Studies on the mode of action of hexobendine, a prospective anti-anginal drug

How does dipyridamole elevate extracellular adenosine concentration? Predictions from a three-compartment model of adenosine formation and inactivation

Steady-state mathematical models are developed according to which adenosine is formed in the cytoplasm of a group of cells, arises in the extracellular space via the symmetric nucleoside transporter and is inactivated in the adenosine forming cells and after rate-limiting transport into other cell-types. Dipyridamole increases the Km and Vmax. of the transporter symmetrically with respect to influx and efflux. Models incorporating differing degrees of compartmentation are used to predict intracellular and extracellular adenosine concentration as a function of dipyridamole concentration and adenosine formation rate. The vasodilator action of dipyridamole is explained since it is predicted to elevate interstitial fluid adenosine concentrations at all rates of adenosine formation provided that washout of the interstitial compartment is restricted.

Adenosine and dipyridamole: actions and interactions on the contractile response of guinea-pig ileum to high frequency electrical field stimulation

The action of adenosine on the myenteric plexus-longitudinal muscle strips from guinea-pig ileum to high frequency electrical field stimulation (10 Hz) was investigated. Electrically induced contractions were reduced markedly by tetrodotoxin (0.2 microM) and atropine (1 microM), and partially by noradrenaline (3 microM) and morphine (3 microM). Adenosine, adenosine 5'-monophosphate (AMP) and adenosine triphosphate (ATP) produced a concentration-dependent inhibition of the high frequency contractions over the range of 0.1-100 microM, the most potent being adenosine. The concentration-response curve for adenosine was significantly shifted to the left by dipyridamole (10 nM), while dipyridamole at higher concentrations (30 nM-10 microM), depressed the contraction markedly by itself. Dipyridamole decreased [3H]-adenosine uptake into strips of ileum in a concentration-dependent manner. There was a significant correlationship between the reduction of adenosine uptake and the inhibition of the contraction induced by dipyridamole (r = 0.970). In strips desensitized to adenosine or treated with adenosine deaminase, the inhibitory effect of dipyridamole was significantly reduced. The present investigation revealed that adenosine depressed responses of guinea-pig ileum to high frequency electrical stimulation and suggested that the inhibitory effect of dipyridamole may be closely associated with the behaviour of endogenous adenosine or related compounds.

Effects of purine compounds on cholinergic nerves. Specificity of adenosine and related compounds on acetylcholine release in electircally stimulated guinea pig ileum

The action of 21 purine compounds on the twitch response of the electrically stimulated guinea pig isolated ileum has been investigated. Adenosine and related compounds produced a dose-dependent depression of the response. Adenosine was the most potent and 2'-deoxyadenosine had one hundredth the potency of adenosine. Adenine, hypoxanthine, inosine, IMP, ITP, xanthine, xanthosine, XMP, XTP, guanine, GMP and GTP were ineffective at concentrations less than 1 mM. Adenosine (30 microgram) reduced the electrically induced ACh output from the ileal strips. The dose--depression curve for adenosine (0.1--30 microgram) was shifted to the right in the presence of xanthine derivatives and of these, theophylline was the most potent inhibitor of adenosine. On the other hand, dipyridamole (0.1--1 microgram) and hexobendine (0.1--1 microgram) shifted the curve to the left. They markedly inhibited 3H-adenosine uptake into the ileum. Theophylline (0.1 mM), dipyridamole (0.3 microgram) and hexobendine (0.3 microgram) did not affect tetrodotoxin-, adrenaline-, strychnine- and morphine-induced inhibition of the twitch response. The present investigations have revealed that adenosine and related compounds reduce ACh release from the intramural cholinergic nerves in the guinea pig ileum possibly in a specific manner (or through a specific receptor site) different from that of other inhibitors such as morphine.

The response of isolated cardiac muscle to acute anoxia: protective effect of adenosine triphosphate and creatine phosphate

A standardized and reproduceable preparation is described that enables the effect of drugs to be examined on the response of isolated cardiac muscle to acute anoxia. There was a linear relation (r = 0·963) between tension developed in isolated, electrically-driven guinea-pig atria and the oxygen tension of the fluid surrounding the muscle. For any one atrial preparation the time taken, from the beginning of the anoxic period, for tension to be reduced by 50 % was constant for consecutive anoxic periods. ATP and creatine phosphate significantly increased this time and protected cardiac muscle against the consequences of anoxia; in concentrations that were without direct cardiac effects, a greater degree of protection was possible with creatine phosphate.

