Coronary vasoconstriction in the conscious rabbit following intravenous infusion of L-NG-nitro-arginine

Cardioprotective action of fentanyl in a model of central sympathetic overactivity in rabbits: antiarrhythmic and anti-ischemic effects

BACKGROUND

Sympathetic overactivity resulting from perioperative noxious stimuli elicits hyperdynamic cardiovascular responses that may lead to myocardial ischemia and/or ventricular arrhythmia, especially in patients presenting with coronary artery disease. In the present study we investigated the cardioprotective effects of clinically relevant doses of fentanyl in an experimental model of sympathetic overactivity associated with myocardial ischemia in anesthetized rabbits.

METHODS

Central sympathetic stimulation was achieved through intracerebroventricular (i.c.v.) injection of L-glutamate (10 micro mol), with simultaneous inhibition of nitric oxide (NO) synthesis through i.v. administration of N(omega)-nitro-l-arginine methyl ester (L-NAME; 40 mg kg(-1)).

RESULTS

L-glutamate triggered ventricular arrhythmia and electrocardiographic alterations indicative of myocardial ischemia. The intravenous administration of fentanyl (5, 10 or 50 micro g kg(-1)) reduced the incidence of ST-segment shift (70, 20 and 10%, respectively, vs. 66.7% in controls) as well as of T-wave inversion from 58.3% to 30, 20 and 10%, respectively. The total number of ventricular premature complexes per minute fell from 65.2 +/- 16 in the control group to 6.8 +/- 3, 3.5 +/- 2 and 2.6 +/- 1.5, respectively. The occurrence of ventricular tachycardia and bigeminy was completely abolished by fentanyl. Finally, the i.v. administration of fentanyl did not induce significant hemodynamic effects (except for dP/dt(max) in the 50 micro g kg(-1)-dose).

CONCLUSION

Fentanyl elicits significant cardioprotective effects in a model of arrhythmia resulting from the association of central sympathetic overactivity with myocardial ischemia in rabbits, independently from its systemic hemodynamic actions.

Protective effects of centrally acting sympathomodulatory drugs on myocardial ischemia induced by sympathetic overactivity in rabbits

It is recognized that an imbalance of the autonomic nervous system is involved in the genesis of ventricular arrhythmia and sudden death during myocardial ischemia. In the present study we investigated the effects of clonidine and rilmenidine, two centrally acting sympathomodulatory drugs, on an experimental model of centrally induced sympathetic hyperactivity in pentobarbital-anesthetized New Zealand albino rabbits of either sex (2-3 kg, N = 89). We also compared the effects of clonidine and rilmenidine with those of propranolol, a beta-blocker, known to induce protective cardiovascular effects in patients with ischemic heart disease. Central sympathetic stimulation was achieved by intracerebroventricular injection of the excitatory amino acid L-glutamate (10 micro mol), associated with inhibition of nitric oxide synthesis with L-NAME (40 mg/kg, iv). Glutamate triggered ventricular arrhythmia and persistent ST-segment shifts in the ECG, indicating myocardial ischemia. The intracisternal administration of clonidine (1 microg/kg) and rilmenidine (30 microg/kg) or of a nonhypotensive dose of rilmenidine (3 microg/kg) decreased the incidence of myocardial ischemia (25, 14 and 25%, respectively, versus 60% in controls) and reduced the mortality rate from 40% to 0.0, 0.0 and 12%, respectively. The total number of ventricular premature beats per minute fell from 30 +/- 9 in the control group to 7 +/- 3, 6 +/- 3 and 2 +/- 2, respectively. Intravenous administration of clonidine (10 micro g/kg), rilmenidine (300 microg/kg) or propranolol (500 microg/kg) elicited similar protective effects. We conclude that clonidine and rilmenidine present cardioprotective effects of central origin, which can be reproduced by propranolol, a lipophilic beta-blocking agent.

Hypertension and impairment of endothelium-dependent relaxation of arteries from spontaneously hypertensive and L-NAME-treated Wistar rats

Effects of chronic treatment of normotensive Wistar rats with N(omega)-nitro-L-arginine methyl ester (L-NAME) on blood pressure and on endothelium-dependent relaxation of the aorta, carotid and iliac arteries were studied. The endothelium-dependent relaxation was compared in arteries from normotensive Wistar Kyoto rats (WKY) and genetically hypertensive rats (stroke-prone spontaneously hypertensive rats, SHRSP). Chronic treatment of normotensive Wistar rats with L-NAME caused an elevation of blood pressure. The elevated blood pressure at 15 weeks of age was significantly higher in these animals than that of untreated Wistar rats, but lower than that of SHRSP. Endothelium-dependent relaxation of the arteries induced by acetylcholine (ACh) was almost abolished by chronic treatment with L-NAME. The remaining small relaxation in arteries from L-NAME-treated rats was completely inhibited by application of L-NAME (10(-4) M). In such preparations, higher concentrations of ACh induced a contraction, which was abolished by removal of the endothelium or by an application of indomethacin (10(-5) M). Endothelium-independent relaxation induced by sodium nitroprusside was similar between preparations from untreated and L-NAME-treated Wistar rats. Endothelium-dependent relaxation was significantly impaired in preparations from SHRSP, when compared with that in those from WKY. However, the impairment was less prominent in preparations from SHRSP than in those from L-NAME-treated rats. These results suggest that the impairment of endothelium-dependent relaxation in the arteries from L-NAME-treated rats is not due to the elevated blood pressure resulting from the chronic treatment, and that impairment of NO synthesis by the endothelium does not play a major role in the initiation of hypertension in SHRSP.

Influence of secretin and L-NAME on vascular permeability in the coronary circulation of intact and diabetic rats

The permeability in the intact and diabetic rat coronary circulation after administration of secretin (3.0 micromol/kg i.v.), an inhibitor of NOS (nitric oxide synthase), and L-NAME (N(G)-nitro-L-arginine-methyl ester hydrochloride) (1 mg/kg i.v.), and both substances given together, were studied. To measure protein extravasation Evans blue dye was used as a marker of vascular permeability. The vascular permeability of the left ventricle did not differ in intact and diabetic rats. In the diabetes state increased permeability of atria was observed. Administration of secretin did not influence the coronary vascular permeability in either the intact or the diabetic rats. L-NAME increased the atria permeability and did not change left ventricle permeability. In diabetes, injection of L-NAME caused a decrease in the permeability in both the atria and left ventricle. In intact rats secretin diminished the L-NAME effect in the atria. In diabetic rats co-administration of secretin+L-NAME increased the permeability of the atria and left ventricle, but L-NAME administered alone decreased them. Secretin modified the effect of L-NAME on coronary permeability in intact and diabetic rats.

