Influence of heart rate on the effects of prenalterol on regional myocardial blood flow and function during coronary stenosis in dogs

Alleviation of myocardial dysfunction and abnormal lactate metabolism during coronary stenosis in dogs by ICI 118,551

Hemodynamic parameters, segment shortening in the ischemic myocardium and cardiac lactate extraction were estimated in the presence of a critical coronary stenosis, before and after administration of the selective beta 2-adrenoceptor antagonist, ICI 118,551, or the beta 1-adrenoceptor antagonist, atenolol, to anesthetized dogs. ICI 118,551 (0.2 and 0.5 mg/kg i.v.) and atenolol (0.2 mg/kg i.v.) produced significant decreases in both heart rate (by 6, 14 and 20% of the predrug value, respectively) and maxLVdP/dt (by 15, 26 and 24% of the predrug value, respectively). ICI 118,551 (0.5 mg/kg) and atenolol significantly improved the impaired shortening of the myocardial segment when compared with the change seen after saline administration. ICI 118,551 at both doses and atenolol significantly increased depressed cardiac lactate extraction while saline did not. Increasing heart rate by pacing abolished the beneficial effects of ICI 118,551 and atenolol on ischemic myocardial segment shortening and lactate metabolism. The data suggest that not only beta 1- but also beta 2-adrenoceptor blockade may contribute to the amelioration of myocardial ischemia in a model of coronary stenosis.

Nisoldipine and perfusion of post-stenotic myocardium in conscious pigs with different degrees of concentric stenosis

1. The effects of oral nisoldipine on the perfusion and wall function of a myocardial segment distal to a fixed coronary artery stenosis were studied in 2 groups of conscious pigs with different degrees of stenosis. In group 1 (n = 8) systolic wall thickening (SWT) of the post-stenotic segment was more than 15% (27 +/- 4%); in group 2 (n = 7) SWT was less than 10% (7 +/- 1%). 2. The systemic haemodynamic profiles at baseline and during nisoldipine were similar in both groups. Dose-titrations of nisoldipine (0.24 +/- 0.02 mg kg-1 and 0.47 +/- 0.04 mg kg-1) were performed to obtain increases in heart rate of 25% and 50%, respectively. These increases were accompanied by increases in cardiac output (up to 50%) and left ventricular (LV)dP/dt max (60%), while systemic vascular resistance (35%) and mean arterial blood pressure (10%) were reduced. Left ventricular systolic and end-diastolic blood pressure and stroke volume were not affected. 3. In both groups, nisoldipine caused increases in blood flow to the non-stenotic area which favoured the subepicardium more than the subendocardium. Blood flow to the post-stenotic area of group 1 was normal at baseline and was only slightly enhanced (preferentially to the subepicardium) by nisoldipine. In the post-stenotic area of group 2 transmural and subendocardial blood flow were lower at baseline compared to the control area. Nisoldipine did not affect subepicardial blood flow but reduced subendocardial blood flow. 4. In spite of the reflex-mediated positive chronotropic actions of nisoldipine, the acute poststenotic systolic wall thickening was not affected by nisoldipine in either group. 5. We conclude that, under the experimental conditions employed (concentric stenosis, no coronary collaterals and acute drug administration), nisoldipine does not have a useful effect on post-stenotic myocardial blood flow, particularly in animals with severe stenosis. In view of a possible resetting of the baroreceptors (subsiding of the tachycardia) with chronic treatment and the presence of eccentric stenosis in many patients, additional studies are warranted.

Effects of nipradilol on myocardial ischaemia produced by coronary stenosis in dogs

Effects of nipradilol which is a new beta-adrenoceptor blocking agent endowed with nitroglycerin-like vasodilator actions, its denitrated derivative (denitro nipradilol) and propranolol on abnormalities of regional myocardial shortening produced by partial occlusion of the left circumflex coronary artery (LCX) were studied in anaesthetized open-chest dogs. In the presence of LCX stenosis, nipradilol (0.1 mg kg-1, i.v.) produced marked decreases in heart rate and LVdP/dt without a significant increase in left ventricular end-diastolic pressure (LVEDP). It improved impaired myocardial segment shortening and restored normal cardiac lactate metabolism. Denitro nipradilol (0.2 mg kg-1 i.v.) and propranolol (0.2 mg kg-1 i.v.) both caused similar haemodynamic changes to nipradilol but also produced a significant increase in LVEDP. However, improvement by these two agents of regional dysfunction in the ischaemic myocardium was comparable to those seen with nipradilol. All three agents markedly inhibited isoprenaline-induced tachycardia, but vehicle did not. Atrial pacing abolished the beneficial effect of nipradilol on myocardial shortening in the ischaemic region without affecting other haemodynamic parameters. These results indicate that nipradilol alleviates acute myocardial ischaemia produced by coronary stenosis with similar efficacy to denitro nipradilol and propranolol suggesting, that a major part of the beneficial effect of nipradilol may be attributable to its beta-adrenoceptor blocking action.