Spontaneous pneumomediastinum (Hamman's syndrome): a rare cause of postpartum chest pain

We present a case of a 28-year-old primiparous woman with facial swelling followed by acute chest pain immediately after delivery. Chest radiograph revealed pneumomediastinum and surgical emphysema. She recovered well within 24 h of observation and conservative management. Postpartum spontaneous pneumomediastinum should be considered in the differential diagnosis of sudden-onset postpartum chest pain immediately or a few hours after delivery. It is a rare benign condition and usually resolves spontaneously without serious consequences. Chest X-ray is the single most important diagnostic test. It is important to rule out other serious and life-threatening conditions. Prolonged pushing, difficult labour and use of inhalational drugs place young primiparous women at higher risk. Recurrence is uncommon in subsequent pregnancy and management is unclear, although expectant management with epidural analgaesia to prevent recurrence in subsequent pregnancy is suggested.

The Effects of Opioid Peptides on Cardiovascular Function and Sympathetic Neurotransmission in Rats

The haemodynamic effects of three opioid receptor agonists, with some preferential activity on δ-, μ- and κ-receptors were investigated in anaesthetized and pithed rats, and effects on sympathetic neurotransmission were also investigated in pithed rats.

In anaesthetized rats, d-Ala2-d-Leu5-enkephalin (DADLE) (a predominantly δ-receptor agonist, 10 μg kg−1), glyol (μ, 0·5 mg kg−1) and R,S-N-C2-N-methyl-3,4,dichloro-phenylacetamido-2-C3-carboxy phenylethyl-pyrrolidine (ICI 204448) (·, 0·1 mg kg−1) by intravenous administration transiently decreased heart rate from 462 ± 12 to 432 ± 14, 460 ± 12 to 448 ± 13 and 460 ± 12 to 448 ± 11 beats min−1, respectively, and mean arterial blood pressure from 142 ± 6 to 111 ± 9, 141 ± 6 to 122 ± 5 and 148 ± 7 to 121 ± 6 mmHg, respectively. The effects of DADLE, but not those of glyol or ICI 204448, were blocked by M8008, a δ-receptor antagonist. In pithed rats, none of the opioid agonists had any significant effects on heart rate or mean arterial blood pressure; however, acetylcholine significantly reduced both heart rate and mean arterial blood pressure. All three opioid agonists reduced the positive chronotropic response to thoracic (C7-T2) spinal cord stimulation in pithed rats, by 17 ± 4,30 ± 2 and 20 ± 10% for DADLE, glyol and ICI 204448, respectively. This compared with a 48 ± 15% reduction with clonidine (5 μ kg−1). This effect of DADLE was almost abolished by M8008.

It is concluded that the haemodynamic effects of the opioid agonists studied are mediated via actions on the central nervous system and that a decrease in sympathetic neurotransmission may account for, at least in part, the bradycardia produced by opioid agonists in intact anaesthetized rats. It seems that the sympathetic nervous system is unlikely to be involved in the arrhythmogenic effects of opioid peptides.

Degradation of bradykinin, a cardioprotective substance, during a single passage through isolated rat-heart

Angiotensin converting enzyme (ACE) inhibitors have cardioprotective effects in different species including human. This cardioprotective effect is mainly due to the inhibition of bradykinin (BK) degradation rather than inhibition of the conversion of angiotensin I to angiotensin II. Bradykinin, a nonapeptide, has been considered to be the potential target for various enzymes including ACE, neutral endopeptidase 24.11, carboxypeptidase M, carboxypeptidase N, proline aminopeptidase, endopeptidase 24.15, and meprin. In the present study, the coronary vascular beds of Sprague Dawley rat isolated hearts were perfused (single passage) with Krebs solution alone or with different concentrations of BK i.e. 2.75x10(-10), 10(-7), 10(-6) and 10(-5) M solution. Percent degradation of BK was determined by radioimmunoassay. The degradation products of BK after passing through the isolated rat-hearts were determined using RP-HPLC and mass spectroscopy. All the four doses of BK significantly decreased the perfusion pressure during their passage through the hearts. The percentage degradation of all four doses was decreased as the concentration of drug was increased, implying saturation of a fixed number of active sites involved in BK degradation. Bradykinin during a single passage through the hearts degraded to give [1-7]-BK as the major metabolite, and [1-8]-BK as a minor metabolite, detected on HPLC. Mass spectroscopy not only confirmed the presence of these two metabolites but also detected traces of [1-5]-BK and arginine. These findings showed that primarily ACE is the major cardiac enzyme involved in the degradation of bradykinin during a single passage through the coronary vascular of bed the healthy rat heart, while carboxypeptidase M may have a minor role.

A species comparison of the effects of changing perfusion pressure on blood flow and metabolic heat production in the myocardium

1. A heated thermocouple technique (internal calorimetry) has been used to investigate blood flow and heat production responses in the myocardium to changing blood pressure in monkeys, dogs, rabbits, guinea-pigs and rats. 2. Perfusion experiments (isolated guinea-pig and dog hearts) showed linear relations between conductivity increment and coronary blood flow up to 1 x 5 ml. g(-1) min(-1) (dog heart perfused with blood) and 5 ml. g(-1) min(-1) (guinea-pig heart perfused with saline). This relation was the same in the beating and non-beating heart. 3. In rabbits, guinea-pigs and rats the relation between flow (conductivity increment) and blood pressure was linear. In contrast, in monkeys and dogs, over the range 100-40 mm Hg, little change in blood flow occurred. After the injection of procaine around the stellate ganglion, the relation between pressure and flow in both species was linear. It is suggested that the constancy of blood flow displayed in the monkey and dog over this blood pressure range was due to the activity of adrenergic rasodilator fibres. 4. Evidence is adduced to show an increased myocardial heat production when systemic arterial pressure fell below about 60 mm Hg. This was not affected by cardiac denervation. The possible significance of this effect is discussed.

Isosorbide-2-mononitrate reduces the consequences of myocardial ischaemia, including arrhythmia severity: implications for preconditioning

The effects of the intracoronary administration of isosorbide-2-mononitrate (ISMN; 3 microg kg(-1) min(-1)), a major metabolite of isosorbide dinitrate, were examined in chloralose-urethane anaesthetized dogs before and during a 25 min, acute occlusion of the left anterior descending coronary artery. The only significant haemodynamic effects of ISMN administration were a slight (-11 +/- 2 mmHg) decrease in arterial blood pressure and a decrease (< 12%) in diastolic coronary vascular resistance. Coronary occlusion in the presence of ISMN led to a markedly reduced incidence and severity of ventricular arrhythmias compared to those in control, saline-infused dogs. There were fewer ectopic beats (62 +/- 35 versus 202 +/- 72; p < 0.05), a lower incidence (25% versus 75%; p < 0.05) and number of episodes (0.7 +/- 0.4 versus 4.3 +/- 2.1; p < 0.05) of ventricular tachycardia and fewer dogs fibrillated during the ischaemic period (17% versus 82%; p < 0.05). More dogs given ISMN survived the combined ischaemia-reperfusion insult (50% versus 0%; p < 0.05). Changes in ST-segment elevation (recorded by epicardial electrodes) and in the degree of inhomogeneity of electrical activation within the ischaemic area were much less pronounced throughout the occlusion period in dogs given ISMN. These results add weight to the hypothesis that the previously reported antiarrhythmic effects of ischaemic preconditioning, and of the intracoronary administration of nicorandil, involve nitric oxide.