Effect of Ca2+ channel blockers on arterial hypertension and heart ischaemic lesions induced by chronic blockade of nitric oxide in the rat

The effects of the Ca2+ channel blockers diltiazem, nifedipine and amlodipine were investigated on both arterial hypertension and myocardial changes induced by chronic blockade of nitric oxide synthesis. Control male Wistar rats received Nomega-nitro-L-arginine methyl ester (L-NAME; 20 mg rat(-1) day(-1)) in the drinking water for 8 weeks; blood pressure and body weight were monitored weekly. The Ca2+ channel blockers were given concomitantly to L-NAME, as follows: diltiazem (13.5 mg rat(-1) day(-1)) and amlodipine (6.25 mg rat(-1) day(-1)) were administered in the drinking water whereas nifedipine (6.25 mg rat(-1) day(-1)) was given in the chow. Nomega-nitro-L-arginine methyl ester induced a time-dependent increase in blood pressure which was significantly attenuated by diltiazem (154+/-1.6 vs. 139+/-1.6 mm Hg, p < 0.05), nifedipine (166+/-2.7 vs. 150+/-2.1 mm Hg, p < 0.05) and amlodipine (208+/-5.8 vs. 158+/-1.8 mm Hg, p < 0.05) at the last week of the treatment. Rats treated with the L-NAME also developed myocardial ischaemia, as indicated by the increased percentage of fibrous tissue found in the left ventricles of these animals (10.9+/-0.1%, p < 0.01) when compared to control ones (6.3+/-0.1%). Neither diltiazem (14.9+/-1.2%) nor nifedipine (11.1+/-1.5%) prevented this effect whereas amlodipine (6.9+/-1.1%, p < 0.01) virtually abolished the increase in fibrous tissue induced by L-NAME. The plasma concentration of the Ca2+ channel blockers was measured by liquid chromatography coupled to mass spectrometry at two different time points (morning and afternoon). Only amlodipine treatment was able to maintain constant levels (186+/-46 ng ml(-1) in the morning and 110+/-19 ng ml(-1) in the evening) compared to nifedipine (3003+/-578 ng ml(-1) in the morning and 436+/-100 ng ml(-1) in the evening) and diltiazem (77+/-51 ng ml(-1) in the morning and not detectable in the evening). In conclusion, our results indicate that amlodipine (but not diltiazem and nifedipine) can efficiently control myocardial ischaemia in nitric oxide deficient rats, probably due to its intrinsically long half-life.

Role of nitric oxide and endothelium in the flow-induced dilation of rat coronary arteries under two preconstriction conditions

1. Pressure-induced tone and flow-induced dilations were studied in a rat perfused epicardial coronary artery mounted in an arteriograph. Spontaneous tone was assessed in arteries submitted either to 60 or 90 mmHg intraluminal pressure either under control conditions, after incubation with NG-nitro-L-arginine methyl ester (L-NAME; 100 mumol/L) or after endothelial denudation. Flow-induced dilation was quantified under these conditions in preparations either submitted to 60 mmHg and preconstricted with 10 mumol/L 5-hydroxytryptamine (5-HT) or exhibiting spontaneous tone at 90 mmHg. 2. Spontaneous tone was greater at 90 mmHg compared with tone obtained at 60 mmHg (21 +/- 2 vs 10 +/- 2% reduction of the fully dilated diameter after sodium nitroprusside incubation, respectively). Incubation with L-NAME or removal of the endothelium significantly increased spontaneous tone at both pressures compared with control. 3. In arteries submitted to 60 mmHg and preconstricted with 10 mumol/L 5-HT, flow (0-800 microL/min) induced a continuous dilation (maximal value 63 +/- 4%). As a function of flow, shear stress first increased and then plateaued at values of approximately 76 +/- 6 dyn/cm2. After L-NAME incubation or endothelial denudation, the flow-induced dilation was reduced to the same extent and was obtained for higher values of shear stress (172 +/- 14 and 150 +/- 14 dyn/cm2, respectively). 4. In arteries exhibiting spontaneous tone, starting flow led, first, to a constriction followed by a dilation up to 76 +/- 4% of the initial tone. Incubation with L-NAME greatly altered flow-induced dilation. Endothelium removal further reduced the dilation obtained for very high values of shear stress (up to 300 dyn/cm2). 5. The present study shows that different patterns of vasodilation induced by flow can be observed, depending on the initial vasoconstrictor stimulus. In 5-HT-preconstricted arteries, flow-induced dilation appears to be fully dependent on the synthesis and release of nitric oxide. In arteries with spontaneous tone, a vasoconstrictor substance could be released for low values of flow. Nitric oxide is mainly, but not exclusively, responsible for the vasodilation. For both experimental conditions, removal of the endothelium greatly reduced the response, but a dilation was still observed.

Bilateral kidney ligation abolishes pressor response to N(G)-nitro-D-arginine

We have shown that N(G)-nitro-D-arginine (D-NNA) is 50% as potent as N(G)-nitro-L-arginine (L-NNA) in causing pressor response and 2-3% as potent as L-NNA in inhibiting endothelium-dependent relaxation in vitro. These results suggest in vivo activation of D-NNA. Furthermore, the potency of D-NNA was markedly increased after it had been incubated with homogenate of the kidney, but not plasma or homogenate of the aorta, lungs or liver. This study examined if bilateral ligation of the kidneys attenuated the biological action of D-NNA. I.v. bolus of D-NNA (16 mg/kg), L-NNA (3 mg/kg) and norepinephrine (0.25-16 microg/kg) increased arterial pressure in sham-operated rats. Bilateral ligation of the kidneys abolished pressor response to D-NNA, but not L-NNA and norepinephrine. I.v. bolus D-NNA in sham-operated rats, but not kidney-ligated rats, inhibited relaxation response to acetylcholine in pre-constricted aortic rings ex vivo. These results indicate that the kidney is the primary organ which activates D-NNA.

Sex differences and role of nitric oxide in blood flow of canine urinary bladder

Continuous measurements were made of bladder blood flow by laser Doppler flowmetry in anesthetized dogs during bladder filling and emptying. In both mucosa and muscle, perfusion was inversely proportional to intravesical pressure. There was significantly greater perfusion in the bladder mucosa of males than females at baseline and up to 10 cm water filling pressure but not in the muscle. Intra-arterial infusion of the nitric oxide synthase inhibitor NG-nitro-L-arginine produced a significant decrease in resting bladder perfusion in the mucosa only, with no differences seen in the response to intravesical pressure. Intra-arterial infusion of L-arginine produced a significant increase in the level of perfusion in the mucosa seen immediately after the bladder was drained. No changes were observed in muscle perfusion after L-arginine. These results suggest that the perfusion of the bladder mucosa differs by gender and is regulated differently than the bladder muscle, possibly related to the different function of the two layers.

The cardiovascular effects of the administration of L-NAME during the early posthemorrhagic period

1. The effects of the various doses of NG-nitro-L-arginine methyl ester (L-NAME, 10 and 30 mg/kg) on some cardiovascular and biochemical parameters during the early posthemorrhagic period were studied in anesthetized rabbits subjected to hemorrhagic hypovolemia. 2. Hemorrhagic shock was produced by intermittent bleeding of 40% of the estimated blood volume for 15 min. Blood samples were taken before and after bleeding (0, 15 and 60 min). Simultaneously, the mean arterial pressure (MAP) and the heart rate (HR) were measured. Hemorrhaged rabbits were treated by L-NAME10 or L-NAME30 (10 or 30 mg/kg, i.v. bolus injection, respectively) or the corresponding volumes of saline (0.6 ml, i.v. bolus) immediately after the end of bleeding. 3. The observed cardiovascular parameters (MAP, HR) were significantly reduced after the end of bleeding in all rabbits. 4. The rise of the MAP was significantly more pronounced 30 min after the injection of L-NAME30 in comparison with the corresponding values in the saline (S) group. In contrast, L-NAME10 produced only a small, insignificant increase in the MAP in hemorrhaged rabbits. 5. The L-NAME30-induced rise of the MAP was accompanied by a severe bradycardia, hyperkalemia and an aggravated metabolic acidosis, more severe than the corresponding disturbance of the acid-base status in the S group. The changes in the acid-base parameters were observed both in arterial (pH, excess base) and in venous blood (pH) of hemorrhaged rabbits. 6. In conclusion, the i.v. bolus injection of L-NAME30 (immediately after the end of bleeding) produced a significant increase in the MAP during the first hour after the injury, but the presumable inhibition of the endothelial constitutive nitric oxide synthase during the early posthemorrhagic period resulted in severe cardiovascular and metabolic disturbances.

