Effects of muscle fatigue and temperature on electromechanical delay.

The effect of repeated maximal isometric knee extensions on electromechanical delay (EMD) and associated muscle temperature changes were investigated on seven college aged subjects. The exercise produced a significant reduction in muscle contraction force, rate of force development and muscle conduction velocity, whilst the muscle temperature increased by 2.1 degrees C. The EMD increased from a pre-exercise value of 38.4 (SEM 3.4) ms to 55.7 (SEM 3.4) ms post-exercise. In an attempt to evaluate the effect of muscle temperature on EMD, hot and ice-water bags were placed on the quadriceps muscle to alter muscle temperature. The EMD in isometric maximal knee extension was measured at 38, 36, 34, 32 and 30 degrees C. The results showed that the EMD elongated at muscle temperatures either lower or higher than 36 degrees C. It was speculated that the increased muscle temperature might contribute to 20-25% of the EMD elongation found during the fatiguing intermittent exercise. The information of the effects of muscle temperature on EMD could be useful when evaluating the effects of strenuous exercise, in which a substantial muscle temperature change might occur, on the time delay between myoelectrical activity and force generation.

Effects of exercise-induced oxidative stress on nitric oxide release and antioxidant activity.

This study shows that acute exercise in healthy subjects is a modest oxidative stress, which may be related to an increase in antioxidant activity and down-regulation of nitric oxide formation.

Studies of vascular tolerance to nitroglycerin: effects of N-acetylcysteine, NG-monomethyl L-arginine, and endothelin-1.

Development of vascular tolerance to nitroglycerin (NTG) has been attributed to sulfhydryl (SH) depletion, guanylate cyclase desensitization, or both. Controversy regarding the precise contribution of these mechanisms may be due to variations in experimental design. To examine further the biochemical basis of NTG tolerance, norepinephrine (NE)-precontracted rat aortic rings were exposed to NTG (10(-5)M), which resulted in 84 +/- 6% relaxation. Other rings were first superfused with NTG (10(-6)M) and then contracted with NE. These rings showed a marked tolerance to the vasorelaxant effects of NTG (maximal relaxation 20 +/- 5%, n = 15, p < 0.001 vs. control rings). Similar tolerance to NTG was observed when the vascular rings were first superfused with acetylcholine (ACh 10(-6)M), indicating cross-tolerance between ACh and NTG. Treatment of NTG-tolerant rings with N-acetylcysteine (NAC) (10(-5)M) did not restore vascular smooth muscle (VSM) relaxation in response to NTG (maximal relaxation 23 +/- 5%, n = 8), suggesting that SH depletion may not be the basis of NTG tolerance in these experiments. Parallel sets of NTG-tolerant aortic rings were contracted with endothelin-1 (ET-1, n = 5) or the endothelium-derived relaxing factor (EDRF) synthase inhibitor NG-monomethyl L-arginine (L-NMMA, 10(-4)M, n = 8). In both ET-1- and L-NMMA-contracted rings, vascular relaxation in response to NTG was preserved (80 +/- 6 and 88 +/- 8% relaxation, respectively). Measurement of cyclic GMP in aortic rings showed marked accumulation on initial exposure of tissues to NTG (310 +/- 10 fmol/mg), whereas the NTG-tolerant rings showed much less cyclic GMP accumulation (48 +/- 29 fmol/mg). Rings contracted with L-NMMA or ET-1, but not NE, accumulated cyclic GMP when exposed to NTG (280 +/- 20 fmol/mg). These data indicate that NTG tolerance develops on exposure of vascular rings superfused with NTG or ACh and is probably not related to tissue SH depletion. Contraction of NTG-tolerant rings with ET-1 or L-NMMA restores NTG-mediated relaxation.

Effects of fatigue and sprint training on electromechanical delay of knee extensor muscles.

