Chronotropic response of isolated atria to acid base alterations

The effect of acid-base alterations on spontaneous rate was analysed using isolated atria exposed to cumulative degrees of acidosis produced either by adding HCl or by increasing PCO2 in the incubation medium. Frequncy vs. pH curves were made to assess chronotropic response to acid-base changes. Heart rate was increased in alkalosis and decreased when the pH of the medium was lowered. Both "respiratory" and "metabolic" alterations affected the contraction rate to the same extent. Decreasing pH from normal values seemed to decrease heart rate more than the enhancement produced by the same change in pH towards the alkalotic side. When frequency was plotted as a function of hydrogen ion activity (aH+) a more linear relationship was obtained, either with pure "metabolic" or with "respiratory" acid-base alterations. Increasing (aH+) from normal values seemed to decrease heart rate to the same extent (respiratory alterations) or even less (metabolic alterations) than the enhancement produced by the same change in (aH+) towards the alkalotic side. Neither the increase in rate produced by alkalosis nor the decrease induced by acidosis were prevented by blocking the neurotransmitters by atropine or propranolol.