Tissue variation of acute haemodynamic changes by NG-nitro-L-arginine in stroke-prone spontaneously hypertensive and Wistar-Kyoto rats

1. The acute effects of nitric oxide synthase inhibition on the haemodynamics in stroke-prone spontaneously hypertensive (SHRSP) and normotensive Wistar-Kyoto (WKY) rats were investigated using radiolabelled microspheres. 2. Intravenous administration of 3 and 6 mg/kg NG-nitro-L-arginine (L-NNA) caused a significant increase in total peripheral resistance, a decrease in cardiac output and an increase in blood pressure in both SHRSP and WKY rats. 3. Significant decreases in regional blood flow (RBF) in the lung, muscle and stomach of WKY rats were observed following L-NNA administration. 4. NG-nitro-L-arginine produced a 70% increase in brain regional blood flow at a dose of 6 mg/kg only in SHRSP. 5. There was a variation in the involvement of nitric oxide (NO) in different tissues. 6. It is concluded that hypertension in SHRSP augments NO-mediated vasodilation.

Physostigmine and modulators of nitric oxide system on the mean arterial pressure of the spontaneously hypertensive rat

1. A slow intravenous infusion of L-arginine (3 mg kg-1) lasting one hr produced significant hypotension in urethane-anaesthetized spontaneously hypertensive rats (SHRs). 2. A slow intravenous infusion of NG-nitro-L-arginine methyl ester (L-NAME) (3 mg kg-1 h-1) did not produce any significant change in the mean arterial pressure during infusion. After stopping infusion of L-NAME, a slowly developing increase of the mean arterial pressure was observed during the following 40 min. 3. The pressor response to physostigmine (20, 40 and 80 micrograms kg-1, IV), injected during a slow intravenous infusion of either L-arginine or L-NAME, was not changed. 4. L-arginine and L-NAME depressed the pressor responses to physostigmine, if physostigmine was injected after the end of a 1-hr infusion. 5. Acute pretreatment with increasing doses of physostigmine markedly affected the blood pressure response to L-arginine (i.e., L-arginine-caused hypotension was more pronounced), but only slightly that to L-NAME. 6. In conclusion, L-arginine, as a donor of NO, produced hypotension by itself and also decreased, but not significantly, the central cholinergically-mediated hypertension (CCMH) produced by physostigmine. It is quite possible that the peripheral NO released by L-arginine antagonized the increased adrenergic activity in the CCMH. This does happen in normotensive rats, but to a lesser degree than in SHRs, as shown in the current experiments. 7. Also, our results show that inhibition of endogenous NO biosynthesis using L-NAME does not necessarily lead to pressor response in vivo, at least in SHRs. It is concluded that L-arginine-nitric oxide pathways operate in SHRs, as well as in normotensive Wistar rats, but their role in modulating cholinergically-mediated regulation of the mean arterial pressure is less pronounced in SHRs than in normotensive animals.

Dissociation between the increase in systemic vascular resistance induced by acute nitric oxide synthesis inhibition and the decrease in cardiac output in anesthetized dogs

The decrease in cardiac output (CO) that follows nitric oxide (NO) synthesis inhibition is thought to be the result of an increase in systemic vascular resistance (SVR). We investigated whether sodium nitroprusside (SNP) and iloprost prevent the decrease in CO induced by short-term administration of N omega-nitro-L-arginine methyl ester (L-NAME) in anesthetized dogs. The left femoral artery and vein were cannulated for mean arterial blood pressure (MABP) measurement and drug administration, respectively. A Swan-Ganz thermodilution catheter was inserted into the right femoral vein and allowed the determination of CO and the calculation of SVR, expressed as the cardiac index (CI) and the index of systemic vascular resistance (ISVR), respectively. L-NAME (0.01-10.0 mg/kg; n = 13) induced dose-dependent increases in MABP and in the ISVR. These changes were accompanied by significant decreases in both the CI and the heart rate. SNP (1 microgram/kg/min; n = 6) virtually abolished L-NAME-induced hypertension and significantly attenuated both the increase in the ISVR (< 3.0 mg/kg) and the decrease in CO. Iloprost (50 ng/kg/min; n = 6) also abolished L-NAME-induced hypertension and markedly attenuated the increase in SVR. However, the decrease in CO was not prevented by this vasodilator. These results clearly demonstrate that the increase in SVR is not the major factor accounting for the decrease in CO after short-term NO synthesis inhibition in anesthetized dogs.

Response of spinal cord blood flow to the nitric oxide inhibitor nitroarginine

OBJECTIVE

The extent to which nitric oxide (NO) is involved in the modulation of spinal cord blood flow (SCBF) in the uninjured and injured cord is unknown. To elucidate these questions, the following experiments in anesthetized rats were conducted.

METHODS

Because NO is an unstable free radical with a half-life of seconds, its role can be understood through the study of the NO synthase inhibitor L-NG-nitroarginine (L-NOARG). L-NOARG was administered intravenously for 30 minutes at a dose of 100 or 500 micrograms/kg/min in 12 and 10 uninjured animals, respectively. SCBF fluctuations at C7-T1 were measured using laser doppler flowmetry. In a second set of 12 rats, L-NOARG (500 micrograms/kg/min) was administered 10 minutes before spinal cord injury using a modified aneurysm clip at C7-T1 and continued for 30 minutes thereafter.

RESULTS

In the uninjured animals, L-NOARG was associated with a dose-dependent increase in mean arterial pressure of 20 to 80% above baseline (P = 0.0001), together with a dose-related decrease in SCBF (P = 0.0373). In the injured animals, L-NOARG was associated with a 48% increase in mean arterial pressure. With L-NOARG, the changes in SCBF from baseline after injury were similar to those of noninjured controls (n = 25) and significantly less than injury controls (n = 18) or those receiving phenylephrine (n = 8).

CONCLUSION

NO synthase inhibitors, by reducing available NO, cause systemic vasoconstriction and a decrease in SCBF in the uninjured spinal cord. In the injured spinal cord, the administration of L-NOARG results in a redistribution of blood flow with an augmentation in posttraumatic SCBF at the injury site.

Possible involvement of endothelial leukotrienes in acetylcholine-induced contraction in rabbit coronary artery

In endothelium-intact preparations of rabbit coronary artery, acetylcholine (3 x 10(-8)-10(-6) M) caused a contraction in the presence of N omega-nitro-L-arginine methyl ester (10(-4) M). Removal of endothelium significantly attenuated the contraction. The present experiments were undertaken to elucidate whether the acetylcholine-induced contraction in endothelium-intact preparations was due to arachidonic acid metabolites, endothelin or superoxide. The acetylcholine-induced contraction in endothelium-intact preparations was attenuated by manoalide (10(-6) M and 3 x 10(-6) M) or oleyloxyethyl phosphorylcholine (10(-5) M) (phospholipase A2 inhibitors), BAY x 1005 (3 x 10(-6) and 10(-5) M) or L 663,536 (10(-6) and 10(-5) M) (leukotriene synthesis inhibitors) and ONO-1078 (10(-6) and 3 x 10(-6) M) or SK&F 104353 (10(-6) and 3 x 10(-6) M) (leukotriene antagonists). The contraction was not affected by aspirin (10(-4) M) or indomethacin (10(-6) M) (cyclooxygenase inhibitors), S-1452 (10(-8) M) or ONO-3708 (10(-7) M) (thromboxane A2 antagonists), FR139317 (10(-6) M) (endothelin receptor antagonist) or superoxide dismutase (150 u/ml) combined with catalase (1000 u/ml), or allopurinol (10(-5) M) (antioxidants). In contrast, the endothelium-independent contraction induced by acetylcholine was unaffected by any of these inhibitors and antagonists listed above. The present experiments demonstrate that in the presence of N omega-nitro-L-arginine methyl ester, acetylcholine partly causes an endothelium-dependent contraction in rabbit coronary artery which is probably due to leukotriene C4 and D4, but does not involve production of thromboxane A2, endothelin or superoxide.