Electromechanical delay (EMD) of knee extensors in isometric contraction was investigated in six healthy men before and after four periods of 30-s allout sprint cycling exercise, conducted pre and post a 7-week sprint cycling training programme. The EMD was lengthened from 40.4 (SEM 3.46) ms at rest to 63.4 (SEM 7.80) ms after the fatiguing exercise (P < or = 0.05) in the pre-training test. During maximal voluntary contractions (MVC) conducted after the fatiguing exercise, the peak contraction force (Fpeak) and peak rate of force development (RFDpeak) were reduced by 51%-56% and 38%-50%, respectively (both P < or = 0.05). The mechanisms of EMD lengthening during fatigue could have been due to the deterioration in muscle conductive, contractile or elastic properties and require further study. The training programme increased the total work performed during the four periods of sprint exercise (P < or = 0.05). However, no significant training effects were found in the resting or postexercise EMD, Fpeak and RFDpeak during isometric MVC. These unchanged isometric contraction variables but enhanced dynamic performance suggest that isometric tests of muscle are insensitive to the neuromuscular adaptations to sprint training.

Accumulated oxygen deficit during supramaximal all-out and constant intensity exercise.

Two studies were conducted to test the validity of an all-out procedure for the assessment of the maximal accumulated oxygen deficit (AOD). Subjects in study 1 (N = 9; VO2max = 57 +/- 3 ml.kg-1.min-1 [+/- SEM]) completed three supramaximal efforts on a cycle ergometer. Exhaustive exercise during an all-out isokinetic procedure (mean intensity of 149% VO2max) was compared with constant intensity exercise at approximately 110% and 125% VO2max. Subjects in study 2 (N = 12; VO2max = 55 +/- 3 ml.kg-1.min-1) completed a constant intensity test to exhaustion at approximately 110% VO2max and a 90 s all-out test on a Monark friction loaded cycle ergometer (mean intensity of 143% VO2max). The AOD within each study were not significantly different (study 1:43.9, 44.1, and 42.0 ml.kg-1 for the 110%, 125%, and all-out tests; study 2: 52.1 and 51.2 ml.kg-1 for the 110% and all-out tests, respectively; P > 0.05). The total amount of work was significantly greater the longer the test, the additional work being attributed to aerobic processes. The rate of both aerobic and anaerobic energy production in the first 30 s of exercise was directly related to exercise intensity and the protocol used. The results indicate that an all-out procedure provides a valid estimate of the maximal AOD and shows potential for a more complete assessment of anaerobic ability as traditional indices of high intensity exercise performance are also obtained.

Electromechanical delay in isometric muscle contractions evoked by voluntary, reflex and electrical stimulation.

Electromechanical delay (EMD) in isometric contractions of knee extensors evoked by voluntary, tendon reflex (TR) and electrical stimulation (ES) was investigated in 21 healthy young subjects. The subject performed voluntary knee extensions with maximum effort (maximal voluntary contraction, MVC), and at 30%, 60% and 80% MVC. Patellar tendon reflexes were evoked with the reflex hammer being dropped from 60 degrees, 75 degrees and 90 degrees positions. In the percutaneous ES evoked contractions, single switches were triggered with pulses of duration 1.0 ms and of intensities 90, 120 and 150 V. Electromyograms of the vastus lateralis and rectus femoris muscles were recorded using surface electrodes. The isometric knee extension force was recorded using a load cell force transducer connected to the subject's lower leg. The major finding of this study was that EMD of the involuntary contractions [e.g. mean 22.1 (SEM 1.32) ms in TR 90 degrees; mean 17.2 (SEM 0.62) ms in ES 150 V] was significantly shorter than that of the voluntary contractions [e.g. mean 38.7 (SEM 1.18) ms in MVC, P < 0.05]. The relationships between EMD, muscle contractile properties and muscle fibre conduction velocity were also investigated. Further study is needed to explain fully the EMD differences found between the voluntary and involuntary contractions.

Reduction in human neutrophil superoxide anion generation by n-3 polyunsaturated fatty acids: role of cyclooxygenase products and endothelium-derived relaxing factor.

