The Effects of Pharmacological Manipulations that Influence Vagal Control of the Heart on Heart Period, Heart-Period Variability and Respiration in Rats

Changes in heart period (HP), heart-period variance (HPV), and the respiratory component of HPV in freely moving rats were examined following pharmacological manipulations known to influence vagal control of the heart. Spectral analysis was used to quantify the component of HPV associated with respiration which is proposed to be sensitive to vagal influences on the heart. The respiratory component of HPV is described by a statistic, V̌, which is the sum of the spectral densities of the heart period spectrum across the band of frequencies associated with normal respiration. Vagal tone was reflexively enhanced by phenylephrine or peripherally blocked by atropine. Phenylephrine produced biphasic changes in HP and V̌, with the increases lasting from 0.5 to 2 hrs. Atropine resulted in significant decreases in HP, HPV and V̌. These decreases were immediate and sustained throughout the experiment, although there were signiflcant linear increases in HP and HPV from their initial post-injection values. In the saline condition, HP increased linearly across time. Results were discussed in terms of vagal and non-vagal control of the heart. It was suggested that, while all three variables are altered by manipulations of vagal influences on the heart, V̌ seems to be influenced less by non-vagal control than HP and HPV.