Atrial natriuretic peptide reduces the severe consequences of coronary artery occlusion in anaesthetized dogs

The aim of the present study was to examine the effects of atrial natriuretic peptide (ANP) on the responses to coronary artery occlusion. In chloralose-urethane anaesthetised mongrel dogs either saline (controls) or human synthetic ANP was infused intravenously (10 microg kg(-1) + 0.1 microg kg(-1) min(-1)), starting 30 min before and continuing 10 min during a 25 min occlusion of the left anterior descending coronary artery (LAD). ANP infusion resulted in a fall in mean arterial blood pressure (by 17 +/- 2 mmHg, p < 0.05), a transient (max. at 5 min) increase in coronary blood flow (by 24 +/- 5 ml min(-1), p < 0.05), and a reduction in coronary vascular resistance (by 0.27 +/- 0.05 mmHg ml(-1), p < 0.05). When the LAD coronary artery was occluded, there was a less marked elevation in left ventricular end-diastolic pressure (LVEDP) in the ANP-treated dogs than in the controls (9.0 +/- 0.9 versus 12.2 +/- 0.8 mmHg, p < 0.05). Compared to the controls, ANP reduced the number of ventricular premature beats (VPBs, 26 +/- 12 versus 416 +/- 87, p < 0.05), the number of episodes of ventricular tachycardia per dogs (VT, 0.7 +/- 0.3 versus 12.4 +/- 4.2, p < 0.05), and the incidences of VT (45% versus 100%, p < 0.05) and ventricular fibrillation (VF 18% versus 57%, p < 0.05) during occlusion. Reperfusion of the ischaemic myocardium at the end of the occlusion period led to VF in all the control dogs (survival from the combined ischaemia-reperfusion insult was therefore 0%), but VF following reperfusion was much less in the dogs given ANP (survival 64%; p < 0.05). The severity of myocardial ischaemia, as assessed from changes in the epicardial ST-segment and the degree of inhomogeneity, was significantly less marked in dogs given ANP. We conclude that ANP protects the myocardium from the consequences of myocardial ischaemia resulting from acute coronary artery occlusion and reperfusion in anaesthetized dogs.

Coronary vascular endothelium-myocyte interactions in protection of the heart by ischaemic preconditioning

Because ischaemia preconditioning is a general phenomenon - it is present in almost all tissues and organs - we must look for a mechanism which involves cell types common to all organs. The hypothesis outlined below is that endothelial cells, in response to a brief ischaemic stimulus, release protective mediators which (in the heart) diffuse to cardiac myocytes to increase resistance to a subsequent ischaemic stress. These substances include nitric oxide, generated through initial bradykinin release, and prostacyclin. The evidence includes measurement of mediator release and the pharmacological attenuation of the antiarrythmic effects of preconditioning by blockade of bradykinin receptors and inhibition of nitric oxide production.

Delayed protection against ventricular arrhythmias by monophosphoryl lipid-A in a canine model of ischaemia and reperfusion

Bacterial endotoxin reduces the severity of ventricular arrhythmias which occur when a coronary artery is occluded several hours later. We have now examined in anaesthetised dogs the effects on ischaemia and reperfusion-induced arrhythmias, of a non-toxic derivative component of the endotoxin molecule of the lipid-A (monophosphoryl lipid-A). This was given intravenously, in doses of 10 and 100 microg kg(-1), 24 h prior to coronary artery occlusion. Arrhythmia severity was markedly reduced by monophosphoryl lipid-A. During ischaemia, ventricular premature beats were reduced from 315+/-84 in the vehicle controls to 89+/-60 (with the lower dose of monophosphoryl lipid-A) and 53+/-23 (P<0.05) with the higher dose. The incidence of ventricular tachycardia was reduced from 75% to 25% (P<0.05) and 31% (P<0.05), and the number of episodes of ventricular tachycardia from 13.4+/-4.9 per dog to 1.1+/-1.1 (P<0.05) and 1. 2+/-0.9 (P<0.05) after doses of 10 and 100 microg kg(-1), respectively. The incidence of ventricular fibrillation during occlusion and reperfusion in the control group was 96% (15/16), i.e., only 6% (1/16) dogs survived the combined ischaemia-reperfusion insult. Monophosphoryl lipid-A (100 microg kg(-1)) significantly reduced the incidence of occlusion-induced ventricular fibrillation (from 50% to 7%; P<0.05), and increased survival following reperfusion to 54% (P<0.05). Monophosphoryl lipid-A also significantly reduced ischaemia severity as assessed from ST-segment elevation recorded from epicardial electrodes as well as the degree of inhomogeneity of electrical activation within the ischaemia area. There were no haemodynamic differences prior to coronary occlusion between vehicle controls and monophosphoryl lipid-A-treated dogs. These results demonstrate that monophosphoryl lipid-A reduces arrhythmia severity 24 h after administration. Although the precise mechanisms are still unclear, there is some evidence that nitric oxide and prostanoids (most likely prostacyclin) may be involved because the dual inhibition of nitric oxide synthase and cyclooxygenase enzymes by administration of aminoguanidine and meclofenamate abolished the marked antiarrhythmic protection resulted from monophosphoryl lipid-A treatment 24 h previously.

Repeated cardiac pacing extends the time during which canine hearts are protected against ischaemia-induced arrhythmias: role of nitric oxide

Right ventricular pacing in lightly anaesthetized dogs (4x5 min periods at a pacing rate of 220 beats/min) protects against the consequences of coronary artery occlusion when this is initiated 24 h after the pacing stimulus. The main purpose of the present experiments was to determine whether repeating the pacing stimulus, at a time when protection from the initial stimulus had faded (48 h), prolonged the protection afforded against ischaemia-induced ventricular arrhythmias and other ischaemic changes (epicardial ST-segment mapping; changes in the degree of electrical inhomogeneity in the ischaemic region). Dogs were paced on two occasions, with a 48 h period between and, at different times (48, 72 and 96 h) after the second pacing stimulus, were re-anaesthetized and subjected to occlusion of the left anterior descending coronary artery. There was a marked reduction in the severity of ischaemia-induced arrhythmias 48 and 72 h after the second pacing stimulus (reduction in occlusion-induced and reperfusion-induced ventricular fibrillation, e.g. at 72 h 0/11 during occlusion and only 3/11 following reperfusion, compared to 7/21 and 10/21 respectively in the controls P<0.05). The protection had disappeared 96 h following the second pacing stimulus. Changes in ST-segment elevation and in the degree of inhomogeneity largely followed these changes in the severity of ventricular arrhythmias. The results suggest the possibility of maintaining protection against life-threatening arrhythmias following coronary occlusion by repeating a preconditioning pacing stimulus. We also demonstrate that this prolonged protection afforded by repeated cardiac pacing is mediated by nitric oxide, since the marked antiarrhythmic effect observed, e.g. 72 h after the second pacing stimulus, was abolished when S-(2-aminoethyl)-isothiourea (AEST), a particularly selective inhibitor of iNOS, had been administered before coronary artery occlusion.

The influence of maturation and gender on the anti-arrhythmic effect of ischaemic preconditioning in rats

The aim of this study was to investigate the influence of maturation and gender on the anti-arrhythmic effect of myocardial ischaemic preconditioning in rats. Coronary artery occlusion was carried out in either rats anaesthetised with sodium pentobarbitone or in rat isolated hearts. Cardiac arrhythmias occurring in the 30 min post-occlusion period were assessed. In anaesthetised 3 month (m) old male rats ischaemic preconditioning, with a 3 min temporary coronary artery occlusion, significantly reduced the total number of ventricular ectopic beats (VEBs) from 2074 +/- 206 to 490 +/- 139 and the incidence of ventricular fibrillation (VF) from 40 to 0% during a subsequent 30 min occlusion (P < 0.05). In middle-aged male rats (16 m) the anti-arrhythmic effect of preconditioning was unaltered (VEBs were reduced from 1958 +/- 121 to 245 +/- 66 and VF from 70 to 0%). In 3 m old anaesthetised female rats the effect of ischaemic preconditioning was also evident (VEBs reduced from 961 +/- 170 to 154 +/- 48; P < 0.05). In non-preconditioned age-matched female animals the total number of VEBs (961 +/- 170), VF (0%) and mortality (0%) were significantly (P < 0.05) lower than in respective male animals. In female rats, attenuation of ischaemia-induced arrhythmic severity was most pronounced in the oestrus state. In hearts isolated from weight-matched male and female rats the incidence of ventricular tachycardia (81 vs 25%) and the total number of VEBs (351 +/- 73 vs 81 +/- 50) were significantly (P < 0.05) different. It is concluded that in rats neither maturation nor gender influence the anti-arrhythmic effect of ischaemic preconditioning. However, female rats exhibit a lower level of arrhythmic activity during sustained coronary artery occlusion than male rats both in vivo and in vitro.

