EFFECTS OF TEMPERATURE CHANGE, OXYGEN DEPRIVATION AND CATIONS ON THE ATRIAL RESPONSES TO VAGAL STIMULATION

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Abstract
Effects of temperature change, O2 deprivation and cations on the chronotropic and inotropic responses of the isolated rabbit''s and guinea-pig''s atria to stimulation of the vagus nerves and to added acetylcholine were studied in the present experiments. The vagal stimulation at a frequency of 20/sec elicited maximum negative responses in rate and amplitude. The interval of 5 minutes between successive stimulations was enough to reproduce almost the same response. The negative chronotropic response to stimulation of the right vagus was more marked than that to stimulation of the left vagus, while the negative inotropic response caused by stimulation of either nerve did not markedly differ. The fall or rise of the temperature of the nutrient solution from 30[degree] C reduced both negative chronotropic and inotropic responses of the atria to vagal stimulation. The responses to added acetylcholine were reduced by the rise but not by the fall of the temperature. The oxygen deprivation augmented both responses to vagal stimulation, while it did not affect the responses to added acetylcholine. The addition of calcium increased the amplitude of the spontaneous contraction and augmented both responses to vagal stimulation. The addition of magnesium reduced the atrial responses to vagal stimulation only in some preparations. While the addition of potassium decreased the rate and amplitude of the spontaneous contraction, it augmented the atrial responses to vagal stimulation. The vagal stimulation or the addition of acetylcholine sometimes restarted the atria arrested by high concentration of potassium. The reduction of sodium resulted in the increase in amplitude accompanied with the decrease in rate of the spontaneous contraction. Low sodium augmented the negative chronotropic response but reduced the negative inotropic response to vagal stimulation. The vagal stimulation as well as the addition of adrenaline restarted the atria arrested by extreme reduction of sodium. The addition of barium reduced the atrial responses to vagal stimulation.