Post-ischaemic activation of kinases in the pre-conditioning-like cardioprotective effect of the platelet-activating factor.

AIM: Platelet-activating factor (PAF) triggers cardiac pre-conditioning against ischemia/reperfusion injury. The actual protection of ischaemic pre-conditioning occurs in the reperfusion phase. Therefore, we studied in this phase the kinases involved in PAF-induced pre-conditioning. METHODS: Langendorff-perfused rat hearts underwent 30 min of ischaemia and 2 h of reperfusion (group 1, control). Before ischaemia, group 2 hearts were perfused for 19 min with PAF (2 x 10(-11) M); groups 3-5 hearts were co-infused during the initial 20 min of reperfusion, with the protein kinase C (PKC) inhibitor chelerythrine (5 x 10(-6) M) or the phosphoinositide 3-kinase (PI3K) inhibitor LY294002 (5 x 10(-5) M) and atractyloside (2 x 10(-5) M), a mitochondrial permeability transition pore (mPTP) opener respectively. Phosphorylation of PKCepsilon, PKB/Akappat, GSK-3beta and ERK1/2 at the beginning of reperfusion was also checked. Left ventricular pressure and infarct size were determined. RESULTS: PAF pre-treatment reduced infarct size (33 +/- 4% vs. 64 +/- 5% of the area at risk of control hearts) and improved pressure recovery. PAF pre-treatment enhanced the phosphorylation/activation of PKCepsilon, PKB/Akappat and the phosphorylation/inactivation of GSK-3beta at reperfusion. Effects on ERK1/2 phosphorylation were not consistent. Infarct-sparing effect and post-ischaemic functional improvement induced by PAF pre-treatment were abolished by post-ischaemic infusion of either chelerythrine, LY294002 or atractyloside. CONCLUSIONS: The cardioprotective effect exerted by PAF pre-treatment involves activation of PKC and PI3K in post-ischaemic phases and might be mediated by the prevention of mPTP opening in reperfusion via GSK-3beta inactivation.

The platelet activating factor triggers preconditioning-like cardioprotective effect via mitochondrial K-ATP channels and redox-sensible signaling.

Endogenous platelet activating factor (PAF) is involved in heart ischemic preconditioning. PAF can also afford pharmacological preconditioning. We studied whether mitochondrial-ATP-sensitive K(+) (mK(ATP)) channels and reactive oxygen species (ROS) are involved in PAF-induced cardioprotection. In Group 1 control hearts, Langendorff-perfused rat hearts underwent 30 min ischemia and 2 hours of reperfusion. Group 2 hearts, before ischemia, were perfused for 19 min with PAF (2x10(-11) M); Groups 3 and 4 hearts were co-infused with PAF and N-acetyl-L-cysteine or 5-hydroxydecanoate to scavenge ROS or to block mK(ATP) channels, respectively. Left ventricular pressure and infarct size were determined. PAF-pretreatment reduced infarct size (33 +/- 4% vs 64 +/- 4.6 % of the area at risk of control hearts) and improved pressure recovery. Infarct-sparing effect of PAF was abolished by N-acetyl-L-cysteine and 5-hydroxydecanoate. Thus, the cardioprotective effect exerted by PAF-pretreatment involves activation of mK(ATP) channels and redox signaling in pre-ischemic phase.

Endothelial cytochrome P450 contributes to the acetylcholine-induced cardiodepression in isolated rat hearts.

AIMS: Acetylcholine (ACh) is known to reduce the contractility of the heart by acting on myocardial muscarinic M2 receptors. ACh induces also an endothelial-dependent vasodilatation by causing the release of nitric oxide (NO), prostacyclin and endothelium-derived hyperpolarizing factors from the vascular endothelium. It has been proposed that ACh elicits a hyperpolarization of the coronary endothelial cells which may be accompanied by the activation of cytochrome P450 (CYP) and the resulting release of epoxyeicosatrienoic acids (EETs). The study aims at investigating whether endothelial CYP is involved in the cardiodepression by ACh. METHODS AND RESULTS: In isolated rat hearts, cardiodepression by ACh (i.e. 25-30% reduction of developed left ventricular pressure) was partially attenuated either by inhibition of CYP with 1-aminobenzotriazole (ABT) or by endothelial dysfunction obtained with Triton X-100. No attenuation of cardiodepression was seen after nitric oxide synthase and cyclooxygenase inhibition by L-nitro-arginine methyl ester and indomethacin, respectively. CONCLUSION: The results suggest that the negative inotropic effect of ACh depends not only on a direct myocardial effect but also on the endothelial CYP activation.

