DCLHb induced contractions in a dose-related manner (-41.6 +/- 8.1% and -73.3 +/- 8.2%, respectively). At equi-molar concentrations, isoflurane had no inhibitory activity. The relative effect of these volatile anesthetics is consistent with their inhibitory actions on other heme containing proteins. Propofol (10(-5)M) only has inhibitory effects on lower concentrations of DCLHb. Fentanyl did not have inhibitory effects. These results suggest that halogenated anesthetics may interact with the heme iron of DCLHb and inhibit its binding with NO.

Comparative effects of exogenous nitrovasodilators on cGMP levels in different canine blood vessels

Nitrovasodilators, by releasing nitric oxide (NO) in vascular smooth muscle, activate soluble guanylate cyclase (sGC) in vascular smooth muscle. However, there is little information on their relative effectiveness, concentration ranges, or on the incubation times required to produce maximum sGC stimulation. To determine the optimal concentrations and incubation times we measured 3', 5'-cyclic guanosine monophosphate (cGMP) levels in response to different concentrations of NO, S-nitroso-L-cysteine (SNC), and S-nitroso-N-acetylpenicillamine (SNAP), in canine aorta, femoral, and carotid arteries incubated in Krebs. Production of cGMP following incubation of endothelium denuded tissues with NO, SNC, and SNAP peaked close to 20 +/- 5, 90 +/- 20, and 120 +/- 60 seconds respectively. Results indicate that cGMP levels vary with concentration of nitrovasodilators and time of incubation. SNAP was the least effective in increasing cGMP levels among the three nitrovasodilators used. In different vascular beds, the production of cGMP in the presence of nitrovasodilators may depend on variations in the levels of guanylate triphosphate (GTP) and/or sGC.

Vascular effects of halothane and isoflurane: cGMP dependent and independent actions

This study investigated the effects of halothane and isoflurane on cGMP-dependent and independent regulation of vascular contraction of the isolated rat aorta and on NO-stimulated soluble guanylate cyclase (sGC) isolated from the perfused rat liver. For the studies of the aorta, isometric tension of isolated rings, with and without, endothelium was recorded and cGMP content measured. ACh was used to initiate endothelial-dependent relaxation of norepinephrine (NE)-contracted rings while NO was used to directly stimulate isolated aortic ring sGC which catalyzes the isolated aortic ring formation of cGMP. Both halothane and isoflurane interfered with ACh and NO relaxations and with NO-stimulated increases in cGMP. Halothane was more potent, having significant attenuating effects at 0.34 mM (1 MAC) and 0.72 mM (2 MAC) while isoflurane had effects only at 0.53 mM (2 MAC). For the isolated sGC studies, a soluble liver fraction was prepared from perfused rat livers. In the absence of NO stimulation, neither halothane nor isoflurane modified the activity of the sGC. However, during NO-stimulation halothane produced significant, concentration-dependent, inhibition of sGC activity over a wide range of NO concentrations. Isoflurane also inhibited sGC activity, but to a lesser extent than halothane. The mechanism whereby the anesthetics could interfere with sGC from liver and blood vessels is unknown. It could result from anesthetic interaction at hydrophobic sites that may exist in GC. However, the results of both the aorta and liver sGC enzyme studies support the suggestion that these anesthetics can compete with NO for its binding site on the ferrous heme of sGC, with chemical structural differences accounting for the potency variations. Both anesthetics also had cGMP independent effects, causing concentration dependent relaxations of NE-contracted vessels without endothelium. Isoflurane was about 5 times more effective at 1 MAC than halothane. Therefore, the net effects of these anesthetics involve the sum of two opposite effects on tension of vessels with intact endothelium: 1) interference with NO-stimulated cGMP relaxation and 2) direct stimulation of relaxation (not dependent on changes in cGMP).

Vasoconstrictor responses to polymorphonuclear leucocytes from atherosclerotic rabbits