Coronary endothelial dysfunction increases the severity of ischaemia-induced ventricular arrhythmias in rat isolated perfused hearts

In order to determine the role of coronary vascular endothelial cells in generating cardioprotective substances during myocardial ischaemia, rat isolated hearts, perfused at constant flow by the Langendorff technique, were subjected to treatment with the detergent Triton X100 and the responses of these hearts to a 30 or 60 min period of coronary artery occlusion was determined. Endothelial damage or denudation was shown both by histological examination and by the altered vasodilator response to the endothelium-dependent vasodilator bradykinin (100 nM), which was reversed to vasoconstriction in hearts treated with Triton X-100. In contrast, the responses to sodium nitroprusside (100 microM) were unimpaired in these hearts and were not different from control responses. Ventricular ectopic activity was much more pronounced in hearts with endothelial dysfunction (e.g., 3329 +/- 361 ventricular premature beats over a 30 min occlusion period compared to 243 +/- 34 in controls; P < 0.01), and the duration of ventricular tachycardia was greatly increased (1162 +/- 391 s v 9 +/- 12 s in the controls; P < 0.01). Ventricular ectopic activity was still marked when the occlusion was prolonged to 1 h and was still apparent at the end of this 1 h occlusion period. Reperfusion arrhythmias (ventricular tachycardia and ventricular fibrillation) were marked in endothelium-damaged hearts (50%); whereas there were no such arrhythmias after a 30 or 60 min occlusion period in control hearts. Hearts were also preconditioned by a 3 min coronary artery occlusion period 10 min prior to a 30 min coronary artery occlusion. This reduced ventricular ectopic activity in both control and endothelium-damaged hearts to about the same extent (between 80 and 90% suppression). The results suggest that under normal conditions substances generated from endothelial cells protect the myocardium against ventricular arrhythmias both during ischaemia and reperfusion. However, in this species, preconditioning is still possible in hearts from which the coronary vascular endothelium has been removed. If these results can be extrapolated to the clinical situation, it suggests that in patients with endothelial dysfunction ventricular arrhythmias may be more pronounced following a period of ischaemia and especially of reperfusion.

Tolerance to ischaemia and ischaemic preconditioning in neonatal rat heart

Although there is much information on ischaemic preconditioning in the adult myocardium, this phenomenon has not yet been investigated in neonatal hearts. To examine the early development of cardiac tolerance to ischaemia and the possible protective effects of preconditioning, rat hearts isolated on days 1, 4 and 7 of postnatal life were perfused (Langendorff) with Krebs-Henseleit solution at constant pressure, temperature (37 degrees C) and rate (200 beats/min). Developed force (DF) of contraction was measured by an isometric force transducer, and analysed using an on-line computer. Hearts were exposed to 40 or 60 min of global ischaemia followed by 30 min of reperfusion. Preconditioning was induced by three 3-min periods of global ischaemia, each separated by 5-min periods of reperfusion. Developmental changes in expression of protein kinase C (PKC) isoforms, and their activation following preconditioning, were estimated using Western blot analysis. Recovery of contractile function during reperfusion decreased from day 1 (48 +/- 2%) to day 4 (42 +/- 1%) and day 7 (33 +/- 2%). Preconditioning failed to improve ischaemic tolerance on day 1 (46 +/- 2%) and on day 4 (43 +/- 3%), but pronounced effect was observed on day 7 (40 +/- 2%). Prolonging the period of sustained ischaemia from 40 to 60 min on day 1 did not lead to a better recovery of contractile function in preconditioned hearts. PKC isoforms alpha, delta, epsilon and zeta were expressed in the ventricular myocardium during the first week of life, but there was no evidence of translocation following preconditioning on day 7. It may be assumed that the decreasing tolerance of the heart to ischaemia during early postnatal life is counteracted by the development of an endogenous protection.

Monophosphoryl lipid A reduces both arrhythmia severity and infarct size in a rat model of ischaemia

A non-toxic derivative of the active lipid A component of the endotoxin molecule (monophosphoryl lipid A) when given to rats in a dose of 5 mg kg(-1) by intraperitoneal injection 24 h prior to anaesthesia and coronary artery occlusion, markedly decreased the severity of ischaemia-reduced ventricular arrhythmias (ventricular fibrillation reduced from 60 to 21%; P < 0.05) and reduced myocardial infarct size (from 35.8 +/- 1.6% of the area at risk to 22.7 +/- 2.0%; P < 0.05). It did not modify blood pressure or heart rate either before or during the period of ischaemia.

The dopamine receptor agonist Z1046 reduces ischaemia severity in a canine model of coronary artery occlusion

Z1046, (S)-6[[6-[[2-(2-methoxyphenoxy)ethyl]amino]propyl]amino]-5,6,7,8-tetra-h ydro-1,2-naphtalenediol dihydrochloride, is an agonist at both dopamine D1 and D2 receptors. Since stimulation of dopamine D2 receptors inhibits noradrenaline release, and because cardiac noradrenaline release has been implicated in the genesis of early ischaemia-induced, life-threatening ventricular arrhythmias, the effect of Z1046 has been examined for its effects on coronary artery occlusion in chloralose urethane anaesthetised mongrel dogs. Z1046 (10 microg kg(-1) intravenously or 1 microg kg(-1) by local intracoronary injection) decreased heart rate and reduced arterial blood pressure and coronary blood flow, effects prevented by the prior administration of domperidone (40 microg kg(-1) i.v.). The ischaemic changes induced by a 25-min occlusion of the left anterior descending coronary artery (including ST-segment elevation and ventricular ectopic activity) were much less marked in those dogs administered Z1046 and survival from the combined ischaemia reperfusion insult was increased from 7% to 36% (P < 0.05). These effects of Z1046 were partly attenuated by domperidone. We conclude that the anti-ischaemic effects of Z1046 are due to inhibition of cardiac sympathetic responses. Studies using rat isolated perfused mesenteric vascular bed preparations subjected to sympathetic nerve stimulation confirmed that Z1046 inhibits synaptic transmission without modifying vascular responses to noradrenaline.

Lack of involvement of mast cell degranulation in the antiarrhythmic effect of preconditioning in rats

It has been proposed that the cardioprotective effects of myocardial ischaemic preconditioning may involve the release of mast cell mediators. The aim of the study was to determine whether mast cells are involved in the antiarrhythmic effect of ischaemic preconditioning in rat hearts. Preconditioning was achieved, both in anaesthetised rats and in rat isolated hearts, by a 3-min temporary occlusion of the left main coronary artery followed by 10 min of reperfusion before a 30-min permanent occlusion. Preconditioning had a marked antiarrhythmic effect, reducing the number of ventricular ectopic beats from 1,176 +/- 69 to 490 +/- 139 and the incidence of ventricular fibrillation from 40% to 0. Administration of the mast cell-stabilising drugs lodoxamide tromethamine and sodium cromoglycate (20 mg/kg/h i.v. 30 min before and throughout experimental protocol) did not modify the antiarrhythmic effect of preconditioning. Sodium cromoglycate, but not lodoxamide tromethamine, itself significantly reduced the number of ectopic beats that occurred over a 30-min period of ischaemia (from 760 +/- 181 to 153 +/- 33 in nonpreconditioned animals). Both drugs abolished the decrease in arterial blood pressure that occurred on coronary artery occlusion. The decrease in arterial blood pressure produced by the mast cell-degranulating compound 48/80 (50 microg/kg; i.v.) was attenuated to a similar degree by both drugs (decreases in pressure of 53 +/- 7, 31 +/- 1, and 25 +/- 3 mm Hg in control, sodium cromoglycate-treated, and lodoxamide tromethamine-treated animals, respectively). In rat isolated hearts, degranulation of mast cells with three consecutive doses of 50 microg of compound 48/80 had no antiarrhythmic effects and did not modify the antiarrhythmic effect of preconditioning. It is concluded that cardiac mast cells do not play a major role in the protection offered by ischaemic preconditioning on arrhythmogenesis in rat hearts.

