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Acetylcholine diminished the developed tension of contraction in rat atria; simultaneously there was a slight increase in resting potential, no appreciable change in action potential, a decrease in the rate of conduction, only a minor alteration of the form of contraction, and, most strikingly, a decrease in the duration of the action potential or the area enclosed by it. The rate of repolarization subsequent to the action potential was markedly accelerated. Epinephrine, in general, produced the opposite effects and slowed the repolarization rate as it augmented atrial contraction. Cholinesterase does not seem to play a direct role in cardiac membrane potentials or ion transfer since physostigmine had no effect of its own on the electric manifestations. Certain aspects of the mechanisms involved are discussed.

Whole chick embryo blastoderms at zero- to 7-somites have been explanted onto a nutrient medium and incubated at 37°C for 20 hours, in order to develop to 18- to 19-somites, when the heart rate is depressed by acetylcholine and even more strongly by a number of parasympathomimetic drugs and cholinesterase inhibitors. Hence we have concluded that receptors for acetylcholine are present in the early chick embryo heart before innervation by the vagus nerve is initiated, but that the receptors do not possess the specificity of adult acetylcholine cardiac receptors. Incubation of the zero- to 7-somite blastoderms on media containing certain of the parasympathomimetic drugs and cholinesterase inhibitors greatly increased the sensitivity of the hearts to depression of rate by acetylcholine at 18- to 19-somites, and we have concluded that development of acetylcholine receptors has been stimulated by the drugs. A possible stereochemical relationship between acetylcholine receptors and drugs that are able to stimulate the development of the receptors is postulated.

The simultaneous effects of ethanol on the contractility and electrical characteristics of atrial cells were examined. Single cell membrane potentials were determined using microelectrodes; the mechanical activity was recorded from a sensitive strain gauge. Ethanol at concentrations between 24 and 192 mm (110–880 mg/100 ml) depresses the contractility of the rat atria, the relationship between contractile depression and concentration being essentially linear. The effects are reversible in all cases. A definite shortening of the action potential, simultaneous with the reduction in contractility, is the most characteristic change in the electrical properties. This results from a more rapid rate of repolarization. The magnitude of the action potential is decreased slightly, whereas the resting potential and the depolarization rate are unaffected by ethanol. The depression of the contractility can be partially attributed to the shortening of the action potential but, in addition, there would appear to be some more direct action on the contractile mechanism. The decrease in the duration of the action potential may predispose the atria to dysrhythmias, such as ectopic beats and fibrillation.

The simultaneous effects of estradiol, testosterone, and progesterone on the electrical and mechanical characteristics of atrial cells were examined. Intracellular potentials were determined using microelectrodes; the mechanical activity was recorded from a strain gauge. Testosterone from 10–6 to 10–5 m produced a marked stimulation in normally contracting atria; while similar concentrations of progesterone and estradiol produced only contractile depression. All three sex steroids exerted a definite effect on the electrical characteristics, namely a reduction in the rates of depolarization and repolarization, without altering the magnitudes of the resting or action potentials appreciably. As a result, there is an increase in the duration and area of the action potential. They also slowed the conduction rate and increased the latent period, effects presumably associated with the slowed depolarization. The marked reduction in the rates of depolarization and repolarization is believed to be due to decreases in the ionic fluxes brought about by reducing the ionic permeability of the cardiac cell membranes. The effects are similar to those produced by antidysrhythmic drugs and may explain certain observations on the relation of cardiac excitability to steroid levels.

Some of the relationships between metabolism, membrane potentials and contractility in cardiac tissue have emerged from a study of the effects of anoxia, depletion of substrate and dinitrophenol on the rat atrium. The repolarization phase of the action potential was shown to be particularly sensitive to disturbances in metabolism. Depression of metabolism resulted in a more rapid repolarization, which may provisionally be interpreted as an acceleration of K + efflux; restoration of metabolism reversed this effect. A concept relating membrane potentials and metabolism is presented, involving the presence of bound ATP within the membrane.

The block of an enzyme, such as monoamine oxidase, may be simple but the effect of this block upon the tissue functions are clearly complex. The behavior of intracellular multienzyme systems is difficult to express kinetically and, in addition, the quantitative relationship between the tissue function and the concentration of an intermediate is generally unknown. In the present case the situation is complicated by the presence of multiple pathways: there are several pathways of amine formation, there are an unknown number of amines formed and there are different pathways for the elimination of these amines. This field of investigation is one that particularly requires the simultaneous application of biochemical, physiologic and pharmacologie concepts at the most fundamental levels.

IN the course of an investigation of the control of the heart rate in the early chick embryo it was noted that under certain conditions exposure to visible light accelerates the rate. The following report presents a study of this phenomenon. There are many reports in the older literature of effects of visible light, unrelated to ultraviolet radiation, on cellular activity. Cessation of amoeboid activity when Pelomyxa was suddenly exposed to light was noted in 18791, and has been extensively studied more recently2,3. Activity of cells in tissue culture is affected by exposure to light; amoeboid activity of lymphocytes in frog spleen explants4, and the rate of beat5 of explants of 6 day chick embryo hearts and of cilia of explants of frog throat epithelium, are all modified by growth of the cultures in visible light. Recently, there have been reports of increased metabolic activity6 and of accelerated growth rate and early hatching7,8 in chick embryos from eggs incubated in permanent light. The stimulation of the chick embryo heart rate to be reported here was obtained in the visible range without addition of dye or photosensitizing agents.

Simultaneous recording of membrane potentials and contractility in isolated rat atria by means of intracellular micro-electrodes and a sensitive strain gage has been used to establish the normal characteristics of the tissue as well as effects of temperature, stimulus frequency and initial tension on muscle. Preliminary correlations between duration of the action potential and the degree of contraction indicate that the rate of repolarization is of major importance in determination of the contractile response.

The LD50 of chymotrypsin in rats was found to be 65.1 ≠ 2.3 mg./Kg. by intraperitoneal injection. Chronic experiments were also carried out in which chymotrypsin was administered subcutaneously at four different dosage levels on alternate days for a period of four weeks. No evidence of toxicity was observed by gross and histological examination of tissues from these animals.