1. The vascular contractile effects of polymorphonuclear leucocytes (PMN) isolated from control rabbits and from rabbits made atherosclerotic by 1% cholesterol feeding for 8 weeks were examined. 2. Rings of control rabbit thoracic aorta with or without endothelium were mounted at 2 g tension in 10 mL organ baths and were submaximally contracted by phenylephrine (0.1 mumol/L). After 30 min incubation at 37 degrees C, the supernatant of PMN (5 x 10(7)/mL, in Tyrode solution containing 0.25% bovine serum albumin) was obtained by centrifugation for addition to the vascular preparation. 3. Control PMN supernatant (443 microL) caused contraction (0.58 +/- 0.15 g, n = 11) of phenylephrine-contracted aortic rings, which was prevented by removal of the endothelium (0.11 +/- 0.07 g, n = 5, P < 0.05). However, the control PMN supernatant had no contractile effect on aortic rings at resting tension (0.00 +/- 0.00 g, n = 8). 4. By comparison, atherosclerotic PMN supernatant (443 microL) caused a significantly greater contraction of the aortic rings (1.41 +/- 0.13 g, n = 9, P < 0.05 vs control PMN supernatant) that was only partly inhibited by removal of the endothelium (0.45 +/- 0.20 g, n = 9, P < 0.05). Moreover, PMN supernatants from four of seven atherosclerotic rabbits contracted aortic rings at resting tension (3.5 +/- 1.4 g, n = 7). 5. These results suggest that the release of a stable vasoconstrictor substance(s) by PMN is enhanced under conditions of atherosclerosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of halothane on EDRF/cGMP-mediated vascular smooth muscle relaxations

BACKGROUND

Halothane has been reported to inhibit endothelium-dependent relaxation in a variety of vessels. These studies were done to determine whether this inhibition is caused by interference with synthesis, release, or action of endothelium-derived relaxing factor (EDRF) on cyclic guanosine monophosphate (cGMP) levels within the vascular smooth muscle.

METHODS

Rat aortic rings were suspended in aerated Krebs solution (37 degrees C) and were contracted to a stable plateau with EC60-70 norepinephrine (NE). Relaxations caused by acetylcholine (ACh; 1 x 10(-8)-1 x 10(-6) M), nitric oxide (NO; 5 x 10(-9)-1 x 10(-6) M), or nitroglycerin (NG; 2 x 10(-9)-3 x 10(-7) M) in rings contracted with NE were compared in the presence and absence of halothane. Tissue cGMP contents were measured using a radioimmunoassay method.

RESULTS

In the presence of halothane (0.5, 1.0, and 2.0 MAC), the ACh-induced relaxations were significantly attenuated in a concentration-dependent manner, an effect that was reversible. Halothane (2 MAC) significantly attenuated NO-induced relaxations at all concentrations and NG-induced relaxations at low concentrations (5 x 10(-9)-3 x 10(-8) M) but not at higher concentrations (1 x 10(-9)-3 x 10(-7) M) in denuded vessels. Nitric oxide-stimulated (5 x 10(-8)-5 x 10(-6) M) cGMP content was significantly attenuated by halothane (2 MAC) at NO concentrations between 1 x 10(-7) and 5 x 10(-6) M.

CONCLUSIONS

Nitric oxide, either endogenous or exogenous, interacts with the enzyme guanylate cyclase to stimulate the production of cGMP. Halothane interfered with the relaxations caused by NO (in rings without endothelium) and decreased the NO-stimulated cGMP content. These results suggest that the site of action of halothane in attenuating endothelium-dependent relaxation in the rat aorta is within the vascular smooth muscle, rather than on the synthesis, release, or transit of the EDRF from the endothelium and that its action may involve an interference with guanylate cyclase activation.

Neurovascular effects of reactive oxygen intermediates produced by photoradiation

These experiments examined the effect of reactive oxygen intermediates, produced by laser illumination of the photosensitizer hematoporphyrin derivative, on the accumulation and release of norepinephrine from sympathetic nerve terminals. Using an isolated, spirally cut, superfused caudal artery of the rat, basal overflow of norepinephrine (NE) was significantly increased both during and after generation of reactive oxygen intermediates. Generation of reactive oxygen intermediates increased overflow of NE in vascular preparations in which release of NE had previously been elevated by the continuous superfusion of Krebs' solution, containing high concentrations of potassium (40 mM). Calcium free solutions did not block the overflow of norepinephrine augmented by reactive oxygen intermediates. This increase in overflow was due both to an increase in release of NE and an inhibition of accumulation of NE.

Vascular interactions of calcium and reactive oxygen intermediates produced following photoradiation

This study was designed to examine vascular smooth muscle contractile properties following enhanced production of reactive oxygen intermediates (ROIs), which were produced by pretreating rat caudal arteries and aortas with a photosensitizer, hematoporphyrin derivative, and then illuminating them with red laser light. This treatment produced a long-lasting contraction that was dependent on the presence of extracellular calcium. Reduction in extracellular calcium relaxed the smooth muscle and replacement of calcium 30 min later increased the tension. Oxygen radical scavengers did not block the contractile effect postillumination when calcium was returned to the bathing solution; however, verapamil (5.5 microM) and nifedipine (10 microM) attenuated this contraction. The contractions were dependent on oxygen in the aerating gas mixture. Production of ROIs by isolated blood vessels was supported biochemically by a significant increase in both bath and tissue levels of oxidation products, reactive with thiobarbituric acid, and by a reduction in the tissue stain, nitroblue tetrazolium. These ROI-induced contractions were observed in vitro on large conduit arteries and also in vivo on small ear arteries. The vascular response following this acute production of ROIs may be similar to vascular abnormalities in certain pathological conditions where ROI production is reported to be elevated. Therefore, these results could contribute to a further understanding of mechanisms involved in these ROI-dependent vascular changes.