Myocardial and circulatory effects of E. coli endotoxin; modification of responses to catecholamines

1. The predominant acute effect of E. coli endotoxin in anaesthetized, ventilated cats was pulmonary hypertension resulting from a 8-12 fold increase in pulmonary vascular resistance. This was followed by decreases in left ventricular (LV) and systemic arterial pressures and in LV dP/dt max. Recovery occurred within 2-4 min and was dependent upon increased sympathetic drive; recovery did not occur in cats treated with the beta-adrenoceptor blocking drug alprenolol.2. The pulmonary vasoconstriction was reduced in cats given compound 48/80 and evidence is presented that it results primarily from histamine release.3. Over the 2-3 h period following endotoxin injection, systemic arterial pressure tended to decrease and heart rate and myocardial metabolic heat production to increase. Myocardial blood flow and LV dP/dt remained fairly stable until the terminal stages of shock.4. The predominant delayed effect of E. coli endotoxin in cats were a markedly reduced stroke volume, an increase in peripheral vascular resistance and a severe metabolic acidosis (arterial base excess-20 mEq/litre). Arterial pO(2) and pCO(2) were not significantly affected. It is concluded that myocardial contractility is maintained at this time through the release of catecholamines and that endotoxin itself depresses contractility.5. The effects of adrenaline and noradrenaline infusions on systolic and diastolic blood pressures, heart rate, cardiac output, myocardial blood flow and LV dP/dt max were markedly reduced in the period 2-3 h after endotoxin. In a few animals some recovery of the response to noradrenaline occurred and was associated with a general circulatory improvement and a reduced metabolic acidosis.

The effects of dipyridamole on the myocardial vasodilator actions of noradrenaline, isoprenaline and adenosine

1. In anaesthetized cats, intravenous adenosine infusions decreased resistance to blood flow in the myocardial vascular bed. This effect of adenosine was augmented during the 60 min following the intravenous injection of dipyridamole (1 mg/kg).2. In different experiments, intravenous infusions of noradrenaline caused either a slight increase or a slight decrease in myocardial vascular resistance. The dilator component of the action of noradrenaline was not augmented by dipyridamole.3. Isoprenaline infusions decreased the resistance of the myocardial bed. This effect was unaltered after dipyridamole.4. These results do not support the hypothesis that part of the effect of catecholamines on myocardial vascular resistance involves the release of adenosine from hypoxic myocardial cells.

Haemodynamic and coronary effects of quazodine in cats with developing myocardial infarcts

Acute ligation of the descending branch of the left coronary artery in anaesthetized cats resulted, within 1–2 h, in a 30% decrease in local blood flow in the region mainly supplied by the ligated vessel, a fall in systemic blood pressure, in cardiac output, and in left ventricular dP/dt max (LVdP/dt). There was electrocardiographic evidence of myocardial ischaemia (pronounced ST elevation). In these animals with developing myocardial infarcts, intravenous infusions of quazodine (MJ1988; 6,7-dimethoxy-4-ethyl-quinazoline) markedly increased myocardial contractility and local myocardial blood flow in the developing infarct, and decreased systemic arterial pressure, peripheral vascular resistance and left ventricular end-diastolic pressure, effects similar to those observed in normal cats. The increase in cardiac contractility (cardiac output and LVdP/dt) occurred without a concomitant increase in myocardial metabolic heat production. This ‘oxygen sparing effect’ probably results from a decrease in left ventricular wall tension. It is suggested that quazodine warrants further investigation as a cardiac stimulant in power failure following myocardial infarction in man.

Actions of the sympathomimetic bronchodilator, rimiterol (R798), on the cardiovascular, respiratory and skeletal muscle systems of the anaesthetized cat

