Michael J Curtis

An improved perfusion apparatus for small animal hearts.

A perfusion apparatus for mechanical and electrophysiological studies in small animal (e.g., rat, guinea pig) hearts is described. The apparatus consists of a number of individual perfusion chambers connected to the aortic perfusion cannula of a Langendorff perfused heart. The aortic root perfusion pressure is controlled by the oxygenating gas (usually 5% CO2 in O2) at a range of 0-200 mmHg. Different solutions can be placed in different individual chambers. Pressure in the left ventricle is monitored by means of a special compliant, but nonelastic, balloon while special atraumatic electrodes allow high-voltage, noise-free ECG recordings to be made from all parts of the epicardium. The apparatus keeps hearts beating for hours while the multichamber allows multiple drug applications to be made rapidly. Thus, the apparatus can be used for all types of pharmacodynamic analysis.

The mechanism of action of the optical enantiomers of verapamil against ischaemia-induced arrhythmias in the conscious rat

1 The actions of (‐)‐verapamil (0.2–6 mg kg−1) and (+)‐verapamil (0.4–12 mg kg−1) against arrhythmias induced by coronary artery occlusion were studied in conscious rats. 2 Intravenously administered (‐)‐ and (+)‐verapamil dose‐dependently reduced ventricular arrhythmias. (‐)‐Verapamil was consistently 4 times more potent than (+)‐verapamil. 3 In the same animals, (‐)‐verapamil was approximately 4 times more potent than (+)‐verapamil for effects on heart rate and blood pressure. Both enantiomers prolonged P‐R interval, but had no effect on QRS interval. 4 In separate groups of conscious rats, neither enantiomer influenced the threshold voltage and pulse width required to elicit fibrillo‐flutter, or altered the maximum following frequency, during electrical stimulation of the left ventricle. 5 In isolated, paced, Langendorff‐perfused ventricles of the rat, both enantiomers dose‐dependently reduced contractility, (‐)‐verapamil being 8–21 times more potent than (+)‐verapamil; both absolute and relative potencies were dependent on potassium concentration. 6 These results are compatible with the hypothesis that calcium antagonism in the ischaemic ventricular myocardium is antiarrhythmic during acute myocardial ischaemia.

Antiarrhythmic actions of verapamil against ischaemic arrhythmias in the rat

1 The actions of intravenous verapamil against arrhythmias induced by occlusion of a coronary artery were investigated in conscious rats. 2 Verapamil (2–20 mg kg−1, i.v. given pre‐occlusion) dose‐dependently reduced arrhythmias in rats with either large or small occluded zones at an ED50 of 6 mg kg−1. This dose was effective when given immediately post‐occlusion. 3 Severe arrhythmias, as opposed to PVC, were preferentially reduced. 4 In conscious, and pentobarbitone‐anaesthetized rats, verapamil (6 mg kg−1) had different effects on electrically‐induced arrhythmias, and the ECG, from an equi‐effective anti‐arrhythmic dose of quinidine (20 mg kg−1, i.v.). Quinidine decreased following frequency, but increased threshold current and pulse width, whereas verapamil did not. Both drugs increased P‐R interval, but only quinidine increased QRS and Q‐T intervals. 5 Thirty minutes post‐occlusion, the verapamil content of tissue and blood was determined after a 6 mg kg−1 dose given pre‐ or post‐occlusion. Measurable levels of verapamil were found in both normal and ischaemic myocardium. Plasma and plasma water concentrations were 3.6 ± 0.8 μmol l−1 and 0.6 ± 0.1 μmol l−1 (x̄ ± s.e.mean), respectively following post‐occlusion administration vs. 2.7 ± 1.2 and 0.24 ± 0.04 for pre‐occlusion administration. 6 Plasma water concentrations were close to IC50 values for inhibition of contractility in rat atria and ventricles. Similar concentrations depressed slow action potentials induced in rat ventricles by raised K+ 7 We suggest that the ability of verapamil to prevent severe ventricular arrhythmias following myocardial ischaemia in the conscious rat is largely due to the calcium antagonist effects of the drug.