Dietary supplementation with n-3 polyunsaturated acids (PUFAs) results in augumented vasorelaxation and reduction in superoxide anion generation. Augmented vasorelaxation may be mediated by enhanced generation of vasodilator prostaglandins and/or endothelium-derived relaxing factor (EDRF), now thought to be nitric oxide (NO). To determine the importance of enhanced vasodilator prostaglandins or EDRF-NO in reduction in superoxide anion generation during n-3 PUFAs intake, human polymorphonuclear leukocytes (PMNs) were incubated with n-6 PUFA arachidonic acid (AA), or n-3 PUFAs eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (each 10(-7) M) for 1 hr at 37 degrees C. Parallel sets of PMNs were treated with the cyclooxygenase inhibitors indomethacin (10(-5) M), or aspirin (10(-5) M), or the EDRF-NO synthase inhibitor L-NMMA (10(-3) M) prior to incubation with PUFAs. Superoxide anion generation by PMNs was determined by measuring the superoxide dismutase (SOD) inhibitable reduction of ferricytochrome C. PMNs incubated with EPA or DHA, but not AA, demonstrated marked reduction in superoxide anion generation. This reduction in superoxide anion generation by n-3 PUFAs was abolished by treatment of PMNs with indomethacin or aspirin, but not by L-NMMA. These observations suggest that n-3 PUFAs decrease superoxide anion generation primarily by a prostaglandin-dependent pathway.

Influence of training status on maximal accumulated oxygen deficit during all-out cycle exercise.

The influence of training status on the maximal accumulated oxygen deficit (MAOD) was used to assess the validity of the MAOD method during supra-maximal all-out cycle exercise. Sprint trained (ST; n = 6), endurance trained (ET; n = 8), and active untrained controls (UT; n = 8) completed a 90 s all-out variable resistance test on a modified Monark cycle ergometer. Pretests included the determination of peak oxygen uptake (VO2peak) and a series (5-8) of 5-min discontinuous rides at submaximal exercise intensities. The regression of steady-state oxygen uptake on power output to establish individual efficiency relationships was extrapolated to determine the theoretical oxygen cost of the supramaximal power output achieved in the 90 s all-out test. Total work output in 90 s was significantly greater in the trained groups (P < 0.05), although no differences existed between ET and ST. Anaerobic capacity, as assessed by MAOD, was larger in ST compared to ET and UT. While the relative contributions of the aerobic and anaerobic energy systems were not significantly different among the groups, ET were able to achieve significantly more aerobic work than the other two groups, while ST were able to achieve significantly more anaerobic work. Peak power and peak pedalling rate were significantly higher in ST. The results suggested that MAOD determined during all-out exercise was sensitive to training status and provided a useful assessment of anaerobic capacity. In our study sprint training, compared with endurance training, appeared to enhance significantly power output and high intensity performance over brief periods (up to 60 s), yet few overall differences in performance (i.e. total work) existed during 90 s of all-out exercise.

Variable resistance all-out test to generate accumulated oxygen deficit and predict anaerobic capacity.

A supramaximal variable resistance test over varying time intervals was evaluated as an instrument for the assessment of a number of anaerobic parameters, including the accumulated oxygen deficit (AOD). Eight active men [age, 22 +/- 1 (SEM 1) years, peak oxygen uptake, 53.1 (SEM 2.1) ml x kg-1 x min-1] completed three randomly ordered all-out sprints of 45-, 60- and 90-s duration. Two incremental pretests consisting of three 5-min stages at power outputs of 45, 135, 225 W and 90, 180, 270 W were performed to establish individual efficiency relationships [r = 0.996 (SEE 1.1) ml x kg-1 x min-1]. These relationships were used to estimate energy demand (millilitres per kilogram of oxygen equivalents in 15-s time intervals) during the supramaximal tests. The AOD for the 45 [47.6 (SEM 1.5) ml x kg-1], 60 [49.0 (SEM 1.8) ml x kg-1] and 90 s [49.6 (SEM 1.7) ml x kg-1] tests were significantly different only for the 45 and 90-s tests. Evaluation of the 90-s test indicated that maximal or near-maximal (98%) anaerobic energy release was achieved in 60 s, with the AOD beginning to plateau after this time. No significant differences among tests were found for peak power, time to peak power and peak pedalling rate. Differences in mean power, total work and relative power decrement were related to the length of the test.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased production of thromboxane A2 by coronary arteries after thrombolysis.