Role of the fuel utilized by tissues on coronary vessel response to physical stimuli in isolated rat hearts.

In isolated rat hearts which can or cannot utilize fatty acids (FA) as substrates the coronary responses to an increase in flow were studied under three different conditions: a) control, during perfusion with glucose-enriched Tyrode solution which allowed the hearts to utilize long-chain FA from the endogenous pool, b) during forced utilization of glucose obtained with oxfenicine, an inhibitor of long-chain FA oxidation, and c) during restored utilization of FA obtained with the addition of hexanoic acid which bypasses the blockade induced by oxfenicine. A step increase in coronary flow (50 %) induced an increase in coronary perfusion pressure whose initial slope (first 60-80 s) was similar in all the conditions of buffer perfusion, thereafter the pressure tended to further increase under control conditions (buffer a), but to decrease during oxfenicine (buffer b). The addition of hexanoic acid to the perfusion solution (buffer c) abolished the effect of oxfenicine. Steady-state conditions were reached after four minutes of increased flow, when perfusion pressure increased by about 70 and 65 % under control conditions and during hexanoate, respectively, but only by 45 % during oxfenicine. In isolated rat hearts during inhibition of FA utilization, an increase in flow elicited a reduced increase in perfusion pressure that resulted in delayed coronary dilation. It follows that the resulting shear stress is substrate-sensitive.

Fatty acids are important for the Frank-Starling mechanism and Gregg effect but not for catecholamine response in isolated rat hearts.

In some pathophysiological conditions myocardial metabolism can switch from mainly long chain fatty acid (LCFA) oxidation to mainly glucose oxidation. Whether the predominant fatty acid or glucose oxidation affects cardiac performance has not been defined. In a buffer perfused isovolumetrically contracting rat heart, oxidation of endogenous pool LCFA was avoided by inhibiting carnitine-palmitoyl-transferase I (CPT-I) with oxfenicine (2 mM). In order to restore fatty acid oxidation, hexanoate (1 mM), which bypasses CPT-I inhibition, was added to the perfusate. Three groups of hearts were subjected to either an increase in left ventricular volume (VV, +25%) or an increase in coronary flow (CF, +50%), or inotropic stimulation with isoproterenol (10(-8) and 10(-6) m). The increase in VV (the Frank-Starling mechanism) increased rate-pressure product (RPP) by 21 +/- 2% under control conditions, but only by 6 +/- 2% during oxfenicine-induced CPT-I inhibition. The contractile response to changes in VV recovered after the addition of hexanoate. Similar results were obtained in hearts, in which an increase in CF was elicited (the Gregg phenomenon). Isoproterenol caused a similar increase in contractility regardless of the presence of oxfenicine or hexanoate. In all groups, a commensurate increase in oxygen consumption accompanied the increase in contractility. The fatty acid oxidation is necessary for an adequate contractile response of the isolated heart to increased pre-load or flow, whereas the inotropic response to adrenergic beta-receptor stimulation is insensitive to changes in substrate availability.

Mitochondrial ATP-sensitive channel opener does not induce vascular preconditioning, but potentiates the effect of a preconditioning ischemia on coronary reactive hyperemia in the anesthetized goat.

