The effect of tachycardia on right atrial dynamics and plasma atrial natriuretic factor in anaesthetized rabbits.

1. The effect of tachycardia on right atrial pressure (RAP) and dimensions (RAD) and plasma immunoreactive atrial natriuretic factor (IR-ANF) was examined in anaesthetized rabbits before and after the administration of atenolol. 2. Small increases in plasma IR-ANF occurred during pacing at 6 Hz and after the administration of atenolol (0.4 mg kg-1). A significantly greater increase in IR-ANF occurred during pacing in the presence of atenolol. 3. Despite a significant rise in mean RAP, systolic RAD (SRAD) and diastolic RAD (DRAD) did not change during tachycardia. Systolic RAD increased after the administration of atenolol while both SRAD and DRAD increased during pacing in the presence of atenolol. 4. Systolic right atrial wall stress (SRAS) increased during tachycardia, did not change after the administration of atenolol, and increased during pacing in the presence of atenolol. Minute SRAS followed a similar pattern of changes except that it decreased after atenolol. 5. Diastolic right atrial wall stress (DRAS) did not change during tachycardia, and increased both after atenolol and after pacing in the presence of atenolol. Tachycardia led to an increase in minute DRAS; a significantly greater increase in minute DRAS occurred during tachycardia in the presence of atenolol. 6. The absence of alterations in RAD during tachycardia suggests that the release of ANF in response to tachycardia is not due to simple mechanical stretch of the atria. Both systolic and diastolic atrial wall stress may be determinants of ANF release: the influence of systolic factors appears to predominate during tachycardia while diastolic factors appear to be the major determinants of the effects of atenolol.