Pharmacological aspects of the coronary circulation

The beta-adrenoceptor of pig coronary arteries: determination of beta 1 and beta 2 subtypes by radioligand binding

beta-Adrenoceptors of pig coronary arteries were investigated by the use of a new ligand, 125iodocyanopindolol (ICYP) in binding studies. Inhibition of ICYP binding by betaxolol (a selective beta 1-antagonist), zinterol (a selective beta 2-agonist) and ICI 118551 (a selective beta 2-blocking drug) resulted in non-linear Scatchard plots, suggesting that both beta-adrenoceptor subtypes are present in pig coronary arteries. Computer analysis of the data gave a beta 1:beta 2-adrenoceptor ratio of approximately 65:35.

Lidoflazine in the early stages of acute myocardial ischaemia

1 Pretreatment of anaesthetized rats with intravenously administered lidoflazine (an antianginal agent) reduced the incidence and severity of ventricular arrhythmias which resulted from acute coronary artery ligation. Ventricular fibrillation was completely prevented by doses of 50 micrograms/kg and 2 mg/kg and no animal so treated died ( contrast 50% incidence of fibrillation in the controls and 30% mortality). 2 In anaesthetized greyhound dogs, lidoflazine (2 mg/kg) administration resulted in transient reductions in systemic arterial pressure, LV dP/dt max and cardiac output. Coronary sinus Po2 was markedly increased, indicating pronounced coronary vasodilatation. 3 Lidoflazine pretreatment inhibited the increase in epicardial ST-segment elevation which resulted, in dogs, from short (3 min) occlusions of the left anterior descending coronary artery. This effect was especially marked at sites where, in control occlusions, ST-segment elevation was most pronounced. 4 Lidoflazine greatly reduced the number of ventricular ectopic beats which usually resulted from more prolonged (30 min) periods of acute coronary artery occlusion. There was no ventricular fibrillation in these dogs (contrast 25% incidence in the controls). 5 Lidoflazine did not modify the ventricular fibrillation which results from reperfusion of a previously ischaemic area of the left ventricular wall.

Catecholamine-induced vasodilator metabolite release from guinea-pig hearts is not due to increased myocardial activity

Guinea-pig isolated hearts were perfused in series, the donor heart perfusate supplying the recipient heart. Isoprenaline increased the rate and force of contraction and oxygen consumption of donor hearts and produced a coronary vasodilatation. This was accompanied by the release of vasodilator metabolite as demonstrated by vasodilatation of the recipient heart, the beta-adrenoceptors of which were antagonized by propranolol. During arrest of donor hearts by either carbachol or application of a fibrillating current, isoprenaline still released vasodilator metabolite and increased oxygen consumption but without changes in rate or tension. This release was prevented by beta-adrenoceptor blockade. It is concluded that the sympathomimetic-induced coronary vasodilatation is mediated via the release of a vasoactive metabolite, the trigger for which is not the concomitant mechanical hyperactivity.

Myocardial pharmacokinetics and pharmacodynamics of dipyridamole in the isolated rabbit heart

The myocardial accumulation and disposition pharmacokinetics of the antithrombotic drug dipyridamole were investigated in isolated perfused and spontaneously beating rabbit hearts. The rabbit myocardium behaved pharmacokinetically as a two-compartment system with regard to the drug. The half-lives of the alpha-phase of distribution and of the beta-phase of disposition were about 1.4 and 6.3 min., respectively. Perfusion with a modified Krebs-Henseleit solution containing 5.1 microgram ml-1 of dipyridamole caused an accumulation of about 140 micrograms g-1 of myocardial tissue at steady state. The initial measured pharmacokinetic parameters were unchanged after perfusion with the drug for 60 min. The accumulating dipyridamole in the rabbit heart caused a progressive decrease in myocardial contractility to about 65%. This was accompanied by a decrease in the ratio of contraction rate to oxygen consumption to about 0.7 as an expression of reduced myocardial efficiency. The coronary flow rate was not significantly increased. The heart beating frequency decreased only slightly and no dromotropic effects were observed.

Pharmacokinetics of dipyridamole

Dipyridamole, though originally introduced as a coronary vasodilator, has lately been increasingly investigated in the treatment of thromboembolic diseases because of its inhibitory influence upon blood platelet function. The compound is eliminated from the organism by hepatic biotransformation to the monoglucuronide, which almost exclusively is subjected to biliary and faecal excretion with simultaneous partial enterohepatic circulation taking place. Only minute amounts are excreted through the kidneys. In experiments on four normal human volunteers it was found that the serum concentration curve after intravenous administration of dipyridamole rather closely fits the pharmacokinetics of an open two-compartment model with first order, linear disposition kinetics and elimination taking place from the central compartment. Experiments with oral ingestion of the compound could be described by the use of a corresponding pharmacokinetic model with two consecutive first order input steps, representing the dissolution and absorption processes. The disposition rate constants (beta) were within the range of 0.0051--0.0083 min.-1 corresponding to biological half-lives of 84--145 min. The absorption rate constant was about 0.07 min.-1, and the systemic availability of an oral dose of 100 mg dipyridamole in tablets varied from 37 to 66%.