Preconditioning ischemia (PI) increases the speed of the initial vasodilatation (vascular preconditioning) of a subsequent coronary reactive hyperemia (CRH) and reduces total hyperemic flow (THF). We investigated whether changes in CRH similar to those induced by PI are obtained with diazoxide, a mitochondrial ATP-sensitive K+ channel opener, and whether diazoxide influences the effects of a subsequent PI on CRH. In anesthetized goats, flow was recorded from the left circumflex coronary artery (LCCA). CRH and PI were obtained with 15-s and 5-min LCCA occlusions, respectively. CRH was studied before and after PI, before and after diazoxide (2.5 mg/kg i.v.) as well as before and after PI was induced after diazoxide pre-treatment. After PI, the time to peak (ttp) of CRH and THF decreased by 51+/-13% and 23+/-8%, respectively. Diazoxide did not change CRH. After diazoxide and PI, when basal flow had returned to the control level, the ttp of CRH was reduced as after PI alone (-45+/-12%), whereas THF was reduced to a greater extent (-41+/-9% versus -23+/-8%; P<0.01). In conclusion, PI alters CRH by decreasing THF and reducing the ttp of CRH. Whilst diazoxide does not reproduce the effects of PI on CRH, pre-treatment with diazoxide potentiates the effects of PI on THF.

Involvement of nitric oxide in ischemic preconditioning.

In ischemic preconditioning, nitric oxide (NO) limits the extension of a subsequent infarct and protects against ischemia/reperfusion-induced endothelial dysfunction, arrhythmias and myocardial stunning. The protective activity concerns both the first and the second window of protection. The antiarrhythmic effect is attributed to microvessel dilation and to the production of cyclic guanosine monophosphate in the myocardium. The limitation of the infarct size is likely to depend on the opening of the mitochondrial adenosine triphosphate-sensitive potassium channels, to which NO participates via the activation of a protein kinase C (PKC). The endothelial protection involves an NO-mediated reduction in neutrophil adherence to the coronary endothelium and platelet aggregation and is accompanied by an enhanced response to vasodilator stimuli. During preconditioning ischemia, NO is released from the coronary endothelium as a result of bradykinin-induced activation of B2 endothelial receptors. In addition to the early protection, endothelium-derived NO is also responsible for a signaling cascade which leads to the activation of myocardial inducible NO synthase, which in turn is responsible for the release of NO involved in the delayed protection. The signaling cascade includes the activation of PKC-epsilon, tyrosine kinase and some mitogen-activated protein kinases. It has been suggested that the activation of PKC-epsilon is mediated by peroxynitrite produced by the combination of NO and the superoxide anion, the latter being generated during reperfusion which follows preconditioning ischemia.

Ischemic preconditioning: from the first to the second window of protection.

In many species one or more brief coronary occlusions limit the injuries which a subsequent ischemia-reperfusion can produce in the myocardium. A similar protection has been observed in the majority of organ systems. A first period or window of protection can lasts up to 3 hours and is followed by a second window of protection (SWOP) which begins about 24 hours after the brief coronary occlusions and lasts about 72 hours. Increase of the release of endogenous agents such as adenosine and nitric oxide (NO) may be responsible for both windows through the activation of a protein-kinase C (PKC) which in turn activates ATP sensitive potassium (K+(ATP)) channels. Nitric oxide is also reported to act directly on K+(ATP) channels. Recently, it has been suggested that the channels involved in the protection are mitochondrial rather than sarcolemmal. In SWOP the origin of NO is attributed to the activity of an inducible NO-synthase. Free oxygen radicals released during preconditioning are likely to take part in the delayed protection through the production of peroxynitrite which activates PKC and through the increase of the activity of antioxidant enzymes such as Mn superoxide-dismutase. The production of heat shock proteins is considered a marker rather than a mechanism of SWOP.

Cytochrome P-450 metabolite of arachidonic acid mediates bradykinin-induced negative inotropic effect.