The effects of ablations in the central nervous system on arrhythmias induced by coronary occlusion in the rat

1 The role of the central nervous system (CNS) in arrhythmogenesis in the 4 h period following occlusion of a coronary artery was investigated in rats by use of CNS ablations and other procedures. 2 Ablations in the CNS included pithing, spinalization and decerebration combined with acute and chronic surgical preparation and noradrenaline/adrenaline infusions. 3 All procedures involving acute surgery reduced the incidence and severity of the arrhythmias induced by occlusion. Such reductions were most marked in the second (0.5‐4 h post‐occlusion) arrhythmic period. 4 The observed reductions in arrhythmias could not be explained in terms of involvement of the CNS or adrenoceptor activation. 5 When circulating leucocytes, platelets and serum potassium were measured in a group of pithed rats before and after occlusion, reduced levels (20–50%) of both leucocytes and platelets occurred while serum potassium levels rose by 50–100%. 6 Arrhythmias following coronary occlusion may depend in part on factors in the blood such as leucocytes, platelets and serum potassium and these factors may be altered by acute surgery.

Actions of the verapamil analogues, anipamil and ronipamil, against ischaemia-induced arrhythmias in conscious rats

1 Two analogues of verapamil, ronipamil and anipamil, were tested for their ability to reduce arrhythmias induced by occlusion of the left anterior descending coronary artery in conscious rats. 2 Only anipamil (50 and 150 mg kg−1 orally) produced a statistically significant reduction in arrhythmias; it was most effective against ventricular fibrillation. Ronipamil at the same doses had limited antiarrhythmic actions. 3 Only anipamil delayed the development of ECG signs of ischaemia, while both drugs reduced the magnitude of such changes. 4 Anipamil has a more favourable ratio of antiarrhythmic to hypotensive effects when compared with verapamil.

The mechanism of action of calcium antagonists on arrhythmias in early myocardial ischaemia: studies with nifedipine and DHM9.

1 Nifedipine and DHM9 (carboxymethyl methyl 1,4‐dihydro‐2,6‐dimethyl‐4‐(3‐nitrophenyl)−3,5‐pyridinedicarboxylate) were studied for their effects on arrhythmias resulting from regional myocardial ischaemia in conscious rats, and for their effects on left ventricular developed pressure in vitro. 2 Nifedipine possessed antiarrhythmic activity at a high dose of 10 mg kg−1 i.v., but not at 0.5 or 2 mg kg−1. Ventricular fibrillation (VF), tachycardia (VT), and ventricular premature beats (VPB) were all attenuated to a similar degree; nifedipine did not have a selectivity of action for high frequency arrhythmias. 3 Before coronary occlusion, the three doses of nifedipine reduced arterial blood pressure by a similar magnitude, indicating a similar (maximal) degree of systemic vasodilatation. The reductions in blood pressure were accompanied by reflex tachycardia. Heart rate and blood pressure did not correlate with the incidence or severity of arrhythmias. 4 DHM9 had no influence on arrhythmias, haemodynamic variables or the ECG, even at 20 mg kg−1 i.v. 5 Nifedipine concentration‐dependently reduced contractility in perfused paced (5 Hz) rat ventricles in vitro. Raising the concentration of K+ in the perfusion fluid from 3 to 10 mequiv.1−1 increased the potency (‐log10 EC50) of nifedipine up to four fold, and caused a significant depression in excitability. 6 DHM9 at up to 3 × 10‐5 m had no significant influence on ventricular contractility in vitro. 7 The results provided indirect evidence in support of the hypothesis that calcium antagonists inhibit ischaemia‐induced arrhythmias by virtue of inhibition of the slow inward current (Isi) in the ischaemic ventricular myocardium.

The actions of felodipine on arrythmias and other responses to myocardial ischaemia in conscious rats

The antiarrhythmic activity of felodipine was investigated in conscious chronically prepared rats subjected to coronary artery occlusion. By deliberate high or low placement of an occluder, either large or small occluded zones were produced. In rats with small occluded zones (SOZ), felodipine i.v. (0.2 mumol/kg slow injection + 0.01 mumol/kg per min infusion, and 12.2 mumol/kg slow injection) possessed slight antiarrhythmic activity. In rats with large occluded zones (LOZ), similar doses of felodipine had little antiarrhythmic activity. Felodipine produced a dose-dependent reduction in blood-pressure and delayed the development of ECG signs of ischaemia (ST elevation). The occluded zone size was not reduced by felodipine, although some myocardial salvage (8-17%) may have occurred in SOZ rats. It is concluded that felodipine, at doses producing severe hypotension associated with vasodilatation, possesses little antiarrhythmic activity.