Nitric oxide in sepsis and endotoxaemia

Nitric oxide (NO) is one of many vasoactive substances released, from a variety of cells, under conditions of endotoxaemia and sepsis. Under physiological conditions it is produced by two constitutive calcium-dependent enzymes (nitric oxide synthase; NOS) in neurones (nNOS) and endothelial cells (eNOS) and has functions ranging from neurotransmission and vasodilatation to inhibition of platelet adhesion and aggregation. Following bacterial infection, especially with Gram-negative organisms, its formation from L-arginine is enhanced due to the cytokine-mediated induction of a NOS enzyme (iNOS) in cells (e.g. cardiac myocytes, vascular smooth muscle) that do not normally have the ability to synthesize NO. The result of this excessive NO production is enhanced bacterial lysis by activated macrophages, vasoplegia and myocardial depression. These cardiovascular effects can be alleviated by inhibitors of the L-arginine NO pathway, which results in elevated perfusion pressure, restored responsiveness to sympathetic nerve stimulation and to exogenous catecholamines, and to enhanced (endothelin-dependent) myocardial contractility. In patients in shock this approach also leads to detrimental effects (increased systemic vascular resistance, elevated pulmonary artery pressure, reduced cardiac output and oxygen delivery, increased platelet accumulation) and survival is not improved. Because some of these detrimental effects are due to inhibition of eNOS, attempts have been made to examine the effects of substances with a higher selectivity for the induced form of the enzyme. In experimental animals, one of these (L-canavanine) protects endothelial cells from damage, increases survival time and restores vascular responsiveness without increasing blood pressure or peripheral vascular resistance. However, whether even this approach will be of benefit to patients with sepsis remains in doubt since studies in iNOS knock-out mice do not support the concept that eliminating this particular source of NO improves ultimate survival.

Nitric oxide. A key mediator in sepsis and endotoxaemia?

A very large number of biologically active substances are released into the circulation under conditions of endotoxaemia and sepsis. One of the most important of these is nitric oxide. Under these conditions nitric oxide is produced through an induced enzyme (nitric oxide synthase) in a variety of tissues and the nitric oxide so generated is largely responsible for the loss of vascular reactivity, which occurs under these conditions, for the resulting unrelenting hypotension associated with the hypodynamic phase of septic shock. Nitric oxide also contributes to the myocardial depression in this condition. The question as to whether it is a worthwhile therapeutic approach to inhibit nitric oxide synthase is discussed with particular reference to the generation of inhibitors selective for the induced form of the enzyme. This approach has certain benefits but may also be detrimental. The fact that nitric oxide is not the key mediator involved in ultimate mortality in this condition is suggested by the failure to improve mortality in iNOS knockout mice given endotoxin.

Genistein, an inhibitor of tyrosine kinase, prevents the antiarrhythmic effects of preconditioning

The possible involvement of tyrosine kinase in the signal transduction processes associated with the antiarrhythmic effects of ischaemic preconditioning was assessed by the administration, prior to the preconditioning stimulus, of the tyrosine kinase inhibitor genistein (5 mg/kg i.v.) to pentobarbitone anaesthetised male rats. The hearts were then removed, perfused by the Langendorff technique and preconditioned by a single brief (3 min) coronary artery occlusion prior to the 30 min test ischaemic insult. Preconditioning reduced ventricular arrhythmias during occlusion (e.g., premature ventricular beats from 333 +/- 60 in controls to 52 +/- 5 in preconditioned hearts; P < 0.05) but this protective effect was not observed in the hearts of rats given genistein (272 +/- 37 premature beats). A similar abolition of the protective effects of preconditioning could be demonstrated in hearts perfused in vitro with genistein (100 microM). This suggests the involvement of tyrosine kinase activation in this powerful form of endogenous cardioprotection.

Does the opening of ATP-sensitive K+ channels modify ischaemia-induced ventricular arrhythmias in anaesthetised dogs?

These experiments were designed to determine whether there is any change in the severity of ventricular arrhythmias resulting from coronary artery occlusion in anaesthetised mongrel dogs if ATP-sensitive potassium channels were already open at the time of coronary occlusion. To achieve this we locally infused the K(ATP) channel opener levcromakalim, in a total dose of 3 microg/kg, and given by slow infusion over a 30 min period directly into a side branch of the left anterior descending coronary artery. This dose increased blood flow in that main artery by 30% (and by 7% in the adjacent left circumflex artery). The degree of inhomogeneity of electrical activation, measured from the left ventricular wall distal to the occlusion site, was unaffected by levcromakalim administration but there was significant epicardial ST-elevation, perhaps indicating K+ egression from cells. Following coronary artery occlusion there was no marked difference in the severity of arrhythmias between control and levcromakalim-treated dogs, except for an increased number of episodes of ventricular tachycardia due entirely to effects in two of the nine treated dogs. We conclude that opening cardiac K(ATP) channels with levcromakalim, at this one dose level, and administered directly to the left ventricular wall, does not significantly modify arrhythmia severity during ischaemia. These results cannot be extrapolated to studies in which such drugs markedly reduce coronary perfusion pressure.

Bradykinin and endothelial-cardiac myocyte interactions in ischemic preconditioning

Myocardial ischemia results in the release of a variety of vasoactive substances from coronary vascular endothelial cells and/or from cardiac myocytes. Some of these substances appear to be protective and include nitric oxide and bradykinin. One hypothesis for the pronounced antiarrhythmic effects of preconditioning involves the early generation of bradykinin and, subsequently, nitric oxide. Evidence for early bradykinin release has come from clinical studies involving patients undergoing coronary angioplasty where, in 4 of 5 patients, there was evidence for elevated kinin levels in coronary sinus blood either during balloon inflation (i.e., ischemia) or deflation (reperfusion). The levels reached are sometimes considerable (increases 10-20 fold). The second piece of evidence comes from dogs subjected to a preconditioning stimulus (2 x 5 min periods of ischemia), followed 20 min later by occlusion of the same artery for a 25-min period. This preconditioning procedure markedly reduces ischemia-induced ventricular arrhythmias and, although under resting conditions there was little difference between arterial and coronary sinus bradykinin levels (125 +/- 22 and 157 +/- 41 pg/mL, respectively), there was a marked increase in coronary sinus levels in preconditioned dogs before the prolonged occlusion (637 +/- 293 pg/mL compared with 114 +/- 18 pg/mL in nonpreconditioned dogs); levels at the end of the prolonged occlusion in the preconditioned dogs were also higher (577 +/- 305 pg/mL compared with 162 +/- 34 pg/mL in control dogs). Other evidence for the involvement of bradykinin and nitric oxide comes from studies in which the generation, or effects, of these mediators have been suppressed (e.g., with the bradykinin B2 receptor blocking agent icatibant, with inhibitors of the L-arginine-nitric oxide pathway, and by methylene blue). The conclusion is that early bradykinin release is protective under conditions of ischemia, is presumably enhanced during therapy with angiotensin-converting enzyme (ACE) inhibitors and is suppressed under conditions of endothelial dysfunction.

Modification by bradykinin B2 receptor blockade of protection by pacing against ischaemia-induced arrhythmias

In dogs, rapid cardiac pacing, by way of a pacing electrode in the right ventricle, protects against ventricular arrhythmias when a coronary artery is occluded immediately after cessation of the pacing period. This represents a form of ischaemic preconditioning. The role of bradykinin in mediating the protective effects of rapid cardiac pacing in this model was investigated using a selective antagonist of bradykinin at B2 receptors (icatibant; HOE 140). In the presence of icatibant cardiac pacing (220 beats min(-1)) resulted in more severe ischaemia (as assessed by ST-segment elevation from the pacing electrode at the end of the stimulus) and to a higher incidence of ventricular arrhythmias during the pacing protocol. When the coronary artery was occluded under such conditions the antiarrhythmic protection afforded by cardiac pacing was not seen although other indices of reduced ischaemia severity (epicardial ST-segment mapping; changes in the degree of inhomogeneity of electrical activation within the ischaemic area) were not affected by icatibant treatment. These results suggest that bradykinin is an important trigger mediator involved in the protective effects of cardiac pacing. Whether this is due to the generation of endothelium-derived protective substances (such as nitric oxide and prostacyclin) or whether it results from a direct effect on B2 receptors in cardiac myocytes is unclear.

The case of low-dose aspirin for the prevention of myocardial infarction: but how low is low?