This study focused on the mechanisms of the negative inotropic response to bradykinin (BK) in isolated rat hearts perfused at constant flow. BK (100 nM) significantly reduced developed left ventricular pressure (LVP) and the maximal derivative of systolic LVP by 20-22%. The cytochrome P-450 (CYP) inhibitors 1-aminobenzotriazole (1 mM and 100 microM) or proadifen (5 microM) abolished the cardiodepression by BK, which was not affected by nitric oxide and cyclooxygenase inhibitors (35 microM NG-nitro-L-arginine methyl ester and 10 microM indomethacin, respectively). The CYP metabolite 14,15-epoxyeicosatrienoic acid (14,15-EET; 50 ng/ml) produced effects similar to those of BK in terms of the reduction in contractility. After the coronary endothelium was made dysfunctional by Triton X-100 (0.5 microl), the BK-induced negative inotropic effect was completely abolished, whereas the 14,15-EET-induced cardiodepression was not affected. In hearts with normal endothelium, after recovery from 14,15-EET effects, BK reduced developed LVP to a 35% greater extent than BK in the control. In conclusion, CYP inhibition or endothelial dysfunction prevents BK from causing cardiodepression, suggesting that, in the rat heart, endothelial CYP products mediate the negative inotropic effect of BK. One of these mediators appears to be 14,15-EET.

The endothelium-derived hyperpolarizing factor: does it play a role in vivo and is it involved in the regulation of vascular tone only?

Several investigations performed in vitro have shown that vascular endothelia can release diffusible compounds capable of inducing hyperpolarization of the smooth muscle fibers. Experiments in vitro have shown that these compounds can cause coronary vasodilation and alter cardiac performance. Experiments in vivo only showed the occurrence of vasodilation. While it has been shown that the release of these endothelium-derived hyperpolarizing factors (EDHFs) is not impaired by the inhibition of nitric oxide synthase and cyclooxygenase, the precise nature of the compound(s) has not yet been identified. It is possible that they vary depending on the organ and animal species. However, a common feature of the activity of EDHFs is the activation of calcium-dependent potassium channels, inhibitable by charybdotoxin and apamin. Furthermore in the coronary circulation of many species EDHF seems to be a cytochrome P450-dependent non-prostanoid metabolite of arachidonic acid activated by a number of chemical and physical stimuli similar to those which are known to activate endothelial nitric oxide synthase. Using compounds which inhibit cytochrome P450 and blockers of the calcium-dependent potassium channels, researchers can study the physiological and pathophysiological relevance of EDHF in vivo thus disclosing the potential therapeutic applications of the basic knowledge in this field.

Systolic and diastolic changes in human coronary blood flow during Valsalva manoeuvre.

Valsalva manoeuvre is reported to be sometimes successful for the relief of angina pectoris. The present study investigated how haemodynamic changes produced by Valsalva manoeuvre can interact to improve the relationship between cardiac work and coronary blood flow. Ten male subjects aged 53 +/- 12 years (SD) were considered. Blood velocity in the internal mammary artery, previously anastomosed to the left descending coronary artery, was studied with Doppler technique. The subjects performed Valsalva manoeuvres by expiring into a tube connected to a mercury manometer, to develop a pressure of 40 mmHg. The arterial blood pressure curve was continuously monitored with a Finapres device from a finger of the left hand. During expiratory effort, an increase in heart rate and a decrease in arterial pulse pressure were followed by a more delayed and progressive increase in mean and diastolic pressures. Systolic blood velocity markedly decreased along with the reduction in pulse pressure and increase in heart rate. By contrast, diastolic and mean coronary blood velocities did not show any significant change. Since it is known that the Valsalva manoeuvre strongly reduces stroke volume and cardiac output, it is likely that a reduction in cardiac work also takes place. Since in diastole, i.e. when the myocardial wall is better perfused, coronary blood velocity did not show any significant reduction, it is likely that unchanged perfusion in the presence of reduced cardiac work is responsible for the relief from angina sometimes observed during Valsalva manoeuvre. It is also likely that the increase in heart rate prevents the diastolic and mean blood coronary velocity from decreasing during the expiratory strain, when an increased sympathetic discharge could cause vasoconstriction through the stimulation of the coronary alpha-receptors.

New insights into nitric oxide and coronary circulation.