The effect of a nitric oxide donor and an inhibitor of nitric oxide synthase on blood flow and vascular resistance in feline submandibular, parotid and pancreatic glands

The aim was to examine whether (1) blood flow and vascular resistance are altered in response to exogenous nitric oxide and (2) whether endogenous synthesis of nitric oxide participates in the haemodynamic regulation of the submandibular, parotid and pancreatic glands. Experiments were performed on anaesthetized, artificially ventilated cats. Mean arterial blood pressure, heart rate, blood gases, cardiac output and tissue blood flow were determined before and 15 min after intravenous administration of either the nitric oxide donor SIN-1 (3-morpholinosydnonimine, 1 mg/kg, n = 10) or the competitive nitric oxide synthase inhibitor NOLA (NG-nitro-L-arginine, 30 mg/kg, n = 9) blood flow was measured by a radioactive-labelled microsphere method. In the SIN-1 group, in spite of a serious decrease in mean arterial blood pressure (p < 0.001), the blood flow in the glands remained unchanged. The vascular resistance decreased after SIN-1 in the submandibular and pancreatic glands (p < 0.001 and p < 0.05, respectively), and was slightly reduced in the parotid. The NOLA increased mean arterial blood pressure (p < 0.01) and reduced the blood flow in the submandibular and pancreatic glands (p < 0.01 and p < 0.001, respectively), but the decrease in the parotid was not significant. Vascular resistance increased after NOLA in all three glands (p < 0.05, p < 0.001 and p < 0.05). These findings suggest that basal nitric oxide production in these exocrine glands is sufficient to modulate vascular resistance. Moreover, the release of endogenous NO from the nerves and/or endothelium is probably involved in the regulation of vascular tone. The nitric oxide-dependent component of blood-flow regulation, however, seems to be less pronounced in the parotid gland.

Human dermal fibroblasts produce nitric oxide and express both constitutive and inducible nitric oxide synthase isoforms

Nitric oxide (NO) is produced by a variety of human and animal cells and is involved in a broad array of physiological and pathophysiological processes. It can cause vasodilation, serve as a neurotransmitter, and have anti-neoplastic, anti-microbial, and anti-proliferative effects. In this study, we have demonstrated that fibroblasts derived from human skin spontaneously produce NO and that this production can be enhanced by stimulating the cells with interferon-gamma and lipopolysaccharide. The production of NO by human dermal fibroblasts can be blocked by NG-monomethyl-L-arginine (L-NMMA). The inhibitory effect of L-NMMA on NO production was restored by addition of L-arginine but not D-arginine. By measuring the rate of conversion of [14C]L-arginine to [14C]L-citrulline, we show that unstimulated cells expressed only Ca2+-dependent NO synthase (NOS) activity (1.36 +/- 0.57 pmol/mg/min; n = 4) whereas stimulated cells expressed both Ca2+-dependent (2.60 +/- 0.54 pmol/mg/min; n = 4) and -independent (1.59 +/- 0.14 pmol/mg/min; n = 4) NOS activities. With reverse transcription polymerase chain reaction (RT-PCR), the 422-bp RT-PCR product for human endothelial constitutive NOS and the 462-bp RT-PCR product for human hepatocyte inducible NOS were detected in proportion to the amount of mRNA-related RT-cDNA added to the reaction mixture. Further evidence by immunocytochemistry demonstrated that human dermal fibroblasts express both constitutive and inducible NOS proteins. These data collectively suggest that in addition to macrophages and other inflammatory cells, nitric oxide production by dermal fibroblasts could be important during the inflammatory stages of wound healing and possibly also in the later stages of proliferation and tissue remodeling after skin injury in humans.

A proposed mechanism for the cardioprotective effect of oestrogen in women: enhanced endothelial nitric oxide release decreases coronary artery reactivity

1. During their reproductive years women have a much lower incidence of coronary heart disease than men of similar age. A cardioprotective effect of circulating oestrogen appears to be responsible for this decrease in cardiovascular mortality in women. 2. Oestrogen can enhance nitric oxide (NO) production by the vascular endothelium, possibly through enhanced production of the enzyme NO synthase. 3. Pressure-induced constrictions in isolated coronary arteries from rats with physiological circulating levels of oestrogen are reduced compared to oestrogen-deficient animals. This difference is abolished by endothelial removal or inhibition of NO synthase. 4. NO through stimulation of guanylyl cyclase increases levels of the cytosolic second messenger cyclic GMP (cGMP) which activates a cGMP-dependent protein kinase in vascular smooth muscle cells. 5. Potassium currents through calcium-activated channels in vascular smooth muscle cells are increased in response to NO or upon exposure to cGMP-dependent protein kinase. 6. In rat coronary arteries dilations to NO are reduced by agents which inhibit calcium-activated potassium channels. NO can also hyperpolarize this tissue, suggesting membrane potential changes are involved in the response to NO. 7. We propose that oestrogen increases NO production leading to more negative membrane potentials and decreased calcium entry in coronary vascular smooth muscle cells.

Haemodynamic effects of NO-synthase inhibitors, NG-monomethyl-L-arginine, NG-nitro-L-arginine and NG-nitro-L-arginine methyl ester, and NO-donor, glyceryl trinitrate, in conscious SHRSP and WKY

1. The experiments were carried out by an ordinary and a microsphere method in order to clarify the effects of three nitric oxide-synthase inhibitors and -donor on the circulating system between SHRSP and WKY. 2. Intravenous administration of each compound possessing the nitric acid-synthase inhibiting action markedly elevated the systolic blood pressure both in SHRSP and WKY, but more prominently in the former. 3. The hypotensive and tachycardic responses to glyceryl trinitrate were more enhanced after treatment with NG-nitro-L-arginine and NG-nitro-L-arginine methyl ester but not with NG-monomethyl-L-arginine compared with the non-treated one. 4. All peripheral organs except the brain decreased their regional blood flow after administration of NG-nitro-L-arginine, indicating a crucial involvement of nitric oxide in the peripheral circulation. 5. It was suggested that the nitric oxide system worked more sensitively and actively in SHRSP than in WKY in order to maintain the peripheral blood flow and systemic blood pressure.

Effects of NO modulation on cardiac arrhythmias in the rat isolated heart

It has been proposed that NO may function as an endogenous cardioprotectant. We have investigated whether modulation of NO levels (detected in coronary effluent by chemiluminescence) by a blocker of its synthesis, by supplementation of its precursor, and by administration of an NO donor can influence reperfusion arrhythmias in the isolated rat heart. Rat hearts were perfused with modified Krebs' solution and subjected to 5, 35, or 60 minutes of left regional ischemia followed by 10 minutes of reperfusion. NG-Nitro-L-arginine methyl ester (L-NAME), which blocks NO synthase, increased the incidence of reperfusion-induced ventricular fibrillation (VF) from 5% in the control condition to 35% after 60 minutes of ischemia (n = 20, P < .05). The profibrillatory effect of L-NAME was prevented in hearts coperfused with 1 or 10 mmol/L L-arginine (an NO precursor) but persisted in hearts coperfused with D-arginine (1 mmol/L). L-NAME did not increase VF susceptibility in hearts reperfused after 5 or 35 minutes of ischemia. L-NAME caused sinus bradycardia (264 +/- 10 versus 309 +/- 5 bpm in control groups, P < .05) and reduced coronary flow before ischemia (6.2 +/- 0.6 versus 9.2 +/- 0.6 mL.min-1.g-1 tissue in controls, P < .05). L-NAME reduced coronary effluent NO levels after 60 minutes of ischemia; during the first minute of reperfusion, values were reduced from 1457 +/- 422 to 812 +/- 228 pmol.min-1.g-1 (P < .05). This effect was prevented by coperfusion with L-arginine (10,344 +/- 1730 pmol.min-1.g-1, P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of the inhibitor of nitric oxide synthase, NG-nitro-L-arginine methyl ester, on cerebral and myocardial blood flows during hypoxia in the awake dog

