Atrial systolic pressure, as well as stretch, is a principal stimulus for release of ANF

The mechanism for increased secretion of atrial natriuretic factor (ANF) during tachycardia and atrial fibrillation has remained unsettled. In seven open-chest pigs, the plasma concentration of ANF increased from 49.8 +/- 12.4 to 131.8 +/- 15.7 pg/ml when heart rate was increased from 133 +/- 13 to 212 +/- 4 beats/min by atrial pacing. Right atrial maximal diameter, recorded by ultrasonic technique at the maximal atrial filling, did not increase. During pacing tachycardia, the atrial contraction (a wave) occurs during atrial filling, and the a wave becomes superimposed on the v wave. In the present study the systolic atrial pressure (a wave) increased from 5.8 +/- 0.8 to 9.6 +/- 0.5 mmHg. The significance of this pressure rise was subsequently examined. After complete atrioventricular (AV) block, the AV delay was progressively increased, without altering heart rate, until the a wave was similar to the the a wave during the preceding tachycardia. Plasma ANF increased to 113.8 +/- 14.7 pg/ml, which showed that the increase in atrial pressure during atrial systole is a stimulus for ANF release. In the second part of the study, atrial fibrillation was induced in six open-chest pigs by rapid atrial pacing after complete AV block. Plasma ANF increased from 83.5 +/- 7.2 to 269.0 +/- 45.4 pg/ml during atrial fibrillation. No increase in atrial dimensions occurred during atrial fibrillation, but atrial pressure was substantially elevated. Thus, although passive atrial stretch stimulates ANF release during blood volume expansion, the present study shows that the increase in atrial pressure during atrial contraction is a stimulus for release of ANF during tachycardia and atrial fibrillation.