The coronary artery produces large amounts of prostacyclin (PGI2) and a small amount of thromboxane A2 (TXA2); this high PGI2/TXA2 ratio contributes to the antithrombotic properties of the coronary artery. This study was designed to determine whether this ratio changes after coronary artery thrombosis and thrombolysis and accounts for coronary artery reocclusion. Anesthetized dogs (N = 12) were subjected to electrically induced coronary artery thrombosis and tissue plasminogen activator-induced thrombolysis. Thrombolysis was achieved in 11 dogs, and the coronary artery reoccluded in five of these dogs after the initial reperfusion. Spontaneous and ionophore A23,187-stimulated PGI2 and TXA2 synthesis in normal circumflex and ischemic-reperfused left anterior descending coronary artery segments was measured by radioimmunoassay of thromboxane B2 and 6-keto-prostaglandin F1 alpha, respectively. Production of TXA2 was 413% to 656% greater in left anterior descending segments (from the region of thrombosis and sites proximal and distal to the thrombus) compared with normal circumflex segments (p < 0.001). Production of PGI2 was also increased but only by 46% to 80% in the left anterior descending segments compared with normal circumflex segments (p < 0.05). TXA2 production was greater in coronary artery segments that reocculded compared with segments that stayed open (p < 0.02). Scanning electron microscopy revealed platelet deposition in thrombosed left anterior descending segments but not in segments proximal or distal to the thrombus site, indicating that the vascular wall per se may be a source of increased production of TXA2.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelets enhance anoxic contraction of rat aortic rings through platelet-activating factor-dependent mechanism.

Previous studies indicate that anoxia results in vascular smooth muscle contraction, and this phenomenon is mediated in part by a decrease in release of endothelium-derived relaxing factor (EDRF). The role of platelets that relax blood vessels by eliciting EDRF release on anoxic contraction is not known. To examine anoxic contraction in the presence of platelets, we exposed norepinephrine (NE)-precontracted rat aortic rings to washed platelets and then induced anoxia (switch from 95% O2 to 95% N2). Platelets transiently relaxed the precontracted aortic rings (decrease in tension 49 +/- 4%, n = 18). The subsequent anoxia-induced contraction was augmented, however (magnitude of contraction 71 +/- 12 vs. 49 +/- 6% in parallel buffer-treated rings, n = 18, p < 0.01). To determine the mechanism of platelet-mediated augmentation of anoxic contraction, vascular rings were treated with indomethacin, the thromboxane A2 (TxA2)/endoperoxide receptor antagonist SQ29,548, lipoxygenase inhibitor U60,257B, free oxygen radical scavengers superoxide dismutase (SOD) plus catalase, serotonin (S2) receptor blocker LY53,857, or the ADP scavenger apyrase. Although apyrase, U60,257B, SQ29,548, LY53,857, and SOD plus catalase had no effect on augmentation of anoxic contraction in the presence of platelets, treatment with indomethacin markedly decreased (p < 0.02) the augmented anoxic vasoconstriction. Because indomethacin decreases the activity of platelet-activating factor (PAF), vascular rings were treated with two different specific PAF receptor antagonists: L-659,989 and WEB 2170 BS. Both these agents blocked the platelet-augmented anoxic vasoconstriction. In addition, synthetic PAF (10(-7) and 10(-6) M) in itself potentiated anoxic contraction, mimicking the activity of platelets.(ABSTRACT TRUNCATED AT 250 WORDS)

Variable effects of human and canine polymorphonuclear leucocytes on vascular smooth muscle tone.