The increase in cerebral blood flow (CBF) elicited by moderate hypoxia in anesthetized animals is little attenuated by nitric oxide (NO) synthase inhibitors. However, in previous studies, the effects of NO synthase inhibitors may have been altered by anesthetics. Consequently, we studied the effects of the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), on cerebral and myocardial blood flows during hypoxia in the awake dog. Regional CBF and myocardial blood flows (MBF) were measured under normoxia and hypoxia in 16 beagle dogs after an intravenous (IV) injection of either saline (control, n = 8) or L-NAME 20 mg/kg (n = 8). One week after thoracotomy for catheter insertion, awake dogs were studied during three periods: normoxia and after 2 and 4 h of normocapanic hypoxia in an environmental chamber (FIO2 = 0.10, FICO2 = 0.035, balance N2). At each stage, a bolus injection of L-NAME or saline was followed 15 min later by left atrial injection of radiolabeled microspheres (141Ce, 103Ru, 46Sc) for regional CBF and MBF. After the dogs were killed, the brain and the heart were fixed in 10% formaldehyde, dissected by region and weighed, and radioactivity was measured in a gamma counter. During hypoxia, Pao2 was approximately 45 mm Hg with normal Paco2. In the control group, CBF increased by 45% after 2 h and 48% after 4 h of hypoxia; MBF increased by 69% and 60%, respectively. L-NAME prevented the CBF increase during hypoxia and the MBF increase after 2 h of hypoxia; after 4 h of hypoxia the measurement of MBF was confounded by cardiac dysfunction. These results suggest that NO plays a role in cerebral vasodilation during hypoxia in the awake animal.

Modulation of vasoconstriction by endothelium-derived nitric oxide: the influence of vascular disease

1. The endothelium makes a significant contribution to the regulation of vascular tone through the release of potent vasodilator agents such as nitric oxide (NO) and prostacyclin (PGI2) as well as vasoconstrictor compounds such as endothelin. Recognition of this function of the endothelium has created a new focus for the investigation of vasoconstrictor activity under physiological and pathological conditions. 2. It has been well established that removal of the endothelium enhances responses to a variety of contractile agents in conductance arteries and that such modulation is predominantly due to the release of NO. The use of selective inhibitors of NO synthesis has confirmed that the endothelium-derived nitric oxide also modulates constriction in resistance vessels. 3. In a number of cardiovascular disease states there is impairment of endothelial function. Thus one of the consequences of atherosclerosis, hypertension and ischaemia is a reduction in endothelium-dependent vasodilatation, both at a basal level and in response to endogenous and exogenous stimuli. In addition, enhanced responses to vasoconstrictors have been reported in those disease states. Such observations have led to the attractive hypothesis that enhanced constriction in vascular disease results from attenuate NO-induced dilatation. However, whilst there is some evidence that pathological impairment of endothelial function is accompanied by increased constrictor activity, particularly where serotonergic mechanisms are involved, it is inappropriate to make the general assumption that where disease impairs NO activity there will also be increased sensitivity to all constrictor stimuli.

Evidence for the role of nitric oxide in the circulation of the dental pulp

Many authors have studied the hemodynamics of the dental pulp; however, there are scarcely any data regarding the involvement of the L-arginine/nitric oxide pathway in the regulatory mechanism. Thus, we have examined the physiological effects of (1) NG-nitro-L-arginine as an inhibitor of nitric oxide synthesis and (2) the nitric oxide donor 3-morpholinosydnonimine on blood flow and vascular resistance in the canines of anesthetized cats to study the potential involvement of nitric oxide in the regulation of dental vascular homeostasis. Mean arterial blood pressure, heart rate, blood gases, pH, cardiac output, and tissue blood flow were determined prior to and 15 min after i.v. administration of either NG-nitro-L-arginine (30 mg/kg, n = 9) or 3-morpholinosydnonimine (1 mg/kg, n = 7). Blood flow was measured by radioactive-labeled microspheres. There were no significant differences in baseline parameters between the two groups of cats. The dental pulp blood flow decreased to 53 +/- 13% (p < 0.01) of the control level after NG-nitro-L-arginine administration, while it decreased only slightly (to 82 +/- 12%) after 3-morpholinosydnonimine administration. The dental pulp's vascular resistance increased to 367 +/- 69% (p < 0.01) of the control level after NG-nitro-L-arginine, while it decreased to 73 +/- 10% (p < 0.05) of control after 3-morpholinosydnonimine. We found that the L-arginine/nitric oxide pathway plays an important role in the regulation of pulpal blood circulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of coronary blood flow by endothelial release of nitric oxide

1. Nitric oxide (NO) is released from vascular endothelium following conversion of L-arginine to L-citrulline by calcium-calmodulin-dependent 'constitutive' NO-synthase. 2. Nitric oxide release occurs under basal conditions, in response to chemical stimuli (acetylcholine, bradykinin, thrombin, prostacyclin, serotonin, etc.) and in response to changes in shear stress (effects of blood velocity on vascular endothelium). 3. Analogues of L-arginine inhibit NO and are widely used to study the effects of NO on the cardiovascular system: in intact animals, these inhibitors cause vasoconstriction, leading to an increase in arterial blood pressure (ABP) and bradycardia. 4. Bradycardia induced by NO inhibitors is due, in part, to baroreceptor activity following the increase in ABP and in part to a direct effect on the sino-atrial node. 5. In the intact animals and isolated perfused heart, NO inhibitors cause coronary vasoconstriction and hence a reduction in basal coronary flow. This effect, however, is not seen in isolated coronary vessels. 6. From experiments in which ABP did not change, NO does not appear to have an important role in regulating coronary vasomotor tone under basal conditions. 7. Nitric oxide appears to be involved in the duration of reactive hyperaemia following coronary vascular occlusion but is not involved to any significant extent in the peak amplitude of hyperaemia. 8. Responses to vasodilator stimuli which do not involve NO in the initiation of the vasodilation may be prolonged by the effect of increased blood flow (shear stress) which releases NO and potentiates hyperaemia.

Enhancement of noradrenergic constriction of large coronary arteries by inhibition of nitric oxide synthesis in anaesthetized dogs

1. Coronary vascular responses to bilateral carotid occlusion (BCO) and the intravenous infusion of tyramine (Tyr, 20 micrograms kg-1 min-1) and noradrenaline (NA, 0.5 microgram kg-1 min-1) were examined after bilateral vagotomy and antagonism of beta-adrenoceptors. BCO, Tyr and NA decreased large coronary artery diameter and increased mean coronary resistance and systemic arterial pressure without affecting heart rate. 2. Inhibition of nitric oxide (NO) synthase with NG-nitro-L-arginine (L-NNA, 5 and 15 mg kg-1) significantly increased mean arterial pressure and decreased heart rate and large coronary artery diameter. Mean coronary resistance was unaffected by either dose of L-NNA. L-NNA significantly reduced depressor and coronary vasodilator responses to the endothelium-dependent vasodilator acetylcholine (ACh, 10 micrograms kg-1, i.v.). Systemic and coronary vasodilator responses to sodium nitroprusside (SNP, 5 micrograms kg-1) were unaffected by L-NNA with the exception that the dilatation of the large coronary artery was significantly enhanced by the higher dose. 3. L-NNA significantly enhanced constriction of the large coronary arteries caused by BCO, Tyr and NA but did not affect the increases in mean coronary resistance or systemic arterial pressure. 4. Inhibition of NO synthesis enhances adrenergic constriction of large coronary arteries caused by both neuronally released and exogenous noradrenaline. In contrast, L-NNA did not affect adrenergic constriction of coronary or systemic resistance vessels. Endothelium-derived NO may play an important role in the modulation of noradrenergic vasoconstriction in coronary conductance arteries.