Since its discovery over 20 years ago as an intercellular messenger, nitric oxide (NO), has been extensively studied with regard to its involvement in the control of the circulation and, more recently, in the prevention of atherosclerosis. The importance of NO in coronary blood flow control has also been recognized. NO-independent vasodilation causes increased shear stress within the blood vessel which, in turn, stimulates endothelial NO synthase activation, NO release and prolongation of vasodilation. Reactive hyperemia, myogenic vasodilation and vasodilator effects of acetylcholine and bradykinin are all mediated by NO. Ischemic preconditioning, which protects the myocardium from cellular damage and arrhythmias, is itself linked with NO and both the first and second windows of protection may be due to NO release. Exercise increases NO synthesis via increases in shear stress and pulse pressure and so it is likely that NO is an important blood flow regulatory mechanism in exercise. This phenomenon may account for the beneficial effects of exercise seen in atherosclerotic individuals. Whilst NO plays a protective role in preventing atherosclerosis via superoxide anion scavenging, risk factors such as hypercholesterolemia reduce NO release leading the way for endothelial dysfunction and atherosclerotic lesions. Exercise reverses this process by stimulating NO synthesis and release. Other factors impacting on the activity of NO include estrogens, endothelins, adrenomedullin and adenosine, the last appearing to be a compensatory pathway for coronary control in the presence of NO inhibition. These studies reinforce the pivotal role played by the substance in the control of coronary circulation.

Ischaemic preconditioning changes the pattern of coronary reactive hyperaemia in the goat: role of adenosine and nitric oxide.

OBJECTIVES: After ischaemic preconditioning (IP), obtained by short episodes of ischaemia, cardiac protection occurs due to a reduction in myocardial metabolism through the activation of A1 adenosine receptors. The antiarrhythmic effect of IP is attributed to an increase in the release of nitric oxide (NO) by the endothelium. On the basis of the above consideration the present investigation studies the changes induced by preconditioning in coronary reactive hyperaemia (RH) and how blockade of A1 receptors and inhibition of NO synthesis can modify these changes. METHODS: In anaesthetised goats, an electromagnetic flow-probe was placed around the left circumflex coronary artery. Preconditioning was obtained with two episodes of 2.5 min of coronary occlusion, separated by 5 min of reperfusion. RH was obtained with a 15 s occlusion. In a control group (n = 7) RH was studied before and after IP. In a second group (n = 7), 0.2 mg kg-1 of 8-cyclopentyl-dipropylxanthine, an A1 receptor blocker, and in a third group (n = 7) 10 mg kg-1 of NG-nitro-L-arginine (LNNA), an NO inhibitor, were given before IP. Reactive hyperaemia was again obtained before and after IP. RESULTS: In the control group, after IP, the time to peak hyperaemic flow and total hyperaemic flow decreased by about 50% and 25%, respectively. The A1 receptor blockade alone did not change RH. During A1 blockade, IP reduced the time to peak of RH similar as in control (45%), but did not alter total hyperaemic flow. LNNA alone reduced resting flow and total hyperaemic flow. After NO inhibition, IP only reduced total hyperaemic flow by about 15%, but the time to peak flow was not affected. CONCLUSIONS: IP alters RH by decreasing total hyperaemic flow and reducing the time to peak hyperaemic flow. While the former effect is attributed to a reduction in myocardial metabolism through the activation of the A1 receptors, the latter is likely to be due to an increased endothelial release of NO, suggesting that in addition to a protective effect on the myocardium, IP also exerts a direct effect on the responsiveness of the coronary vasculature (vascular preconditioning).

The effects of ischemic preconditioning on resting coronary flow and reactive hyperemia: involvement of A1 adenosine receptors.