OBJECTIVE: Previous studies have shown variable effects of human and canine polymorphonuclear leucocytes (neutrophils) on vascular tone. The aim of this study was to identify whether these variations in neutrophil function are due to species differences. METHODS: Canine and human arterial rings (with and without endothelium) were contracted with the thromboxane A2 analogue U46619, and then exposed to isolated neutrophils. RESULTS: Human neutrophils caused a significant relaxation of the human mammary arterial rings, and the relaxation was unaffected by the cyclo-oxygenase inhibitor indomethacin, enhanced by superoxide dismutase (SOD), and inhibited by oxyhaemoglobin. The relaxant effect of human neutrophils was also diminished upon pretreatment with NG-monomethyl-l-arginine (L-NMMA), indicating that the vasorelaxant material released by the neutrophils was nitric oxide (NO). Human neutrophils also relaxed canine femoral arterial rings, and the relaxant effect was potentiated by SOD and inhibited by pretreatment with oxyhaemoglobin or L-NMMA, confirming that the vasorelaxation was via release of NO. Canine neutrophils, on the other hand, caused an endothelium dependent contraction of autologous femoral arterial rings. This vasoconstriction was not affected by indomethacin, SOD, oxyhaemoglobin, or L-NMMA. However, treatment of canine neutrophils with the 5-lipoxygenase inhibitor piriprost attenuated (p < 0.02) their contractile effect on vascular rings, suggesting that neutrophil generated 5-lipoxygenase products were probably responsible for smooth muscle contraction. Presence of the leukotriene C4 and D4 receptor antagonist FPL 55,712 totally blocked the contractile effects of canine neutrophils, indicating that femoral arterial ring contraction was mediated by peptido-leukotrienes. CONCLUSIONS: The endothelium dependent nature of the canine neutrophil induced contraction suggests that the 5-lipoxygenase product leukotriene A4 is taken up by endothelial cells for conversion to peptido-leukotrienes. Since SOD had no effect and FPL 55,712 totally blocked the vasoconstrictor effects of canine neutrophils, it appears that the vasoconstrictor effects of the latter are mediated primarily through peptido-leukotrienes. In contrast, the vasorelaxation by human neutrophils is mediated through release of NO.

Modulation of vascular tone by endothelin-1: role of preload, endothelial integrity and concentration of endothelin-1.

1. Endothelin-1 (ET-1) has been shown to exert both arterial relaxant and constrictor effects. To examine the mechanisms of these divergent effects, rat aortic rings were suspended in an organ bath (baseline preload, 5 g) and exposed to ET-1 (10(-11) to 10(-7) M). ET-1 contracted these rings in a concentration-dependent fashion. 2. When aortic rings were contracted with noradrenaline (NA) to 1 g of tension, ET-1 caused further contraction of these rings. In rings precontracted to 2 to 4 g of tension, low concentrations of ET-1 (10(-11) to 10(-9) M) caused a significant relaxation, but high concentrations (greater than or equal to 5 x 10(-9) M) caused a marked contraction, indicating both relaxant and contractile effects of ET-1 depending on the preload and ET-1 concentration. 3. To determine the mechanism of ET-1-induced relaxation, aortic rings were pretreated with the cyclo-oxygenase inhibitor indomethacin, NG-monomethyl-L-arginine (L-NMMA) an inhibitor of synthesis of endothelium-derived relaxing factor (EDRF), or oxyhaemoglobin (Hb) which decreases the activity of EDRF, prior to their exposure to ET-1. Both indomethacin and L-NMMA markedly (P less than 0.01) attenuated ET-1-induced relaxation, whereas Hb totally abolished it. Removal of the endothelium from aortic rings also abolished ET-1-mediated relaxation. 4. The relaxant effect of ET-1 in NA-precontracted rings was associated with marked accumulation of guanosine 3':5'-cyclic monophosphate (cyclic GMP), whereas ET-1-induced contraction of quiescent rings was not. 5.In manually stretched rings (4 g of tension), ET-l caused only concentration-dependent contraction, but no cyclic GMP accumulation.6. Thus, ET-1 contracts rat aortic rings with intact endothelium and those which are passively stretched. However, stimulation of rat aortic rings with NA to modest tension alters the contractile effect of ET-1 to a potent relaxant effect. The ET-l-mediated relaxation in this setting appears to be endothelium-dependent and is related to release of both cyclo-oxygenase products and EDRF.