Although there is firm consensus that aspirin reduces the incidences (and severity) of reinfarction if given to patients after a myocardial infarction or in patients with unstable angina, there is disagreement about the optimum dose that should be used. We make the case that it could be considerably lower than the "medium dose" (75-320 mg daily) in current usage and put forward the view that a dose of 30 mg daily is sufficient. The arguments are based on the presystemic, exclusively pharmacokinetic inhibition of thromboxane synthesis in platelets by very low-dose aspirin and the importance of maintaining prostacyclin production by endothelial cells. Little attention has been in the past to the endogenous myocardial protection that results from prostacyclin generation. Such low doses have the additional advantage of not inducing gastrointestinal ulceration. We conclude that more extensive clinical trials with such "low-dose" aspirin preparations are warranted.

Endotoxin-induced inhibition of mesenteric vasodilator responses to acetylcholine, bradykinin, and post-occlusion hyperemia in anesthetized rats

The effects of endotoxin (20 mg kg-1 i.p.) on the mesenteric vascular responses to acetylcholine, bradykinin, sodium nitroprusside, and to transient occlusion of the superior mesenteric artery were examined in rats anesthetized with pentobarbitone. Mesenteric vasodilator responses to close arterial injections of acetylcholine and bradykinin were reduced at 1.5 h after endotoxin and almost abolished by 4 h; responses to sodium nitroprusside were unaffected. Occlusion of the superior mesenteric artery for 30, 60, or 120 s produced, on release of the occlusion, a time-dependent vasodilator response in the mesenteric circulation (post-occlusion hyperemia). This hyperemia was markedly reduced by nitro-L-arginine methyl ester (L-NAME); L-NAME did not modify acetylcholine-induced vasodilation. Endotoxin-pretreatment did not modify mesenteric post-occlusion hyperemia 1.5 h after administration but markedly reduced the response by 2.5 h. The administration of L-NAME to endotoxin-treated rats did not further attenuate the hyperemic responses. Mesenteric vasoconstrictor responses to phenylephrine were not modified by endotoxin, although systemic pressor responses to this agent were impaired. We concluded that endotoxin impairs endothelium and nitric oxide-dependent vasodilator responses in the mesenteric circulation.

Effect of nitric oxide synthase inhibition on myocardial contractility in anesthetized normal and endotoxemic dogs

Nitric oxide (NO) produced by the induced NO synthase (NOS) enzyme has been implicated in the mechanisms of the circulatory changes that occur in the later stages of sepsis. As NO produced by the constitutive form of the enzyme is known to play a role in the regulation of normal circulation, we have performed a series of experiments to study the early circulatory effects of inhibition of NOS in a hyperdynamic endotoxemic dog model. Pentobarbital-anesthetized animals were used. Cardiac output (CO) was measured by thermodilution. Myocardial contractility (MC) was estimated from the slope of the left ventricular end-systolic pressure-diameter relationship obtained from sonomicrometer- and catheter-tip manometer signals in closed chest animals. All animals received a 15 mL/kg/h infusion of Ringer's lactate. A hyperdynamic response was elicited by a 2 h infusion of a total dose of 5.3 micrograms/kg Escherichia coli O55:B5 endotoxin (ETX). CO increased initially by about 25%, and total peripheral resistance decreased by 35%. These changes subsided in 60-90 min, after which a sustained decrease in CO occurred. MC elevated transiently by 25% after the first 30 min of ETX infusion, then decreased gradually below the control level. Administration of 2 mg/kg of the NOS inhibitor N-nitro-L-arginine (NNA) between the 45th and 55th min of the ETX infusion increased MC to the level in the control group, but accelerated the decline of the initially increased CO and caused a sustained increase in total peripheral resistance to about 50% above the control level. In normal (nonendotoxin treated) dogs, NNA also caused a similar increase in MC which, however, lasted at least 3 h. Left ventricular diameter increased in the NNA-treated groups. This increase also occurred in the endotoxin-only group but with a delay of about 2.5 h. Our results demonstrate the participation of constitutive NOS-produced NO in the early hyperdynamic response of endotoxemia. Suppression of NO is associated with increased myocardial contractility. NNA treatment may be favorable for the restoration of depressed cardiac contractility during endotoxemia, but this treatment is probably detrimental for the compensatory systemic flow (CO) increase.

Effects of L-canavanine, an inhibitor of inducible nitric oxide synthase, on endotoxin mediated shock in rats

The effects of L-canavanine, an inhibitor of nitric oxide synthase, on endotoxin-induced shock was investigated in the pentobarbitone anesthetized rat. Endotoxin infusion (2.5 mg kg-1 h-1 over 6 h) produced progressive and marked hypotension and hypoglycemia. Electron microscopy showed marked changes in the kidney, comprising severe endothelial cell disruption and the accumulation of platelets in the blood vessels. In the lung, there was marked accumulation of polymorphonuclear leukocytes in small blood vessels and endothelial disruption. Treatment with L-canavanine (10 mg kg-1 by bolus injection each hour starting 70 min after endotoxin or saline infusion) significantly reduced endotoxin-induced hypotension, without any effect on the hypoglycemia. This treatment markedly reduced the endotoxin-induced electron microscopical changes in the kidneys and lungs. Although L-canavanine, like L-NAME, inhibited both cerebellar constitute and splenic inducible nitric oxide synthase in vitro, in contrast to L-NAME it did not modify either arterial blood pressure or carotid artery blood flow in control rats. The data are consistent with L-canavanine being a selective inhibitor of inducible nitric oxide synthase, at least in vivo, and suggest that inhibitors of this enzyme may be beneficial in endotoxin-induced shock.

Delayed protection against ischaemia-induced ventricular arrhythmias and infarct size limitation by the prior administration of Escherichia coli endotoxin

1. Bacterial endotoxin (lipopolysaccharide derived from Escherichia coli) was injected intraperitoneally in conscious rats in doses ranging from 0.5 to 2.5 mg kg-1. At various times afterwards the animals were anaesthetized and subjected to a 30 min period of left coronary artery occlusion. 2. Under these conditions the severity of ventricular arrhythmias was markedly suppressed, in comparison with saline-injected controls, but this was particularly marked with the higher doses (1.5 and 2.5 mg kg-1); the number of ventricular premature beats was reduced from 1687 +/- 227 over the 0.5 h coronary artery occlusion period to 190 +/- 46 in those rats administered 2.5 mg kg-1 endotoxin 8 h previously (P < 0.05). The duration of ventricular tachycardia was also significantly reduced (138 +/- 26 s to 8.9 +/- 4.2 s; P < 0.01) and there was a reduction in the incidence of ventricular fibrillation (from 56% to 10%). 3. The time course of this protection was studied following the administration of a single dose of 2.5 mg kg-1 of endotoxin by anaesthetizing rats 4, 8 or 24 h later. Protection was apparent at each time but was particularly marked at 8 h. 4. No rat given the highest dose of endotoxin (32 in all) died as a result of ventricular fibrillation, or from any other cause, during an occlusion, in contrast to a 26% mortality in the controls (P < 0.01). 5. Infarct size, measured following a 30 min period of coronary artery occlusion followed by a 3 h reperfusion period, was reduced both 8 and 24 h after the administration of 2.5 mg kg-1 endotoxin (reductions of 24.3 and 23.1% respectively; P < 0.05). Endotoxin had no significant effect on the area at risk. 6. The beneficial effects of endotoxin on infarct size and on ventricular arrhythmias were markedly attenuated by the prior administration of dexamethasone, 3 mg kg-1 given 1 h prior to endotoxin administration. Dexamethasone itself reduced infarct size (P < 0.05) but had no direct effect on arrhythmia severity following coronary artery occlusion. 7. The mechanisms of this "cross-tolerance' induced by bacterial endotoxin against ischaemia-reperfusion injury remain to be elucidated but the most likely mechanisms appear to be the induction of protective enzymes or proteins (e.g. nitric oxide synthase, cyclo-oxygenase (COX) 2) probably mediated by cytokine release.

