A NOTE ON THE ELECTRICALLY TRANSMURALLY STIMULATED ISOLATED TRACHEA OF THE GUINEA-PIG

The isolated trachea of the guinea-pig was stimulated electrically transmurally to give a biphasic response. The initial contraction was abolished by atropine and was rapidly succeeded by relaxation which was blocked by low concentrations of dichloro-isoprenaline. The relaxation was also blocked by procaine, guanethidine, or by previous treatment of the animal with reserpine, but not with hexamethonium. It was concluded that the effects of transmural stimulation were mediated by post-ganglionic adrenergic nerves innervating beta receptors.

HEART RATE OF THE PREWEANLING RAT AND ITS AUTONOMIC CONTROL

The heart rate of the rat at birth is about 300 beats/min, increases to 360 beats/min within 3 days, and remains there until the animal is about 10 days old. From 10 to 21 days the heart rate again increases to a maximum of about 500 beats/min. When reserpine is used to selectively block adrenergic sympathetic function the rise in heart rate that occurs during the first 3 days is only slightly modified, but the rise in heart rate from 10 days onward is completely prevented. Atropine, however, has no effect on heart rate at all ages tested. When newborn rats are kept at a body temperature of 37 C and then at 40 C, the average heart rate increases 7%. Three-day-old animals have a 19% increase, whereas 6-day-old animals show a 46% increase. When nerve-growth-factor antiserum is used to destroy sympathetic function, the expected hyperthermic tachycardia does not appear. It is concluded that in the first 6 days of the rat's life sympathetic control over the heart rate is minimal, but from then on the sympathetic nervous system continuously exerts a positive chronotropic effect. The parasympathetic nervous system does not seem to be involved.

CORONARY AND MYOCARDIAL ACTIONS OF ANGIOTENSIN

In the dog heart-lung preparation, single injections of 40 µg angiotensin II produced consistently a biphasic response. Initial impairment of cardiac function was observed with decrease of cardiac output and of ventricular force, and with increase of atrial pressures. After 10 to 20 seconds, there was a positive inotropic effect, with an increase of cardiac output and of ventricular force, and a decrease in atrial pressures. Responses were comparable in reserpinized and nonreserpinized preparations. Tachyphylaxis to angiotensin was found consistently in the heart-lung preparation, with a regular decrease in response following second and third injections. A positive inotropic action of angiotensin II was demonstrated in the cat papillary muscle preparation. These studies demonstrated that angiotensin has a direct cardiac action, namely a positive inotropic effect. This action probably does not depend upon myocardial stores of catecholamines.

When the left anterior descending coronary artery of the dog was perfused with blood at a constant flow, single intracoronary injections of angiotensin (1.0 µg) consistently increased coronary resistance (6% to 92%). Continuous intracoronary infusion of angiotensin increased coronary vascular resistance when blood concentrations were as low as 0.42 µg/ liter. No evidence of initial cardiac depression or of increased coronary vascular resistance was found after norepinephrine.

It is concluded that the initial impairment of cardiac performance following angiotensin is caused at least in part by decreased coronary blood flow. Caution is suggested in the use of angiotensin in the treatment of human shock because of its action upon the coronary circulation.