Slow channel calcium blockers attenuate reoxygenation-mediated vascular contraction, but augment anoxia-mediated vascular contraction.

Anoxia and reoxygenation modulate vasomotor tone. To determine the effect of the slow channel calcium blockers verapamil and diltiazem in vascular smooth muscle contraction during states of altered oxygenation, rat aortic rings with intact endothelium were contracted with norepinephrine (NE) or the thromboxane A2 mimic U46,619 and then exposed abruptly to anoxia (switch from 95% O2 to 95% N2) for 30 min and then reoxygenated (switch from 95% N2 to 95% O2). Anoxia caused a transient 40 +/- 9% (mean +/- SE, n = 15) increase in contraction, whereas reoxygenation resulted in an initial decrease followed by a large (83 +/- 21%) increase in contraction. Treatment of vascular rings with verapamil or diltiazem (1 microgram/ml or 2 microM) decreased contractile response to the agonists (p less than 0.01). Both these agents consistently augmented the magnitude and duration of anoxia-induced contraction (p less than 0.01). In other experiments, pretreatment of vascular rings with NG-monomethyl-L-arginine (L-NMMA), an inhibitor of endothelium-derived relaxing factor (EDRF) synthesis, or with oxyhemoglobin, an inhibitor of EDRF activity, or de-endothelialization resulted in marked (p less than 0.01) decrease in anoxic contraction, indicating that anoxia-induced contraction is caused by modulation of EDRF. Treatment of aortic rings with verapamil also reduced acetylcholine-mediated relaxation (from 86 +/- 6% to 45 +/- 5%, p less than 0.02) and cyclic GMP accumulation (from 192 +/- 53 to 111 +/- 35 fmol/mg, p less than 0.02), indicating reduction in EDRF synthesis or activity by verapamil.(ABSTRACT TRUNCATED AT 250 WORDS)

5-Hydroxytryptamine potentiates vasoconstrictor effect of endothelin-1.

Interactions between 5-hydroxytryptamine (5-HT) and endothelin-1 (ET-1) relative to contraction of rat aortic rings were examined in this study. Pretreatment of rings with threshold concentration of 5-HT potentiated the subsequent contractile response to ET-1. However, pretreatment with threshold concentration of ET-1 did not potentiate the contractile response to 5-HT. The 5-HT receptor antagonist LY 53857 blocked the synergistic contractile effects of 5-HT and ET-1 on rat aortic rings. Indomethacin and the thromboxane A2/endoperoxide receptor antagonist SQ 29548 also attenuated (P less than 0.05) the synergistic contractile effects of 5-HT and ET-1, suggesting release of thromboxane A2 or expression of thromboxane A2 receptors during this interaction. The calcium channel blocker verapamil also decreased the synergistic contractile effects of 5-HT and ET-1. Contraction of aortic rings by 5-HT alone was abolished by LY 53857 and attenuated by verapamil, diltiazem, and SQ 29548. Decrease in the force of contraction by verapamil as well as diltiazem indicates activation of voltage-dependent calcium channels during 5-HT-mediated contraction and perhaps during amplification of the vasoconstrictor activity of ET-1 by 5-HT.

Agonist-induced tension determines vascular reactivity during anoxia and reoxygenation.