During the myocardial protection induced by ischemic preconditioning a reduction in myocardial metabolism occurs due to activation of the A1 adenosine receptors. This study investigates whether preconditioning changes both resting coronary flow and the magnitude of coronary reactive hyperemia and whether A1 adenosine receptors are involved in the observed changes. Experiments were performed in 14 goats (30-50 kg body weight). After the animals were anesthetized with ketamine, an electromagnetic flow-probe was used to record blood flow in the left circumflex coronary artery. Distal to the probe, an occluder was placed to produce ischemic preconditioning and reactive hyperemia. Preconditioning was obtained with two periods of 2.5 min of coronary occlusion separated from each other by 5 min of reperfusion. Coronary reactive hyperemia was obtained with 15 s of occlusion of the artery before and after preconditioning. In a group of goats before preconditioning 0.2 mg kg(-1) of 8-cyclopentyl-dipropylxanthine (CPX), an A1 adenosine receptor blocker, were given intravenously. In all animals ischemic preconditioning did not alter resting coronary flow, but, in the absence of A1 adenosine receptor blockade, reduced the reactive hyperemic response. The total hyperemic flow and the excess/debt flow ratio were reduced by about 25% and 30% respectively. The A1 adenosine receptor blockade "per se" did not cause any change in the resting flow and in the parameters of the reactive hyperemia. Unlike what observed in the absence of blockade, after CPX ischemic preconditioning was unable to reduce total hyperemic flow and the excess/debt flow ratio. The results suggest that ischemic preconditioning reduces the coronary hyperemic response by decreasing the myocardial metabolism through the activation of the A1 adenosine receptors.

Systolic coronary flow impediment in the dog: role of ventricular pressure and contractility.

The present study was planned to investigate the effect of left ventricular pressure and inotropic state on coronary arterial inflow in systole in the anaesthetized dog. A wide range of left ventricular systolic pressures, including the physiological range, were studied. Experiments were done under conditions of maximal vasodilatation and low perfusion pressure in order to avoid vascular autoregulative interference and to keep the microvascular pressure within the normal range. In five anaesthetized dogs, perfused with extracorporeal circulation system, ventricular volume was changed from 20 to 50 ml in steps of 10 ml by filling an intraventricular latex balloon, and the related changes in left ventricular pressure and coronary flow were measured. The volume was then extended to 70 ml to obtain an overstretch which induced a transient decrease in cardiac contractility. During the period of low cardiac contractility the volume was brought back to 20 ml in steps of 10 ml. Systolic ventricular pressure changed with volume but was lower during the period of low contractility. For systolic pressures below 100 mmHg there was no significant relationship between pressure and coronary systolic flow, but the relationship shifted to higher flows during low contractility. For systolic pressures above 100 mmHg systolic coronary flow decreased significantly when systolic pressure increased. In this case the slopes of the relationships were not significantly different before and after the reduction in contractility. These findings suggest that for systolic pressures less than 100 mmHg (i.e. below the physiological range) the shielding effect of the contracting ventricle prevents the ventricular pressure from being transmitted in the myocardial wall. When systolic pressure exceeds 100 mmHg the shielding effect is overcome and the amplitude of the systolic flow reduction varies with ventricular pressure.

[Mechanisms of ischemic preconditioning: relation with ischemia-reperfusion injury]. / Meccanismi del precondizionamento ischemico: relazione con i danni da ischemia-riperfusione.

If a coronary occlusion long enough to produce a myocardial infarction is preceded by one or more brief periods of occlusion, the infarct size is reduced with respect to the area at risk. Also the ischaemia reperfusion injury is remarkably reduced. Such effects form the ischaemic preconditioning. Ischaemia-reperfusion injury is attributed to a Ca2+ overload of the myocardial fibres together with an inadequate resynthesis of ATP, a loss of membrane phospholipids and a release of free oxygen radicals. The inadequate resynthesis of ATP is responsible for an increased concentration of nucleosides and purinic bases with swelling of the myocardial fibres. The cell Ca2+ overload depends on a reduced activity of the ionic pumps caused by the oxygen lack during ischaemia. During reperfusion the vascular endothelial cells of the previously ischaemic area release free oxygen radicals in response to the activity of the xanthine-oxidase on hypoxanthine produced by the ischaemic myocardium. This initial release of oxygen radicals is responsible for the adhesion of neutrophils to the endothelium. After adhesion also the neutrophils release free radicals due to the activity of NADPH-oxidase on molecular oxygen. Myocardial, neural and endothelial mechanisms account for the protective effect of preconditioning. Myocardial mechanisms include the release of adenosine as well as of antioxidant enzymes. Adenosine, which activates protein-kinase C, favours the phosphorylation of a protective protein, whereas the antioxidant enzymes impair the activity of the free oxygen radicals. Preconditioning may also involve the synthesis of a heat shock protein. Neural mechanisms are represented by a reduced release of noradrenaline from the sympathetic nerve endings and a reduced sensitivity of myocardium to noradrenaline. Finally, vascular endothelial cells take part in preconditioning by means of an increased production of nitric oxide which seems to exert a protection against arrhythmias.