Delayed cardioprotection is associated with the sub-cellular relocalisation of ventricular protein kinase C epsilon, but not p42/44MAPK

Both noradrenaline administration to rats and rapid cardiac pacing in dogs induces delayed protection of the heart against ischaemia-induced ventricular arrhythmias. In an attempt to establish molecular mechanisms underlying the delayed cardioprotection, we have examined the potential role of two kinases, PKC epsilon and p42/44MAPK. These protein kinases are expressed in the ventricles of the heart and are characterised by their ability to regulate ion-flux and gene transcription. In the rat p42MAPK is predominantly localised in the high-speed supernatant fraction of the ventricle homogenate, whereas p44MAPK is enriched in the nuclear low speed pellet. A small proportion of the p42MAPK is activated even in hearts from control animals. However, neither kinase is relocalised or activated by noradrenaline administration and this provides preliminary evidence the p42/44MAPK may not play a significant role in delayed protection in this species. In contrast, noradrenaline does induce the translocation of PKC epsilon to cell membranes, a response that is sustained for up to 4 h. However, PKC epsilon is down-regulated from the cytoplasm after 24 h post noradrenaline treatment. PKC epsilon is also translocated to the membrane in dogs that have been classically pre-conditioned and cardiac paced. In the latter case, translocation of PKC epsilon from the cytoplasm to the cell membrane is evident 24 h after pacing. These results indicate that the release of endogenous mediators may either inhibit down-regulation or elicit an increase in PKC epsilon mRNA expression. Therefore, in dog heart the subcellular relocalisation of PKC epsilon persists into the 'second window' and may play a central role in the molecular mechanism governing delayed cardioprotection. It is important in the future to identify either the gene products that are induced or the target protein(s) that are phosphorylated by PKC epsilon.

Nicorandil suppressed ventricular arrhythmias in a canine model of myocardial ischaemia

These experiments were designed to explore the possibility that a K+ channel opener which also donates nitric oxide to the myocardium (nicorandil) may modify ischaemia-induced ventricular arrhythmias in a large animal model. In mongrel dogs anaesthetised with chloralose-urethane and thoracotomised, a side branch of the left anterior descending artery was catheterised for the local intracoronary infusion of nicorandil (2.5 micrograms kg-1 min-1 for 20 min prior to coronary artery occlusion and then continuing throughout the 25 min occlusion period). In this dose, nicorandil had no haemodynamic effects, increased coronary blood flow by up to 16% and significantly reduced the severity of ischaemia-induced arrhythmias (e.g. from nearly 500 ventricular premature beats in the controls to 160 +/- 60 in the nicorandil group). There was a significant reduction in the number of episodes of ventricular tachycardia during the ischaemic period and a reduced incidence of ventricular fibrillation following reperfusion resulting in a 42% survival from the combined ischaemia-reperfusion insult (cf. 0% in the control; P < 0.05). The marked changes that occurred in ST-segment elevation (mapped with epicardial electrodes) and in the inhomogeneity of electrical activation within the ischaemic area in control dogs was markedly reduced in those dogs administered nicorandil. We conclude that the local intracoronary administration of nicorandil reduces the severity of both ischaemia and the life-threatening arrhythmias that result from an abrupt reduction in coronary blood flow in this canine model. Possible mechanisms include an increase in coronary blood flow, a reduction in the severity of myocardial ischaemia and an ability of the compound to "donate' nitric oxide to the ischaemic area.

The release of kininase from rat isolated hearts during myocardial ischaemia

BK destroying activity was observed in rat isolated heart perfusates. BK was optimally degraded at pH 8.4 in rat heart. The results indicated that myocardial kinin degradation was due to ACE and a serine protease. These results suggest that bradykinin may have some cardioprotective role during myocardial ischaemia at acidic pH.

Kinin release from normally perfused and ischaemic isolated rat hearts: effect of strain

The release of kinin during 30 min of left descending coronary artery was observed in WKY, SHR and SD rat isolated hearts. The kinin levels were compared with the ECG abnormalities in these strains. It was observed that low level of kinin could not be a reason for increased ECG abnormalities. IRBK in the perfusate was also characterised.

Influence of indomethacin on the haemodynamic effects of lipopolysaccharide in rats

This study was undertaken to evaluate the influence of the cyclooxygenase inhibitor indomethacin on the time course of the haemodynamic effects of lipopolysaccharide (LPS) intravenous infusion (10 mg.kg-1.h-1) in anaesthetized rats. LPS infusion produced a rapid (within 10 min) and prolonged (over the 90 min observation period) fall in mean arterial blood pressure (MABP) and a decrease in the pressor responses to noradrenaline (NA, 0.1, 0.3 and 1 microgram.kg-1, intravenously [i.v.]) elicited 60 and 90 min after the onset of LPS infusion. Indomethacin (7 mg.kg-1 i.v. 30 min prior to the onset of saline or LPS infusion), which by itself did not affect basal MABP nor reactivity to NA in control rats, significantly attenuated the fall in MABP observed within 20 min after the onset of LPS infusion (but did not significantly modify the hypotension observed between 30 and 90 min). Indomethacin also completely prevented the hyporeactivity to NA observed 60 min after the onset of LPS infusion, but it did so only partially at 90 min. Aortic rings removed from LPS or LPS + indomethacin-treated rats showed an identical profile of contractile reactivity (hyporesponsiveness to NA, relaxation to L-arginine, and restoration of the contractile response by methylene blue). These results suggest that in this model, cyclooxygenase products are involved in the early haemodynamic effects of LPS. However, they do not seem to play an obligatory role in the onset of longer term haemodynamic changes.

Captopril and norepinephrine-induced hypertrophy and haemodynamics in rats

We wished to determine whether pretreatment with captopril, an angiotension-converting enzyme (ACE) inhibitor, modified the myocardial and haemodynamic consequences of chronic administration of norepinephrine (NE) in rats. Administration of NE (0.15 mg kg(-1) h(-1) by an osmotic minipump implanted subcutaneously for 28 days) resulted in left but not right ventricular hypertrophy. Captopril (250 but not 52 mu g kg(-1) h(1) administered for 28 days) significantly attenuated the development of left ventricular hypertrophy (weight of left ventricle to body weight ratio was 0.46 +/- 0.01 0.57 +/- 0.02, 0.53 +/- 0.02, and 0.51 +/- 0.01 for vehicle, NE, and NE plus low and high dose of captopril, respectively). Chronic administration of NE caused significant increases in systolic arterial blood pressure (BP: 194 +/- 11 vs. 130 +/- 6 mm Hg), systolic left ventricular pressure, heart rate (HR: 458 +/- 13 vs. 389 +/- 15 beats/min) and dP dt(-1)(max) P(-1), an index of myocardial contractility (202 +/- 29 vs. 91 +/- 3 s(-1)). Captopril (250 mu g kg(-1) h(-1) for 28 days) significantly reduced diastolic arterial BP (from 86 +/- 6 to 53 +/- 3 mm Hg). Concomitant administration of this dose of captopril together with NE prevented the NE-induced increase in systolic arterial BP but did not modify the increases in HR or dP dt(-1) max P(-1) (261 +/- 41 and 202 +/- 29 s(-1) in captopril and NE vs. NE-alone groups). Acute administration of NE (0.1-10 mu g kg(-1) intravenously, i.v.) produced less marked increases in cardiac contractility and in arterial BP in rats chronically pretreated with NE or NE plus captopril than in animals receiving vehicle or captopril alone. Chronic administration of NE and/or captopril did not significantly modify the haemodynamic effects of the acute administration of calcium chloride. We conclude that administration of captopril at 250 but not 52 mu g kg(-1) h(-1) for 28 days attenuates NE-induced cardiac hypertrophy and that this effect is associated with a decrease in systolic arterial BP. Captopril did not modify the reduced effects of acutely administered NE in rats treated with NE for a prolonged period.

