Species differences in the chronotropic response to acid-base alterations

The chronotropic response to acid-base alterations was studied in isolated spontaneously beating atria of rat, cat and rabbit. In the three species, atrial rate was found to be dependent on extracellular pH. Both "respiratory" and "metabolic" alterations affected chronotropism to the same extent. Decreasing pH from 7.8 led to a decrease in rate in the three species, although the rabbit kept atrial rate constant when pH changed between 7.65 and 7.18. There was a curvilinear relationship between rate and pH so that decreasing pH from 7.8 to 7.4 had a lesser chronotropic effect than decreasing pH below 7.4. However, when atrial rate was plotted against hydrogen ion concentration, an almost linear relationship was obtained. Species differences were observed when assessing the absolute decrease of atrial rate for a given change in pH. The rat was the most responsive of the species tested, while the rabbit possessed the least ability to change its rate in response to modifications in pH.