The effects of physostigmine, L-arginine and NG-nitro-L-arginine methyl ester (L-NAME) on the mean arterial pressure of the rat

The effects of physostigmine, L-arginine and NG-nitro-L-arginine methyl ester (L-NAME) were investigated in urethane-anaesthetized rats. The drugs were chosen because physostigmine has been known to produce an increase in peripheral adrenergic activity, whereas L-arginine and L-NAME have been known to modulate nitric oxide (NO) production. Slow infusion of L-arginine produced significant hypotension, but only in animals pretreated by physostigmine. L-NAME applied in the same way produced a slow developing increase in blood pressure, but not in animals pretreated by physostigmine. The pressor responses to physostigmine were potentiated if the drug was injected during infusion of L-NAME, and depressed if the drug was injected after stopping L-NAME infusion (in rats not pretreated with physostigmine). It is concluded that L-arginine-NO pathways act in vivo to oppose peripheral vasoconstrictor influences coupled with central cholinergically mediated activation of the adrenergic system, as produced by physostigmine. In this way, NO is part of a general mechanism for blood pressure regulation.

NG-nitro L-arginine methyl ester: systemic and pulmonary haemodynamics, tissue blood flow and arteriovenous shunting in the pig

The effects of NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of the endothelial nitric oxide (NO) biosynthesis, on systemic and pulmonary haemodynamics, and tissue as well as arteriovenous anastomotic blood flows were investigated in the anaesthetized pig, using simultaneous injections of radioactive microspheres of two different sizes (diameter: 15 and 50 microns). L-NAME (1, 3 and 10 mg.kg-1) reduced systemic and pulmonary artery conductance and cardiac output, but heart rate and mean arterial blood pressure remained unchanged. L-arginine reversed the systemic and pulmonary haemodynamic changes induced by L-NAME. As detected with 15 microns microspheres, L-NAME (1 and 3 mg.kg-1) decreased tissue blood flow to and vascular conductance in the eyes, lungs, atria, kidneys, adrenals and liver. Furthermore, the difference between blood flows simultaneously measured with 15 and 50 microns microspheres, which can be equated to blood flow through arteriovenous anastomoses with a diameter between about 28 and 90 microns, was reduced by L-NAME (3 mg.kg-1) in the skin of head and gluteal regions and, as indicated by the microsphere content of the lungs, in the total systemic circulation. These results suggest that in the anaesthetized pig (i) NO is involved in the regulation of both systemic and pulmonary vascular conductance, (ii) the decrease in systemic vascular conductance is in part due to constriction of systemic arteriovenous anastomoses, and (iii) the decrease in pulmonary vascular conductance, leading to reduction of cardiac output, seems to negate the expected rise in arterial blood pressure observed, for example, in rats and rabbits following inhibition of NO-synthesis.

Suppression by ethanol of pressor response caused by the inhibition of nitric oxide synthesis

The effects of ethanol on mean arterial pressure (MAP) and heart rate (HR) responses to the nitric oxide synthase inhibitor, NG-nitro-L-arginine, and to angiotensin II and noradrenaline were studied in rats. I.V. bolus injections of NG-nitro-L-arginine dose dependently increased MAP in vehicle-pretreated rats, with a maximum increase of 56 +/- 7 mm Hg and an ED50 of 3.8 +/- 0.4 mg/kg, respectively. I.v. infusions of ethanol dose dependently reduced maximum MAP response to NG-nitro-L-arginine, with a Ki of 96 +/- 8 mg/kg per min but did not alter the ED50. Ethanol (48 mg/kg per min) did not modify the MAP response to i.v. bolus injections of angiotensin II (0.02-1.28 micrograms/kg) or noradrenaline (0.25-16 micrograms/kg). However, ethanol attenuated the reflex HR responses of NG-nitro-L-arginine and angiotensin II but not that of noradrenaline. The results demonstrate that ethanol selectively but non-competitively inhibits the MAP response to NG-nitro-L-arginine, suggesting an interaction between ethanol and the L-arginine/nitric oxide pathway.

Nitric oxide-mediated hyporeactivity to noradrenaline precedes the induction of nitric oxide synthase in endotoxin shock

1. The role of an enhanced formation of nitric oxide (NO) and the relative importance of the constitutive and inducible NO synthase (NOS) for the development of immediate (within 60 min) and delayed (at 180 min) vascular hyporeactivity to noradrenaline was investigated in a model of circulatory shock induced by endotoxin (lipopolysaccharide; LPS) in the rat. 2. Male Wistar rats were anaesthetized and instrumented for the measurement of mean arterial blood pressure (MAP) and heart rate. In addition, the calcium-dependent and calcium-independent NOS activity was measured ex vivo by the conversion of [3H]-arginine to [3H]-citrulline in homogenates from several organs obtained from vehicle- and LPS-treated rats. 3. E. coli LPS (10 mg kg-1, i.v. bolus) caused a rapid (within 5 min) and sustained fall in MAP. At 30 and 60 min after LPS, pressor responses to noradrenaline (0.3, 1 or 3 micrograms kg-1, i.v.) were significantly reduced. The pressor responses were restored by NG-nitro-L-arginine methyl ester (L-NAME, 1 mg kg-1, i.v. at 60 min), a potent inhibitor of NO synthesis. In contrast, L-NAME did not potentiate the noradrenaline-induced pressor responses in control animals. 4. Dexamethasone (3 mg kg-1, i.v., 60 min prior to LPS), a potent inhibitor of the induction of NOS, did not alter initial MAP or pressor responses to noradrenaline in control rats, but significantly attenuated the LPS-induced fall in MAP at 15 to 60 min after LPS. Dexamethasone did not influence the development of the LPS-induced immediate (within 60 min) hyporeactivity to noradrenaline. However,dexamethasone pretreatment prevented the hypotension and vascular hyporeactivity at 180 min.5. At 60 min after LPS a moderate increase in the activity of a calcium-independent (inducible) NOS activity was detected in the aorta, but not in any of the other tissues studied. However, at 180 min after LPS, a significant NOS induction was observed in the lung, liver, spleen, mesentery, heart and aorta.This NOS induction was substantially prevented by pretreatment with dexamethasone.6. These results suggest that the immediate hypotension and vascular hyporeactivity to noradrenaline in endotoxin shock is caused by an enhanced formation of NO due to activation of the constitutive enzyme. The delayed hypotension and vascular hyporeactivity, however, is due to enhanced NO formation by the LPS-induced enzyme.