1. The actions of rimiterol [erythro(3,4-dihydroxyphenyl, 2-piperidyl methanol hydrobromide)], a new sympathomimetic bronchodilator, have been compared with those of salbutamol and laevoisoprenaline on the heart and lungs, and on contractions of the soleus muscle of cats under chloralose anaesthesia.2. Rimiterol and salbutamol injected intravenously were about equipotent in all tests, and were about 8 times less potent than laevoisoprenaline both in opposing the bronchoconstrictor action of 5-hydroxytryptamine, and in decreasing the tension and degree of fusion of incomplete tetanic contractions of the cat soleus muscle. They were about 19 times less potent than laevoisoprenaline in increasing heart rate.3. The effect on the soleus muscle is considered to be analogous to the muscle tremor that often occurs in man, and the results therefore suggest that systemic administration of bronchodilator doses of rimiterol, like salbutamol, may produce muscle tremor as an unwanted side-effect.4. When equipotent doses to oppose 5-hydroxytryptamine-induced bronchospasm were compared, rimiterol and salbutamol produced less tachycardia than did laevoisoprenaline. In order to match the tachycardia produced by laevoisoprenaline, the doses of rimiterol or salbutamol had to be increased about two and a half times. This safety margin for salbutamol in the cat is considerably less than that reported by others for different species, which suggests that beta(1)- and beta(2)-adrenoceptors may be less clearly differentiated in the cat than they are in other laboratory animals.

Cardiovascular pharmacology of quazodine (MJ-1988), with particular reference to effects of myocardial blood flow and metabolic heat production

1. The effects of intravenous infusions of quazodine (6,7-dimethoxy-4-ethylquinazoline; MJ-1988) on myocardial blood flow, myocardial metabolic heat production and on general haemodynamics have been studied in cats anaesthetized with sodium pentobarbitone.2. Quazodine (0.25 and 0.5 (mg/kg)/min for 10 min) decreased diastolic blood pressure, peripheral vascular resistance, systolic ejection time and left ventricular end-diastolic pressure. Heart rate, cardiac effort, output and external work and left ventricular dP/dt were markedly increased. These changes are indicative of increased myocardial contractility and peripheral vasodilatation.3. In a dose of (1.0 mg/kg)/min, quazodine had a more marked hypotensive effect, systolic pressure being significantly reduced, and had less effect on left ventricular dP/dt and cardiac effort. Calculated external cardiac work was slightly reduced and there were very occasional nodal arrhythmias.4. Changes in heart rate, aortic dP/dt and diastolic blood pressure induced by quazodine were unaffected by the previous administration of the beta-adrenoceptor blocking agent alprenolol in a dose (1.0 mg/kg) which abolished the effects of isoprenaline.5. In all doses, quazodine markedly increased local blood flow (by 70-540%) around an implanted myocardial heated thermocouple recorder. ;Corrected temperature', an index of local myocardial metabolic heat production, was almost unchanged and it is suggested that increased myocardial contractility, occurring with unchanged metabolic heat production and oxygen consumption, probably results from a concomitant decrease in intramural wall tension.

Myocardial and haemodynamic effects of phentolamine

1. In cats anaesthetized with pentobarbitone, intravenous infusions of phentolamine ((10-50 mug/kg)/min for 5 min) increased heart rate, left ventricular dp/dt max (without increasing end-diastolic pressure), aortic dp/dt, cardiac output, myocardial blood flow and metabolic heat production.2. Phentolamine-induced increases in myocardial contractility occurred irrespective of the direction or magnitude of the blood pressure change and were maintained well beyond the actual infusion period.3. In cats treated with alprenolol, bretylium or reserpine there was no evidence of increased cardiac contractility following phentolamine administration.4. It is concluded that phentolamine, in doses less than those required to produce significant alpha-adrenoceptor blockade, increased myocardial contractility through an effect on the sympathetic nervous system.

The effect of "selective" beta-adrenoceptor blocking drugs on the myocardial circulation

1. A comparison has been made of the effects of a relatively specific beta(1)-adrenoceptor blocking drug (practolol) and a relatively specific beta(2)-adrenoceptor blocking drug (butoxamine) on myocardial and general haemodynamics in anaesthetized cats.2. Practolol, in a dose (10 mg/kg, intravenously) which had little effect on arterial pressure, heart rate, myocardial blood flow or myocardial vascular resistance, markedly reduced the effects of isoprenaline infusions on heart rate, aortic dp/dt, myocardial blood flow, vascular resistance and metabolic heat production, and the cardiac effort index. Isoprenaline induced vasodepression was unaffected.3. Butoxamine (5 mg/kg, intravenously) decreased heart rate, aortic dp/dt, the cardiac effort index and myocardial blood flow and increased myocardial vascular resistance. This is taken as further evidence for the existence of beta(2)-adrenoceptors in the myocardial microcirculation.4. After butoxamine, the effects of isoprenaline on myocardial blood flow, myocardial vascular resistance and heart rate were unaffected but the peripheral vasodilator effect was abolished. The effects on aortic dp/dt and the cardiac effort index were potentiated.5. It is concluded that the effect of isoprenaline in increasing myocardial blood flow is due predominantly to increased cardiac work and oxygen consumption and that practolol, since it has little direct effect on myocardial blood flow yet abolishes the cardiac stimulant and oxygen wasting effects of released catecholamines, has properties which indicate that it should be an effective and safe anti-anginal drug.