To determine the influence of pre-existing pharmacologically-induced tension on vascular reactivity during anoxia and reoxygenation, rat aortic rings were contracted with norepinephrine, epinephrine, endothelin or KCl to 1, 2 or 4 g of tension. The rings were then exposed to anoxia (95% N2) for 10 min followed by reoxygenation (95% O2). The degree of anoxia-mediated contraction varied with the magnitude of tension before anoxia and resembled the length-tension relationship in myocardial fibers. The optimal agonist-induced tension for maximal anoxic contraction was approximately 1 to 2 g. This relationship between tension and anoxic contraction was observed in all but KCl-contracted rings. The agonist- as well as KCl-contracted rings showed normal relaxant response to acetylcholine, suggesting that a decrease in endothelium-derived relaxing factor (EDRF) alone cannot be the basis of anoxic contraction and release of endothelium-derived constricting factors (EDCFs) may relate to anoxic contraction in agonist-preconstricted rings. The relationship between the magnitude of agonist-induced tension and the extent of anoxia-mediated contraction may relate to the ability of endothelium to release EDRF and EDCFs as well as to the degree of phosphorylation in vascular smooth muscle cells. The reoxygenation-mediated contraction was noted to progressively increase in all experiments regardless of the pharmacologic agent used. This increase in reoxygenation-mediated contraction correlated with pre-existing pharmacologic tension, and may relate to calcium influx and restoration of ATP and other mediators in the vascular tissues during reoxygenation.

Effects of activated polymorphonuclear leukocytes on vascular smooth muscle tone.

Unstimulated polymorphonuclear leukocytes (PMNLs) release nitric oxide or a like material that relaxes vascular tissues. To determine the effects of activated PMNLs on vascular tone, precontracted rat aortic rings were exposed to ionophore A23187-treated PMNLs. Whereas "unstimulated" PMNLs caused 29 +/- 4% relaxation, "stimulated" PMNLs caused initial contraction followed by 90 +/- 7% relaxation of aortic rings. Indomethacin or the 5-lipoxygenase blocker piriprost had no effect on PMNL-induced initial contraction or subsequent relaxation. However, initial contraction was abolished and the subsequent vasorelaxation attenuated (22 +/- 5%) by the superoxide radical scavenger superoxide dismutase (SOD), suggesting that release of superoxide radicals may have induced vascular contraction and caused endothelial damage that would permit unopposed vasorelaxant effect of PMNLs. To examine this hypothesis, aortic rings were exposed to superoxide radicals (generated by xanthine plus xanthine oxidase, X + XO) or manually deendothelialized. These rings revealed marked relaxation (78 +/- 6 and 85 +/- 6%, respectively) in response to unstimulated PMNLs. These observations suggest that stimulated PMNLs exert an initial vasoconstrictor effect and a subsequent vasorelaxant effect in response to release of superoxide radicals and nitric oxide, respectively. Arachidonate metabolites or 5-lipoxygenase products do not appear to be important in the actions of PMNLs on vascular smooth muscle.

Interactions between nitroglycerin and endothelium in vascular smooth muscle relaxation.

To evaluate the role of endothelium in nitroglycerin (NTG)-mediated vascular relaxation, epinephrine-contracted rat thoracic aortic segments with and without intact endothelium were exposed to NTG (10(-10) to 10(-5) M). Aortic segments with intact (endo+, n = 15) and denuded endothelium (endo-, n = 9) exhibited typical NTG-induced relaxation. However, the mean effective concentration of NTG was lower for endo- than for endo+ segments (P less than 0.001). To determine if this phenomenon related to nitric oxide (NO) generation by endothelium, six endo+ segments were treated with NG-monomethyl-L-arginine (L-NMMA), an inhibitor of NO production. These endo+ segments exhibited greater (P less than 0.001) relaxation in response to NTG than the untreated endo+ segments. Oxyhemoglobin, an inhibitor of guanylate cyclase activation, greatly diminished NTG-mediated relaxation of all aortic segments. To determine if the enhanced NTG-mediated relaxation of endo- segments was unique to the guanosine 3',5'-cyclic monophosphate-dependent vasodilator NTG, other endo+ and endo- segments were exposed to adenosine 3',5'-cyclic monophosphate-dependent vasodilator papaverine (10(-8) to 10(-4) M), and no difference in EC50 was noted between endo+ and endo- segments. Thus endothelium attenuates NTG-mediated vasorelaxation, and this attenuation is abolished by inhibition of endothelial NO production with L-NMMA. These observations indicate that endothelium is a dynamic modulator of vascular smooth muscle relaxant effects of NTG. This modulation appears to result from a competitive interaction between endothelial NO and NTG.

