Effects of ouabain and vagal stimulation on heart rate in the dog

When the vagus nerves are stimulated with one burst of pulses per minute, but each burst is placed at a different time in the cardiac cycle, a "vagal effect curve" can be derived which displays the changes in cardiac cycle length (CCL) as a function of the elapsed time from the stimulus. This curve consists of a primary and a secondary phase of increased CCL. We performed experiments on open-chest anaesthetised dogs to test the hypothesis that the secondary phase of increased CCL is caused by an increase in the activity of the electrogenic Na+/K+ pump. In animals with a sinoatrial nodal rhythm (SANR), the mean peak values of the primary and secondary phases of increased CCL were 0.48 +/- 0.03 and 0.06 +/- 001 s, respectively, above the prestimulation level (0.52 +/- 0.01 s). Ouabain (40 micrograms X kg-1 iv) did not significantly alter this bimodal response. In animals with an atrioventricular junctional rhythm (AVJR), the peaks of the primary and secondary increases in CCL were 0.55 +/- 0.03 and 1.04 +/- 0.17 s, respectively, above the prestimulation level (0.72 +/- 0.02 s). In animals with AVJR, ouabain markedly attenuated the peak secondary increase in CCL to 19% of the corresponding control value. This attenuation supports the hypothesis that the prominent secondary increase in CCL evoked by a brief vagal stimulus during AVJR is ascribable to an increased activity of the electrogenic Na+/K+ pump.