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In the course of microelectrode studies of rabbit atria in this laboratory it was noted that cells in the region of the sino-atrial node failed to respond to as high frequencies of electrical stimulation as did atrial cells. The present study was undertaken to investigate this phenomenon by means of simultaneous recording of atrial and sino-atrial cell action potentials. An effort was made to correlate the contractile response with the electrical activity of atrial muscle.Methods. ITirgin female albino rabbits were sacrificed by a blow on the neck and quickly exsanguinated. The thorax was opened and the entire heart quickly removed and dropped into oxygenated Tyrode solution. as modified by Hoffman and Suckling . In this solution, through which oxygen (95%)-carbon dioxide (5%) constantly bubbled, the combined atria were carefully separated from the ventricles and then mounted in a Plexiglas bath containing Tyrode solution. The bath was oxygenated through a diffusion dish which was incorporated into the chamber. The nutrient solution constantly flowed through at a rate of 3 cc per minute. The right atrial appendage was fastened to 2 silver hooks which also served as the stimulating electrode. The connective tissue in the septal area was secured to the arm of a sensitive strain gauge (Grass force displacement transducer), and the left atrium was allowed to float freely in the chamber. Stimulation was applied by means of a Grass Model 3C Stimulator. The output was passed through an isolation transformer to reduce the shock artifact. The signal produced by distortion of the strain gauge arm during contraction, was fed into a preamplifier. which in turn supplied the input to a DC amplifier of a Sanborn 4 channel recorder.
Cervoni et al. (Mon,) studied this question.