The effect of catecholamine infusions on myocardial blood flow, metabolic heat production and on general haemodynamics, before and after alprenolol (H56-28), in anaesthetized cats

1. In cats anaesthetized with pentobarbitone sodium, infusions of adrenaline, noradrenaline (0.5 mug/kg per min) and isoprenaline (0.25 mug/kg per min) increased myocardial blood flow, myocardial heat production, left ventricular systolic and end-diastolic pressures, left ventricular +ve and -ve dp/dt max, and calculated cardiac output, effort and oxygen consumption. These effects (apart from the effect of noradrenaline on left ventricular systolic pressure) were markedly reduced by previous administration of alprenolol (0.5 or 1.0 mg/kg).2. Infusions of adrenaline and noradrenaline increased arterial diastolic blood pressure and isoprenaline reduced it. After alprenolol the effects of adrenaline and noradrenaline were potentiated and that of isoprenaline abolished; in some experiments isoprenaline increased arterial diastolic pressure after alprenolol. Alprenolol did not influence the increases in arterial systolic pressure which followed the administration of adrenaline and noradrenaline.3. Isoprenaline-induced tachycardia was markedly reduced and adrenaline tachycardia was converted to bradycardia after alprenolol. The bradycardia which occurred during noradrenaline infusions was unaffected.4. After blockade by alprenolol, recovery of the effects of isoprenaline on left ventricular dp/dt and on heart rate occurred more quickly than recovery of the effects on arterial diastolic pressure. This suggests that alprenolol has a greater affinity for beta(2)- than for beta(1)-adrenoceptors.5. Intravenous administration of acetylcholine decreased arterial blood pressure, left ventricular pressure and +ve and -ve dp/dt max. During recovery from these effects there was a marked increase in +ve dp/dt max. which was absent after the administration of alprenolol (0.5 mg/kg). Because this dose of alprenolol is thus able to block the effects of reflex sympathetic cardiac nerve stimulation but does not completely antagonize the effects of exogenous adrenaline on dp/dt, it is suggested that alprenolol may have some adrenergic neurone blocking activity.6. Increases in liver and myocardial blood flow and heat production produced by noradrenaline, adrenaline and isoprenaline were reduced after alprenolol.7. Isoprenaline reduced air-way resistance and this effect was abolished by alprenolol; increases in air-way resistance produced by adrenaline and nor-adrenaline were augmented. All three amines inhibited intestinal smooth muscle contractions in vivo. Only the effect of isoprenaline was reduced by alprenolol.

Myocardial and haemodynamic effects of the beta-adrenoceptor blocking drug alprenolol (H56/28) in anaesthetized cats

1. The effects of the beta-adrenoceptor blocking drug alprenolol (H56/28) on myocardial and general haemodynamics were studied in anaesthetized cats.2. Alprenolol (0.5 mg/kg and 1.0 mg/kg) reduced femoral systolic and diastolic pressures, heart rate and left ventricular systolic pressure. The rate of rise of the left ventricular pressure pulse (dp/dt) was reduced despite a significant elevation of left ventricular end-diastolic pressure. This is evidence for decreased myocardial contractility. On some occasions there was a transient initial increase in +ve dp/dt max. possibly indicative of moderate beta-adrenoceptor stimulant activity.3. Myocardial and liver blood flows were measured using a heated thermocouple technique. Alprenolol slightly decreased both myocardial and liver blood flows (mean of 17% and 15% respectively with a dose of 1.0 mg/kg). Myocardial and liver vascular resistances were only very slightly increased.4. Alprenolol had no direct effect on calculated myocardial and liver metabolic heat production.5. In doses up to 1.0 mg/kg alprenolol had no effect on airway resistance but occasionally decreased (in vivo) intestinal muscle movement.6. Since alprenolol (although reducing calculated myocardial oxygen consumption and the myocardial and metabolic heat stimulant effects of catecholamines) has no significant effect on myocardial vascular resistance, it is suggested that it would be a useful adjunct in the therapy of angina pectoris.