The release of a coronary vasodilator metabolite from the guinea-pig isolated perfused heart stimulated by catecholamines, histamine and electrical pacing and by exposure to anoxia

1 A procedure involving two guinea-pig isolated hearts perfused in series is described for detecting in the recipient heart the release of a possible coronary vasodilator metabolite from the donor heart. 2 Adrenaline and isoprenaline stimulated the rate and force of contraction and produced a multiphasic coronary vascular response, the predominant phase of which was vasodilatation. When the beta-adrenoceptors of the recipient heart were blocked, stimulation of the donor heart by the catecholamines was associated with a coronary vasodilatation of the recipient heart. 3 Histamine stimulated rate and force of contraction and was predominantly coronary vasodilator. After blockade of histamine H1- and H2-receptors in the recipient heart, coronary vasodilatation followed increases in activity of the donor heart in response to histamine. 4 These vasodilator responses of the recipient heart were attributed to the release from the donor heart of a vasodilator metabolite by the increased activity. This is the proposed mechanism for the predominant coronary vasodilator response to catecholamines and histamine. 5 Periods of electrically-paced tachycardia and anoxia of the donor heart also led to the release of vasodilator activity. 6 The possible identity of the metabolite is discussed.

The role of H1 and H2-receptors in the coronary vascular response to histamine of isolated perfused hearts of guinea-pigs and rabbits

1. The effects of histamine on the isolated perfused hearts of guinea-pigs and rabbits were examined. Records of contractile force, heart rate and coronary perfusion pressure were obtained. 2. Histamine exerted positive inotropic and chronotropic effects which were antagonized by burimamide and attributed to stimulation of H2-receptors. 3. The coronary vascular response to histamine differed between guinea-pigs and rabbits. In guinea-pig hearts, three phases were apparent: (a) An initial vasodilatation preceding any effects on heart force and rate was antagonized by mepyramine and therefore mediated by histamine H1-receptors in the coronary circulation. (b) A secondary vasoconstriction was attributed to the increased myocardial compression during the positive inotropic and chronotropic responses. (c) The final, more predominant, component was a prolonged vasodilatation probably associated with the increased metabolic activity of the heart. 4. The latter two components were abolished together with the myocardial responses by burimamide. The remaining coronary vascular response was biphasic, consisting of a vasodilatation immediately followed by vasoconstriction. Both were antagonized by mepyramine and therefore mediated by H1-receptors. 5. The coronary vascular response of rabbit hearts was similar but no direct vasodilatation was observed and it was concluded that histamine receptors in the coronary vasculature involve only vasoconstriction.

The effects of dipyridamole on blood flow and oxygen handling in the acutely ischaemic and normal canine myocardium

1. The effects of the intravenous administration of dipyridamole (0.25 mg/kg) were examined in a canine preparation that enabled simultaneous measurements to be made of blood flow in ischaemic and in essentially normal areas of the myocardium and also of oxygen handling (availability, consumption and extraction) in both these regions.2. When administered to dogs anaesthetized with trichlorethylene 2-3 h after acute ligation of the descending branch of the left coronary artery, dipyridamole markedly increased blood flow in essentially normal regions (left circumflex flow) but failed to increase flow in the area supplied by the ligated vessel (measured by (133)xenon clearance and by retrograde flow). In five of the six animals definite decreases in flow (; stealing ') were observed in the ischaemic region. These flow changes were related to the decreased trans-ventricular perfusion pressure (diastolic peripheral coronary pressure minus left ventricular end-diastolic pressure) and were accompanied by electrocardiographic evidence of increasingly severe myocardial ischaemia. The results support the suggestion that only increasing the perfusion gradient will usefully improve blood flow (and hence oxygen availability) to the acutely ischaemic myocardium.3. Despite these effects on ischaemic muscle blood flow, the oxygen tension of the blood draining the infarcting muscle was markedly elevated. The conclusion is drawn that dipyridamole decreases the efficiency of the myocardial circulation by opening up vessels that do not take part in tissue exchange.

Characterization of the coronary vascular -adrenoceptor in the pig

The beta-adrenoceptor in pig coronary vascular smooth muscle has been characterized by the use of a selective agonist, salbutamol and a selective antagonist, practolol. In coronary artery strips contracted with KCI, salbutamol had only marginal relaxant activity and was a partial agonist compared with isoprenaline. Practolol had relatively high antagonist potency against isoprenaline (pA(2) 6.59). It is concluded that the pig coronary vascular,beta-adrenoceptor is of the beta(1)-type.

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

1. In cats anaesthetized with pentobarbitone sodium, hexobendine (0.25 mg/kg) markedly increased myocardial blood flow (measured using a heat clearance technique) and usually decreased myocardial metabolic heat production, without influencing cardiac contractility or systemic arterial blood pressure. These effects lasted for about 45 min.2. Larger doses (0.5 and 1.0 mg/kg) decreased systemic arterial blood pressure and the rate of rise of the left ventricular pressure pulse (dp/dt), although left ventricular end-diastolic pressure was usually increased. This is indicative of a decrease in myocardial contractility.3. In a concentration of 5 x 10(-6) g/ml., hexobendine protected isolated rabbit atria against the decrease in contractility that follows the removal of oxygen.4. Hexobendine did not antagonize the systemic and myocardial effects of infusions of adrenaline and noradrenaline, nor (except in concentrations of 10(-5) g/ml.) the increase in contractility induced by these catecholamines on isolated rabbit atria.