Nitric oxide modulates vascular permeability in the rat coronary circulation

1. The objective of the present study was to assess whether inhibition of nitric oxide (NO) production could modulate vascular permeability in the coronary circulation in conscious rats. 2. Intravenous injection of NG-nitro-L-arginine methyl ester (L-NAME, 2 mg kg-1) resulted in a slowly developing hypertension and evoked twofold increases in vascular permeability in the left ventricle and right atrium as measured by the extravasation of Evans blue dye. Maintenance of mean arterial blood pressure at the level observed following L-NAME injection by infusion of noradrenaline (620-820 ng kg-1 min-1) did not induce significant protein extravasation in the coronary circulation. 3. L-NAME treatment markedly enhanced (up to 490%) protein extravasation both in the left ventricle and right atrium in response to platelet-activating factor (PAF, 1.9 nmol kg-1, i.v.) and endothelin-1 (1 nmol kg-1, i.v.). Noradrenaline infusion potentiated (up to 69%) endothelin-1-induced protein extravasation. The permeability effect of PAF was only slightly enhanced by noradrenaline. 4. The present findings indicate that inhibition of endogenous NO synthesis leads to an increase in protein extravasation and to potentiation of the permeability effects of PAF and endothelin-1 in the coronary circulation. These results also suggest that NO may be an important regulator of vascular permeability under physiological and pathological conditions.

A comparison of the inhibitory effects of sodium nitroprusside, pinacidil and nifedipine on pressor response to NG-nitro-L-arginine

1. The inhibitory effects of sodium nitroprusside (SNP), a nitric oxide (NO) donor, on mean arterial pressure (MAP) responses to NG-nitro-L-arginine (L-NNA) (NO synthase inhibitor), angiotensin II (AII) and noradrenaline (NA) were compared with those of pinacidil (KATP channel opener) and nifedipine (L-type calcium antagonist) in conscious, unrestrained rats. 2. Intravenous bolus injections of L-NNA (1-64 mg kg-1), AII (0.02-1.28 micrograms kg-1) and NA (0.25-16 micrograms kg-1) dose-dependently increased MAP to similar maxima. Intravenous infusions of SNP (1, 4 and 16 micrograms kg-1 min-1) dose-dependently increased ED20S of L-NNA, AII and NA. However, the maximum response evoked by L-NNA, but not by AII nor NA, was dose-dependently reduced by SNP. Moreover, the inhibitory effect of SNP on the pressor response to L-NNA ceased when the infusion of SNP was terminated. 3. Pinacidil (80 micrograms kg-1 min-1 for 30 min followed by 5 micrograms kg-1 min-1) increased ED50S of L-NNA, AII and NA but did not decrease the maximum responses to any of these agents. 4. Nifedipine (1 mg kg-1 min-1) non-selectively reduced maximum responses to L-NNA, AII and NA to similar levels and increased ED50S of AII and NA but not L-NNA. 5. The results show that SNP causes a selective, non-competitive and reversible inhibition of the pressor response to L-NNA. This inhibition by SNP is unlikely to be related to hypotension, the opening of ATP-sensitive potassium channels or blockade of L-type calcium channels.

Effects of NG-nitro-L-arginine and L-arginine on regional cerebral blood flow in the cat

1. We studied the effects of NG-nitro-L-arginine (NOLA), a potent inhibitor of the L-arginine-nitric oxide pathway, and L-arginine, the precursor of nitric oxide, on regional cerebral blood flow, electrocortical activity and ex vivo cerebrovascular reactivity in the cat. Flow was measured via radiolabelled microspheres, and vascular responses were studied by measuring isometric tension of isolated middle cerebral arterial rings. 2. NOLA (30 mg kg-1 bolus followed by 1 mg kg-1 min-1 infusion) caused an approximately 40 mmHg elevation in the mean arterial blood pressure, a regionally heterogenous increase of the regional cerebrovascular resistance and a decrease in the regional cerebral blood flow 15 and 40 min after the start of its administration. In contrast L-arginine (30 mg kg-1 bolus followed by 10 mg kg-1 min-1 infusion) did not alter blood pressure, cerebrovascular resistance nor regional cerebral blood flow 15 min after the start of its administration. The NOLA-induced changes in tissue flow were the most pronounced in the cerebellum, pituitary and medulla oblongata, whereas there was no decrease in the flow of the cortex and white matter. 3. NOLA caused characteristic changes in total fronto-occipital EEG power and in power spectra which were unlikely to have been due to cerebral ischaemia. In addition, the ex vivo reactivity of the middle cerebral arteries showed signs of impaired endothelial nitric oxide synthesis: there were enhanced noradrenaline-induced contractions and N-ethoxycarbonyl-3-morpholino-sydnonimine (SIN-1)-induced relaxations and markedly attenuated acetylcholine- and ATP-induced relaxations after NOLA treatment. 4. The present data indicate that resting cerebral blood flow and cerebrovascular resistance are regulated by nitric oxide derived from L-arginine in a regionally heterogenous way and that exogenous L-arginine availability is not a limiting factor in this nitric oxide generation. Possibly, both the vascular endothelium and the neurons contribute to this basal nitric oxide release.

The influence of atropine and atenolol on the cardiac haemodynamic effects of NG-nitro-L-arginine methyl ester in conscious, Long Evans rats

1. In the present study, the extent to which baroreflexes contribute to the cardiac effects of NG-nitro-L-arginine methyl ester (L-NAME) was assessed in conscious, Long Evans rats chronically instrumented with thoracic electromagnetic flow probes for the measurement of cardiac haemodynamics. 2. L-NAME (10 mg kg-1, i.v.) was administered in the absence (n = 6) and in the presence (n = 7) of atropine (1 mg kg-1) and atenolol (1 mg kg-1). 3. L-NAME caused a marked increase in mean arterial pressure and marked reductions in total peripheral conductance, cardiac output, heart rate, stroke volume, peak thoracic flow and the maximum rate of rise of aortic flow. 4. Administration of atropine, after the maximal bradycardic effect of L-NAME was established, restored the heart rate to resting levels. Concurrently, there was a reduction in stroke volume, such that cardiac output, although transiently elevated, did not show a sustained increase. No other variables were significantly affected by atropine. Additional administration of atenolol had no effect other than to cause a slight bradycardia, such that in the presence of atropine and atenolol, heart rate was not different from that in animals receiving atropine and atenolol before L-NAME. 5. In the presence of atropine and atenolol, L-NAME had similar pressor, vasoconstrictor and cardiac haemodynamic effects to those in untreated animals, although the bradycardia was significantly attenuated. However, there was still a significant reduction in heart rate following L-NAME in the presence of atropine and atenolol.6. These results indicate that the major component of the bradycardia following L-NAME is indirect and mediated through an increase in vagal efferent activity. However, the substantial reduction in cardiac function caused by L-NAME is not dependent on the autonomic control of the heart but rather, may depend on the increase in afterload and/or a direct effect of L-NAME on the heart and/or its vasculature.

Effect of removing the endothelium on the vascular responses induced by leukotrienes C4 and D4 in guinea-pig isolated heart

The coronary vascular endothelium of the guinea-pig isolated perfused heart was removed by treatment with 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS), a zwitterionic detergent. After CHAPS treatment of the heart the vasoconstrictor responses of leukotriene (LT) C4, LTD4 and angiotensin II (AII) were significantly attenuated whereas the vascular actions of U46619, a thromboxane (Tx) A2 mimetic, and endothelin-1 (ET-1) were unaltered. The endothelium-dependent vasoconstrictor response of LTC4 and LTD4 could not be attributed to the release of TxA2, platelet-activating factor or AII since indomethacin, WEB 2086 and captopril had no effect on LT actions. However, in the presence of cromakalim, a potassium channel activator, the vasoconstrictor effects induced by LTC4, LTD4 and AII were significantly attenuated to a greater extent than the responses of U46619 and ET-1. The results suggest that in the coronary vasculature of the guinea-pig isolated heart the vasoconstrictor responses of LTC4, LTD4 and AII are endothelium-dependent and may involve a cromakalim-sensitive mechanism.