The effect of endotoxin on sympathetic responses in the rat isolated perfused mesenteric bed; involvement of nitric oxide and cyclo-oxygenase products

1. The effects of endotoxin on the vasoconstrictor responses to sympathetic nerve stimulation (SNS) were investigated in the rat isolated perfused mesenteric bed. 2. Rats received either saline (0.1 ml h-1) or endotoxin (2.5 mg kg-1 h-1) intravenously for 4 h; the mesenteric beds were then isolated, perfused with Krebs and prepared for SNS (50 V, 3 ms, 7-40 Hz). 3. SNS caused a frequency-dependent vasoconstrictor response which was abolished by either tetrodotoxin (10(-7) M), prazosin (2.4 x 10(-7) M) or guanethidine (2.4 x 10(-7) M). 4. In mesenteric vascular beds removed from rats infused with endotoxin, there were markedly impaired vasoconstrictor responses to SNS, although responses to noradrenaline were not modified. 5. Removal of the endothelium with distilled water prevented endotoxin-induced impairment of vasoconstrictor responses to SNS, without modifying these responses in preparations from control rats. 6. Pretreatment with dexamethasone (3 mg kg-1 i.p. 1h before commencing endotoxin or saline infusions) did not modify responses to SNS in control rats but prevented the effects of endotoxin. 7. Both L-NAME (10(-3) M) and indomethacin (10(-5) M) restored responses to SNS in preparations from endotoxin-treated rats without modifying these responses in control preparations. However, co-administration of L-NAME and indomethacin markedly augmented responses in both control and endotoxin-treated preparations. 8. The effects of L-NAME were reversed by addition of L-arginine (10(-3) M). 9. The data suggest that endotoxin impairs the release of noradrenaline and that this effect is secondary to increased production of nitric oxide and prostanoids, possibly by the endothelium.

Changes in intracellular pH close to term and their possible significance to labour

The contractile pattern of uterine smooth muscle is markedly altered by even modest manipulations of intracellular pH (pHi); an acidification can abolish contractions, while alkalinization increases contractility. As at the end of gestation the uterus changes from being relatively quiescent to highly contractile, we investigated whether pHi changes with pregnancy in women. Isolated tissue was loaded with the pH-sensitive fluorophore, carboxy-SNARF. We found a significant alkalinization over the last few weeks of pregnancy. Such an increase in pHi will increase contractility of the uterus, and may therefore contribute to the mechanisms ensuring strong and efficient contractions occur during labour.

Delayed protection of the heart against ischaemia

Ischaemic preconditioning is the protective adaptive mechanism produced by short periods of ischaemic stress that results in a marked resistance of the myocardium to prolonged periods of the same stress; however, this protection is transient. There is now evidence that protection resulting from preconditioning returns several hours later, and here James Parratt and Laszlo Szekeres highlight the possible importance of this concept, which may lead to novel approaches to the long-term protection of the heart against ischaemic injury.

Effects of chronic norepinephrine administration on cardiac function in rats

We assessed the changes in the contractile response of rat hearts in vivo after chronic exposure to a range of doses of norepinephrine (NE) and determined whether free radical production played a role in these changes. Osmotic minipumps were implanted subcutaneously (s.c.) in male rats and delivered either NE (0.15-0.35 mg/kg/h) or acid saline for 10-28 days. The animals were then anaesthetised and prepared for haemodynamic measurement, and dose-response curves to acutely administered NE and calcium chloride were constructed. We analysed plasma for evidence of free radical activity by measuring the levels of thiobarbituric acid-reactive substances (TBARS). All doses of NE studied produced left, but not right, ventricular hypertrophy. Treatment with 0.25 mg/kg/h NE for 28 days produced signs of distress and, by 10 days, treatment with 0.35 mg/kg/h resulted in 33% mortality. Treatment with the two lower doses, but not the highest dose, of NE resulted in increases in basal left ventricular (LV) maximum rate of pressure generation and a marked increase in systolic, but not diastolic, arterial blood pressure (SBP, DBP). All doses of NE caused reduced responses to acutely administered NE but no marked change in the response to calcium chloride. Levels of plasma free radicals were increased only with the highest dose of NE. Over the concentration range studied, chronic administration of NE to rats causes beta-adrenoceptor downregulation and free radical production was associated only with the administration of a dose of NE that resulted in high mortality.

Functional effects of intracellular pH alteration in the human uterus: simultaneous measurements of pH and force

Changes in intracellular pH have differing effects on contraction in different types of muscle, and between species. Because of the large number of women requiring a caesarean section due to failure to progress in labour, it is important to know how the human myometrium responds to the changes in intracellular pH that may occur during labour. The pH-sensitive dye, carboxy-SNARF, was used to measure intracellular pH in small strips of human myometrium. Intracellular pH and tension were simultaneously recorded in pregnant and nonpregnant tissue. Intracellular pH was altered by the addition of weak acids and bases. Intracellular alkalinization caused an increase in the frequency and amplitude of contractions. Intracellular acidification led to an initial increase in the frequency and/or the amplitude of the contractions, followed by abolition of contractions. Alterations in intracellular pH had profound effects on contraction in human uterine smooth muscle. Possible mechanisms are discussed whereby pH could influence force production, and changes in contraction are related to the speed and extent of the change in intracellular pH.

An acidic kininogenase in rat ventricular myocardium

BACKGROUND

Canine ventricles have been reported to contain a cathepsin D-like kininogenase, which might confer protection on the heart during ischaemia. The aim of this study was to investigate the presence and levels of a similar kininogenase in normal and ischaemic rat hearts.

METHODS

Aqueous extracts of rat ventricles were tested for the ability to release bradykinin-like immunoreactivity from human low-molecular-weight kininogen and high-molecular-weight kininogen at acidic pHs. The enzymes involved were separated using gel filtration followed by the testing of fractions for cleavage of D-Val-Leu-Arg-pNA and low-molecular-weight kininogen. Extracts from normal and ischaemic ventricles were compared for the ability to release bradykinin-like immunoreactivity from low-molecular-weight kininogen. Kinin levels in mixed venous blood were compared before and after ischaemia. By assessing their effect on isolated oestrous rat uteri and on protease inhibitors, further characterization of acidic kininogenases in the extracts was performed.

RESULTS

Extracts of rat ventricles released bradykinin-like immunoreactivity only from low-molecular-weight kininogen. Using the isolated oestrous rat uterus, gel filtration and protease inhibitors, the enzyme involved was identified as a cathepsin D-like enzyme with an optimum pH of 4.7 and a molecular weight of 42.8 +/- 4.9 kDa. It is an arginine amidase and releases bradykinin-like immunoreactivity from low-molecular-weight kininogen. Ischaemia reduced the amount of bradykinin-like immunoreactivity released by the ventricular extract (P < or = 0.05) and increased levels of free kinin in venous blood from the right atrium.

CONCLUSION

Rat ventricles contain a cathepsin D-like acidic protease that cleaves low-molecular-weight kininogen to release bradykinin-like immunoreactivity. The acidic protease may protect the heart during ischaemia.

Bradykinin as an endogenous myocardial protective substance with particular reference to ischemic preconditioning--a brief review of the evidence

The present brief review summarizes the evidence for the possibility that endogenously released bradykinin plays a major role in protecting the heart against the consequences of acute myocardial injury. This evidence includes the facts that kinins are generated under myocardial ischemia; that when they are administered, they are cardioprotective (e.g., antiarrhythmic); that drugs that enhance the release of bradykinin from the ischemic heart reduce the ischemic injury and, conversely, drugs that block bradykinin receptors attenuate the reduction in ischemic injury resulting from the release of, or administration of, bradykinin. The possible mechanism of bradykinin in the cardioprotection afforded by ischemic preconditioning is summarized. Ischemic preconditioning can be defined as the marked reduction in the severity of ischemic changes that result from coronary artery occlusion when that occlusion is preceded by brief periods of myocardial ischemia, either regional or global, induced, for example, by complete or partial coronary artery occlusion or by rapid ventricular pacing. The possible mechanisms of cardioprotection elicited by bradykinin (and ischemic preconditioning) are summarized. The most likely is the generation of cyclic GMP within the ischemic myocardium following bradykinin-stimulated nitric oxide generation and release from endothelial cells.