The Gaboon viper, Bitis gabonica: hemorrhagic, metabolic, cardiovascular and clinical effects of the venom.

The effects of Bitis gabonica venom have been studied in several animal species, including the monkey, dog, rabbit, rat and guinea pig. Further information has been provided by observations on the effects of snake bite in man. Bitis gabonica venom exerts a number of cytotoxic and cardiovascular effects: cytotoxic effects include widespread hemorrhage, caused by the presence of two hemorrhagic proteins. These hemorrhagins bring about separation of vascular endothelial cells and extravasation of blood into the tissue spaces. Metabolic alterations include decreased oxygen utilization by tissues and increased plasma glucose and lactate concentrations. Metabolic non-compensated acidosis has also been seen in the rat as a consequence of the cytotoxicity of the venom. Cardiovascular effects include disturbances in atrio-ventricular conduction and reduction in amplitude and duration of the action potential brought about by a decreased calcium membrane conductance. A progressive decrease in myocardial contractility can also be attributed to the decreased calcium conductance, which together with the severe acidosis may cause death in experimental animals. A severe, though reversible, vasodilatation was observed after envenomation due to unidentified compounds in the venom. In man, envenomation causes a variable clinical picture depending on the time course and severity of envenomation. Frequently seen effects include hypotension, hemorrhage at the site of the bite and elsewhere and disseminated intravascular coagulation. Envenomation can be satisfactorily treated with antivenom.

The heart rate after inhibition of nitric oxide release in the anaesthetized dog.

1. The effect of nitric oxide (NO) inhibition on heart rate was studied in anaesthetized vagotomized dogs. 2. The effect of changes of baroreceptor stimulation was prevented using an arterial pressure reservoir. 3. After NO-inhibitor (Nitro-L-arginine), heart rate decreased by 8% in spite of an unchanged pressure. 4. When upstream pressure was increased by constriction of the descending aorta, heart rate decreased by 4% before and after inhibition. Owing to the vagotomy this decrease was attributed to a sympathetic tone reduction following baroreceptor stimulation. 5. The results show that NO-inhibition reduces heart rate independently of an increased baroreceptor stimulation and does not reduce the basal sympathetic control on the sinus-atrial node.

Myocardial, neural and vascular aspects of ischemic preconditioning.

Ischemic preconditioning can be obtained with brief coronary occlusions. It has been studied in different animal species including dogs, pigs, rabbits and rats. The suggested duration of the occlusions ranges from four periods of 5 min, separated from each other by 5 min of reperfusion, to one period of 2.5 min. In addition to the reduction of the size of a subsequent infarction, preconditioning is responsible for the attenuation of the ischemia-reperfusion injury. The protection has a short duration and does not exceed two hours. Myocardial, neural and endothelial factors are involved in preconditioning. The myocardial component includes an increased release of adenosine with activation of A1 adenosine receptors, the activation of a protein-kinase C and possibly of antioxidant enzymes. The neural component includes a reduction in the release of noradrenaline from the postganglionic sympathetic fibers and a reduced myocardial sensitivity to noradrenaline. The increased myocardial release of adenosine, together with the reduced adrenergic activity, is consistent with the reduction in myocardial metabolism which has been observed after preconditioning. The coronary vascular endothelium is concerned in an increased release of nitric oxide which seems to be responsible for a prevention of reperfusion arrhythmias. In addition to the protective effect exerted on the myocardium, ischemic preconditioning seems to be responsible for a change in the coronary responsiveness to short periods of occlusion followed by release. This change in responsiveness is mainly represented by a greater velocity of the increase in flow occurring in the coronary reactive hyperemia.