Pathogenesis of arterial hypotension in cirrhotic rats with ascites: role of endogenous nitric oxide

Nitric oxide is a vasodilator tonically secreted by endothelial cells that is involved in the regulation of arteriolar tone. This study, which includes two protocols, was performed to investigate whether nitric oxide plays a role in the pathogenesis of arterial hypotension in cirrhosis with ascites. In protocol 1, the administration of increasing doses (25, 50, 250, 500 and 1,000 micrograms.kg-1.min-1) of the nitric oxide biosynthesis inhibitor N omega-nitro-L-arginine to 18 conscious rats with cirrhosis and ascites produced, at each dose tested, a significantly greater increase in arterial pressure than in 17 conscious control rats. At the lowest dose of N omega-nitro-L-arginine, arterial pressure significantly rose in cirrhotic rats but not in controls. In protocol 2, arterial pressure, estimated renal plasma flow, glomerular filtration rate and sodium excretion were measured in 12 cirrhotic rats with ascites and 10 control rats before and during the sequential infusion of previously selected doses of N omega-nitro-L-arginine (25, 50 and 250 micrograms.kg-1.min-1). Changes in arterial pressure reproduced those observed in protocol 1. In control rats, N omega-nitro-L-arginine caused a decrease in estimated renal plasma flow without affecting glomerular filtration rate or sodium excretion. In contrast, N omega-nitro-L-arginine administration to cirrhotic animals did not produce any appreciable renal vasoconstrictor effect, and it increased glomerular filtration rate and sodium excretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous prostacyclin preserves myocardial function and endothelium-derived nitric oxide formation in myocardial ischemia

Nitric oxide (NO) and prostacyclin (PGI2) release was determined in effluents of Langendorff-perfused rabbit hearts under control conditions and during reperfusion subsequent to 2 h of global, low-flow ischemia. PGI2 release (6-oxo-PGF1 alpha) was significantly enhanced during early reperfusion and remained enhanced during a total time of 70 min of reperfusion. NO formation was reduced during ischemia but was substantially enhanced during reperfusion. Inhibition of endogenous PGI2 production by indomethacin resulted in severe disturbance of myocardial function and NO release. Inhibition of NO generation by L-N-nitroarginine did not affect myocardial contractility. These data suggest a cardioprotective and endothelium-protective role of PGI2 in myocardial ischemia which also involves protection of NO generation.

Regional coronary haemodynamic effects of two inhibitors of nitric oxide synthesis in anaesthetized, open-chest dogs

1. The role of endothelial nitric oxide synthesis from L-arginine in the regulation of coronary vascular tone and myocardial tissue perfusion was evaluated in anaesthetized, open-chest dogs. Coronary blood flow was measured with an electromagnetic flow probe placed around the left circumflex coronary artery. Coronary vascular resistance was calculated from mean arterial blood pressure and mean coronary blood flow, whereas regional myocardial tissue flow was determined by use of the radioactive microspheres technique. 2. NG-monomethyl L-arginine (L-NMMA) and NG-nitro-L-arginine methyl ester (L-NAME), administered directly into the left circumflex artery, induced a small increase in arterial blood pressure and an increase in coronary vascular resistance. However, myocardial tissue perfusion, assessed by the microspheres technique (whether subendocardial, subepicardial, or transmural), was unaffected by L-NMMA or L-NAME. 3. Acetylcholine, administered intracoronarily, induced an increase in left circumflex coronary blood flow and a decrease in coronary vascular resistance, without affecting systemic haemodynamics. This coronary vasodilator effect of acetylcholine was markedly inhibited by L-NMMA and L-NAME, the latter being a more potent antagonist than the former. 4. These results indicate that the endothelial L-arginine pathway is largely responsible for the coronary vasodilator effect of acetylcholine. However, although basal release of nitric oxide from L-arginine apparently contributes to the regulation of resting coronary vascular tone, blockade of this pathway does not affect myocardial tissue perfusion, possibly because of compensatory mechanisms occurring at the level of small arterioles and/or capillaries.

Haemodynamic changes and acetylcholine-induced hypotensive responses after NG-nitro-L-arginine methyl ester in rats and cats

1. The haemodynamic effects of NG-nitro-L-arginine methylester (L-NAME; 1, 3, 10 and 30 mg kg-1) and its potential ability to attenuate the hypotensive responses to acetylcholine (0.03, 0.1, 1.0 and 3.0 micrograms kg-1) have been investigated in anaesthetized rats and cats. 2. In the rat, L-NAME elicited a dose-dependent pressor effect increasing mean arterial blood pressure from the baseline value of 116 +/- 4 mmHg to a maximum of 156 +/- 6 mmHg with 30 mg kg-1. This increase in blood pressure could be only partly reversed by L-arginine (300 mg kg-1). However, the increase in blood pressure by lower doses (up to 10 mg kg-1) of L-NAME was effectively reversed by L-arginine (1000 mg kg-1). 3. In the cat, L-NAME did not significantly modify systemic haemodynamic variables (heart rate, mean arterial blood pressure, cardiac output, stroke volume or total peripheral resistance), when compared to the changes in saline-treated animals. Administration of L-arginine did not cause any significant effect in cats treated with L-NAME, but some decrease in heart rate and increases in cardiac output and stroke volume were observed in the saline-treated group. 4. With the lowest dose (1 mg kg-1), L-NAME did not affect tissue blood flows in the cat, but higher doses (3 and 30 mg kg-1) significantly reduced blood flows to the mesentery, stomach, spleen, intestines, lungs and the total liver. L-Arginine (300mgkg-1) injected into the control (saline-treated) animals resulted in a significant increase in blood flow to the heart, mesentery, lungs as well as the total liver, particularly its portal fraction. L-Arginine-induced increases in tissue blood flows (mesentery, kidneys, spleen, lungs, total liver and portal blood flow) in saline-treated animals were attenuated in animals treated with L-NAME.5. The acetylcholine-induced peak hypotensive response was not reduced in rats or cats by L-NAME. The duration of acetylcholine response was, however, attenuated in both species by L-NAME. Treatment with L-arginine (10-100mg kg- 1) did not change the acetylcholine-induced hypotension.6. The above results reveal a marked difference between the haemodynamic effects of L-NAME in rats and cats and suggest that in cats, unlike rats, the role of the L-arginine-NO pathway in the regulation of blood pressure is rather limited, although such a pathway may exist in several tissues. Furthermore, the hypotensive response to acetylcholine in both species seems to be mediated largely by NO-independent pathways.

Coronary atherosclerosis: current therapeutic approaches and future trends

Invasive cardiovascular procedures, such as percutaneous translumenal coronary angioplasty (PTCA) and aorto-coronary bypass surgery (ACBS), that are currently employed in treating the coronary stenosis or occlusion caused by atherosclerosis represent a major therapeutic advance for managing coronary heart disease (CHD). However, the cellular proliferative response and associated intimal hyperplasia that can follow the damage to blood vessels that occurs with these procedures leads to late complications which cannot be effectively controlled by presently available drugs. Hence, a new approach is required for managing these complications, termed "restenosis" (in the case of PTCA) or "stenosis" (in the case of ACBS). Existing drug therapy is reviewed and some new approaches to this problem are provided herein. Further studies of growth factors and other substances that influence the cellular proliferative response that follows injury to the blood vessel wall could lead to the development of effective therapy. Inhibition of intimal hyperplasia and/or acceleration of endothelial cell re-growth provide a basis for such new approaches. Platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF), as well as endothelium-derived relaxing factor(s) (EDRF) and calcitonin gene-related peptide (CGRP) are among the substances discussed. Modification of certain currently available drugs (e.g. Ca(2+)-antagonists) could also be of value in meeting this therapeutic demand.