Recovery of vascular smooth muscle relaxation from nitroglycerin-induced tolerance following a drug-free interval. A time-course in vitro study.

Hemodynamic tolerance occurs upon continuous exposure of vascular tissues to nitroglycerin (NTG). This phenomenon is believed to be due to the depletion of the tissue sulfhydryl (SH) group, which is essential for NTG-induced increase in tissue cyclic GMP and vasorelaxation. To determine the effect of an NTG-free interval on recovery of tissue cyclic GMP accumulation and vasorelaxation following development of NTG tolerance, isolated rat aortic rings were kept in Krebs physiologic buffer at 37 degrees, precontracted with epinephrine, and exposed to NTG. The mean concentration of NTG, which relaxed the rings by 50% (EC50) upon first exposure, was 1.1 x 10(-7) M (N = 20), and vascular cyclic GMP levels after NTG increased from 21 to 46 fmol/mg (P less than 0.02). A second exposure to NTG 15 min later increased the EC50 to 1.3 x 10(-4) M and cyclic GMP levels did not change (P less than 0.001 vs first NTG exposure), indicating tolerance to NTG. However, acetylcholine-mediated relaxation of aortic rings was preserved even in NTG-tolerant rings. A second exposure of tissues to NTG separated by 30, 60, and 120 min from the first exposure progressively decreased the EC50, such that at 120 min the EC50 of NTG was 0.4 x 10(-7) M (P = NS vs first NTG exposure). Tissue cyclic GMP levels increased from 14 to 71 fmol/mg (P = NS vs first NTG exposure). These data confirm development of tolerance to the vasorelaxant effects of NTG following initial exposure. An interval of 2 hr between multiple exposures of tissues to NTG results in preservation of the smooth muscle relaxation and an increase in tissue cyclic GMP in response to NTG.

Captopril-induced reversal of nitroglycerin tolerance: role of sulfhydryl group vs. ACE-inhibitory activity.

The angiotensin-converting enzyme (ACE) inhibitor captopril has been shown to reverse vascular tolerance to nitroglycerin (NTG). Whether captopril reverses NTG tolerance by providing sulfhydryl (SH) groups or by inhibiting ACE is not clear. To examine this issue, we treated rat aortic rings with buffer, captopril (SH +, ACE inhibitory activity +), enalaprilat (SH-, ACE inhibitory activity +), or N-acetylcysteine (NAC, SH+, ACE inhibitory activity-) prior to their contraction with epinephrine and subsequent relaxation with NTG. Previous exposure of NTG-treated rings resulted in marked resistance to the vasorelaxant effect of a subsequent exposure to NTG in buffer-treated rings. Both NAC and captopril, but not enalaprilat, potentiated the vasorelaxant effects of NTG during the first exposure of vascular rings to NTG and also prevented the development of tolerance to NTG during a second exposure. Buffer-treated rings showed an inability to accumulate cyclic guanosine monophosphate (GMP) in response to a second exposure to NTG. In contrast, both NAC and captopril-pretreated rings demonstrated a persistence of cyclic GMP accumulation during the second NTG exposure. The endothelium-dependent vasodilator acetylcholine (ACh) caused relaxation of the NTG-tolerant rings and also induced cyclic GMP accumulation in these rings. In other experiments, we found that prior exposure of vascular rings to ACh did not cause resistance to the subsequent vasorelaxant effects of ACh. NAC, captopril, and enalaprilat did not modulate the effects of ACh during either the first or subsequent exposures to ACh. In addition, indomethacin did not influence the "protective" effects of NAC or captopril against NTG tolerance.(ABSTRACT TRUNCATED AT 250 WORDS)