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.