Ischaemic preconditioning in the rat heart: the role of G-proteins and adrenergic stimulation

Since recent findings indicate the involvement of G-proteins in the mechanism of ischaemic preconditioning (PC), the present study was aimed to investigate the role of adrenergic mechanisms, such as G-proteins and stimulation of adrenergic receptors, in this phenomenon. For this purpose, isolated Langendorff-perfused rat hearts were subjected to regional ischaemia (30 min occlusion of LAD) followed by reperfusion. The effect of PC (a single 5 min occlusion/reperfusion before a long occlusion) on ischaemia- and reperfusion induced arrhythmias was studied in conjunction with an assessment of G-proteins in the myocardial tissue by means of Western blotting and ADP-ribosylation with bacterial toxins. To follow the link between G-proteins and adrenergic receptors, their stimulation by exogenous norepinephrine (NE) was applied to test whether it can mimic the effect of PC on arrhythmias. Thirty min ischaemia and subsequent reperfusion induced high incidence of ventricular tachycardia (VT) and fibrillation (VF). PC significantly reduced a total number of extrasystoles, incidence of VT and abolished VF. It was, however, insufficient to suppress reperfusion-induced sustained VF. Measurement of G-proteins revealed that PC led to a reduction of stimulatory Gs proteins, whereas inhibitory Gi proteins were increased. NE (50 nmol) introduced a manner of similar to PC (5 min infusion, 10 min normal reperfusion) reduced ischaemic arrhythmias in the same way, as PC. In addition, in NE-pretreated hearts reperfusion induced mostly transient VF, which was spontaneously reverted to normal sinus rhythm. A transient increase in heart rate and perfusion pressure during NE infusion completely waned before the onset of ischaemia, indicating that antiarrhythmic effect was not related to haemodynamic changes and to conditions of myocardial perfusion.

CONCLUSION

antiarrhythmic effect of PC may be mediated by a stimulation of adrenergic receptors coupled to appropriate G-proteins. Consequently, the inhibition of adenylate cyclase activity and reduction in cAMP level, as well as the activation of protein kinase C may be considered as two possible pathways leading to a final response.

Effects of Bordetella pertussis toxin pretreatment on the antiarrhythmic action of ischaemic preconditioning in anaesthetized rats

1. Bordetella pertussis toxin, which catalyses the ADP-ribosylation of certain guanine nucleotide binding proteins (G proteins), thus functionally uncoupling them from associated receptors, was examined to determine whether it modified the antiarrhythmic effect of ischaemic preconditioning in anaesthetized rats. 2. Pertussis toxin (25 micrograms kg-1, i.p., 48 h prior to heart isolation) attenuated the negative chronotropic effect of acetylcholine (ACh) in rat isolated Langendorff perfused hearts. ACh (10 microM) reduced heart rate by 4% in hearts taken from pertussis toxin-treated animals, compared to a reduction of 57% in hearts taken from animals treated only with vehicle. 3. In anaesthetized rats, ischaemic preconditioning (a single 3 min occlusion of the left main coronary artery followed by 10 min reperfusion) had a pronounced antiarrhythmic effect during a subsequent 30 min period of regional myocardial ischaemia. Compared to hearts receiving only a 30 min period of left coronary occlusion, there was a reduced mortality (67% and 0% for control and preconditioned groups, respectively; P < 0.01) and decreased incidences of ventricular tachycardia (VT) and ventricular fibrillation (VF). Pretreatment with pertussis toxin (25 micrograms kg-1, i.p., 48 h previously) did not modify the arrhythmias associated with a 30 min period of regional myocardial ischaemia, neither did it modify the reduction in mortality (from 56% to 0%; P < 0.05) associated with preconditioning. Furthermore, the decrease in total ventricular premature beat count induced by preconditioning seen in controls (from 427 +/- 130 to 95 +/- 45) was also seen in pertussis toxin-treated rats (from 252 +/- 190 to 57 +/- 25). 4. These results suggest that receptor coupling to pertussis toxin-sensitive G proteins is not necessary for the antiarrhythmic effect of ischaemic preconditioning in this model.

Pronounced antiarrhythmic effects of preconditioning in anaesthetized dogs: is adenosine involved?

Although there is good evidence that adenosine contributes to the ability of ischaemic preconditioning to reduce myocardial ischaemic damage (infarct size) there is no evidence that it contributes to the marked antiarrhythmic effects of this endogenous protective mechanism. We have examined this in anaesthetized open-chest mongrel dogs by administering the non-selective adenosine receptor blocking drug 8-sulfophenyltheophylline (8-SPT) (1 mg/kg) given by intracoronary administration 10 min before both 5 min preconditioning coronary artery occlusions and also before the prolonged 25 min LAD occlusion i.e. in a total dose of 3 mg/kg. The only haemodynamic effect of 8-SPT was a reduction in coronary (LAD) blood flow and an increase in cardiovascular resistance. It was difficult to precondition dogs in the presence of 8-SPT; the number of ventricular premature beats was significantly higher (61 +/- 6 v 8 +/- 4; P < 0.01) during the initial preconditioning occlusion and the incidence of ventricular fibrillation during the preconditioning procedure was higher in the presence of the drug (5/11 v 4/20; P < 0.05). Nevertheless, it was still possible to precondition these dogs in the presence of the drug. No VF occurred during the prolonged occlusion (cf. 50% in the controls) and the number of episodes of ventricular tachycardia, and the number of ventricular premature beats in dogs preconditioned in the presence of 8-SPT was similar to those in dogs preconditioned without 8-SPT and significantly (P < 0.01 or P < 0.05) less than in control, non-preconditioned dogs.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuation by dexamethasone of endotoxin protection against ischaemia-induced ventricular arrhythmias

Ventricular arrhythmias from a 30 min occlusion of the left coronary artery were assessed in Langendorff perfused isolated hearts removed from rats administered either saline, or endotoxin derived from Escherichia coli (2.5 mg kg-1 i.p.) given either 2, 4, 8, 24 or 48 h previously. Arrhythmia severity was markedly reduced in those hearts removed from rats administered endotoxin with a maximum protection at 8h; there was a marked reduction in the incidence of ventricular fibrillation (from 54% to 4%) and in the number of ventricular premature beats during the occlusion period (e.g. from 1165 +/- 144 to 37 +/- 19; P < 0.01). Dexamethasone (3 mg kg-1, given 1 h prior to endotoxin or saline) markedly attenuated the protection afforded by endotoxin.

Prevention by dexamethasone of the marked antiarrhythmic effects of preconditioning induced 20 h after rapid cardiac pacing

Dogs were paced, via a pacing electrode in the right ventricle, for four 5 min periods at a rate of 220 beats min-1. On the following day they were reanaesthetized, thoracotomized and the left anterior descending coronary artery occluded for 25 min. Pacing markedly reduced the severity of ischaemia-induced arrhythmias (e.g. reduction in VF from 45% in unpaced dogs to 10% in paced dogs; P < 0.05), an effect reversed by dexamethasone (4 mg kg-1 i.v., 45 min prior to pacing). This protection may be due to the induction of nitric oxide synthase or cyclo-oxygenase.

Effects of NS-2, a new class 1 antiarrhythmic agent, and AFD-19, its active metabolite, on ventricular arrhythmias induced by coronary artery occlusion and reperfusion in anesthetized rats: comparison with disopyramide and mexiletine

We studied the antiarrhythmic effects of NS-2 (4-diisobutylamino-1,1-diphenyl-1-butanol maleate) and AFD-19 (active metabolite of NS-2) on early stage ventricular arrhythmias induced by coronary artery occlusion and reperfusion in anesthetized male rats. These effects were compared with those of disopyramide and mexiletine. Drugs were intravenously administered either before or after coronary occlusion. When administered 5 min before occlusion, 3 mg/kg of NS-2 and AFD-19 exhibited equivalent anti-arrhythmic activity to that of 5 mg/kg of disopyramide and mexiletine, as assessed by reductions in the number of premature ventricular complexes and in the incidences of ventricular tachycardia and ventricular fibrillation. In a dose of 5 mg/kg, the antiarrhythmic effects of NS-2 and AFD-19 were more pronounced. When administered 5 min after coronary artery occlusion, only NS-2 and AFD-19 (in doses of 5 mg/kg) had significant antiarrhythmic effects. None of the drugs influenced the severe ventricular arrhythmias induced by reperfusion when administered 1 min before reperfusion. In conclusion, NS-2 might be effective in reducing the severity of the life-threatening ventricular arrhythmias that occur during acute myocardial infarction.