Effects of autonomic nerves and their mediators on the coronary circulation and myocardial contraction.

Mean and phasic coronary flows were recorded simultaneously by an electromagnetic meter, and peripheral coronary pressure was recorded distally from the temporarily occluded descending branch of the left coronary artery. From the results of these studies we conclude that: 1. The sympathetic cardiac nerves, as does levarterenol, increase myocardial contraction, (indicated by a shorter, more abrupt systole and more rapid isometric relaxation), and cause a slight degree of coronary arteriolar dilation. 2. Para-sympathetic fibers exert no significant effect, although acetylcholine diminishes coronary arteriolar tone and may slightly diminish ventricular contraction. 3. That measurements of mean coronary flow and resistance are relatively satisfactory for indicating changes in coronary arteriolar tone, but are of relatively little value in evaluating myocardial contraction. C ONFLICTING reports have appeared in the literature regarding the sympathetic control of the coronary circulation; direct constrictor and dilator effects have been described as well as indirect dilator effects resulting from altered contraction. Very little information is available as to whether or not a true parasyrnpathetic control exists. The available reports are conflicting but the general consensus in the published data is that vagal parasympathetic stimulation would, if it had any effect, cause constriction of the coronary arterioles. Most of the early investigators measured only mean flow. Heart rate, aortic pressure, and strength of contraction were usually not evaluated, nor was arteriolar resistance calculated. These various variables may have led to contradictory interpretations. It appeared that a clearer interpretation of these effects could be made if the various flow determining factors could be evaluated separately. This paper is a report of investiga-tioiis designed to demonstrate whether or not either the vagal or the adrenergic nerve fibers have any direct effect on the coronary arteri-oles or on ventricular contraction. METHODS General. The technique used in this study is similar to that described by Denison, Bardhanaba-edya and Green. 3 The studies were done on open-chested mongrel dogs given morphine sulfate, 2 mg./Kg. subcutaneously, followed in one half hour by 0.25 ml./Kg. of a solution containing sodium pentobarbital, diallylbarbiturie acid and urethane. This anesthetic kept the heart rate from increasing excessively as it often will with pentobarbital alone. Intermittent positive pressure artificial respiration was administered by means of a motor driven valve which alternately inflated the lungs wnth compressed air and allowed them to deflate to the atmosphere 30 to 60 times/min. This was supplemented by a continuous flow of oxygen into the …