THE INOTROPIC AND CHRONOTROPIC ACTIONS OF ACETVLCHOLINE ON RABBIT ATRIA SUSPENDED IN SODIUM DEFICIENT MEDIA’

Recent evidence indicates that inorganic cat-ions play an important role in the action of acetylcholine on the heart. Since sodium ions are involved in pacemaker and contractile activity (Brooks ci at., 1955), investigation of the relation-ship of sodium ion to the inotropic and chrono-tropic actions of acetylcholine may be of value in the elucidation of the mechanism of action of acetylcholine. This report describes the action of acetylcholine on frequency and amplitude of contraction of isolated rabbit atria in media deficient in sodium ions. METHODS. Isolated spontaneously beating rabbit atria were suspended in a tissue bath maintained at 27#{176}C.The tissue was attached to a strain gauge and recordings of rate and amplitude of contraction were made on a Sanborn recorder. The normal medium contained 7.6 g NaCl, 0.42 g KCI, 0.24 g CaCl2, 0.117 g NaH2PO4, 2.1 g NaHCO3 and 2.0 g dextrose in distilled water to make 1 liter. The sodium deficient medium was made by replacing one half of the NaCI with an isosmotic amount of sucrose. The media were aerated with a 95 % 02-5 % CO2 gas mixture and adjusted to pH 7.4. Each preparation was allowed to equilibrate for 1 hour, at which time the contrac-tions were uniform in rate and amplitude. The atria were allowed to equilibrate for 5 to 15 minutes in low sodium medium before the effects of acetylcholine were tested. The control values for rate and amplitude were determined during a 10-second period prior to the acetylcholine addi-tion. The response to acetylcholine was deter-mined by calculating the rate and amplitude 1 minute after addition of the drug and expressed as per cent of the control. RESULTS. The effect of sodium ion conceniration on the frequency and amplitude of contraction of 1 This investigation was supported by a researc