The Mechanism of Pacemaker Acceleration Due to Increased Intra-Atrial Pressure in the Isolated Atrium

A closed circuit with a miniature pump was used to perfuse isolated rat atria through the lumen. The rate of spontaneous atrial potentials was measured continuously on a recording potentiometer.The response of the atrium to changes in luminal pressure was observed under a number of different conditions. The acceleration due to increased pressure was greatest when the perfusing fluid contained low bicarbonate concentrations (5–10 mmolar) and was very much reduced or abolished with 28 mmolar bicarbonate. Pacemaker rate at constant pressure varies directly with pH in the presence of high bicarbonate concentrations. The pressure response may be interpreted in terms of a pH gradient across the atrial wall which is dependent on the bicarbonate concentration and may be reduced by stretching the atrium.