Contractile properties of isolated vascular smooth muscle after photoradiation

The purpose of this study was to characterize the responses of various types of vascular smooth muscle to conditions that would be encountered during photodynamic therapy, namely laser illumination of photosensitizer-pretreated tissue. Vascular smooth muscle obtained from representative canine, rodent, and rabbit vascular beds was cut into rings and placed in organ baths (37 degrees C, aerated with 95% O2-5% CO2). These vessels were pretreated for 30 min with the photosensitizer hematoporphyrin derivative (HpD, 3-30 micrograms/ml) washed, and then exposed to red laser light (633 nm, 1-3.5 mW) for up to 20 min. Under basal tension conditions laser illumination of HpD-pretreated vessels resulted in an increase in tension, whereas laser illumination of vessels not exposed to HpD did not contract. This sustained contraction was not reversed by washing the tissue with fresh Krebs-Ringer solution. Responses to norepinephrine, transmural electrical stimulation, and elevated concentrations of KCl were reduced in blood vessels tested after HpD laser illumination. Laser-induced contractions of canine carotid arteries did not require the presence of an intact vascular endothelium. Vascular effect of these photosensitizers appears to involve the formation of oxygen-derived radicals. This preparation could provide a good model for examining the effects of free radicals on vascular physiology.

Adrenergic nerve function and contractile activity of the caudal artery of the streptozotocin diabetic rat

The adrenergic nerve function and contractile responses of the densely innervated caudal artery of the 8-week streptozotocin (SZ) (65 mg/kg i.p.) diabetic rat were investigated. Segments of this artery were removed from diabetic and control rats, placed in Krebs-filled tissue baths (37 degrees C) and isometric tension recorded. Contractile responses to sympathetic nerve activation by electrical stimulation and to cumulative concentrations of norepinephrine (NE) and tyramine were recorded. In order to determine NE content, the NE was extracted from the caudal artery, isolated by adsorption chromatography, and quantified by HPLC with electrochemical detection. NE accumulation and release were also studied by quantifying the amount of tritiated NE [( 3H]NE) and its metabolites in extracts of the tissue or incubation medium. The responses of the caudal artery of SZ diabetic rats to electrical stimulation (4-16 Hz) and to tyramine (1 X 10(-5)-1 X 10(-4) M) were significantly less than those of arteries from control rats and the NE content reduced by 41%, while sensitivity to NE was unchanged. Diabetic arteries also accumulated and released more [3H]NE than did arteries from control rats. These results establish that neurovascular function of the isolated caudal artery of the 8-week SZ diabetic rat is abnormal and suggest that problems in the ability of adrenergic nerves to store and release NE may contribute to this dysfunction. Such changes may play a role in the cardiovascular disturbances associated with diabetic autonomic neuropathy.

Attenuation of endothelium-mediated vasodilation by halothane

To determine whether halothane alters endothelium-mediated vasodilation of vascular smooth muscle, isolated ring preparations of rabbit aorta and canine femoral and carotid arteries were suspended for isometric tension recordings in Krebs-Ringer bicarbonate solution at 37 degrees C. Acetylcholine and bradykinin have been shown to relax these norepinephrine contracted arteries via an endothelium-dependent process. In this study, these relaxations were reversibly and significantly attenuated by 2% halothane. However, halothane did not affect relaxations caused by nitroglycerin, which, in these vessels, acts by an endothelium independent mechanism. These results suggest that halothane is not interfering with cyclic guanylate-monophosphate mediated relaxation of vascular smooth muscle, but may interfere with the synthesis, release, or transport of the endothelium-derived relaxing factor. In addition, during contractions evoked by norepinephrine, halothane caused significant decreases in tension in both the canine carotid and rabbit aortic preparations, but increased tension in the femoral artery rings. These effects were not altered by mechanical removal of the endothelium. These results suggest a direct action of halothane on the vascular smooth muscle, which can result in either an increase or decrease in tension, depending on the specific vessel. In addition to its direct vascular effect, this study suggests a new action of halothane; it interferes with endothelium-derived relaxing factor-mediated relaxation of vascular smooth muscle. This action may contribute in part to the vascular alterations seen clinically during administration of halothane.