Age- and Sex-Related Changes in Heart Rate Variability Under Conditions of Blockade or Stimulation of Peripheral Adrenoceptor in Outbred Rats

Changes in heart rhythm variability were studied in male and female mature and 5-6-week-old rats under conditions of 7-day administration of β 1 -adrenoreceptor blocker atenolol (2.5 mg/kg) and α 1 -adrenoreceptor agonist phenylephrine (0.3 mg/kg). Atenolol administration to mature rats was followed by a slight deceleration of cardiac rhythm, a tendency to heart rate variability decrease in the HF range, and moderate increase in centralization of regulation. In 6-week-old rats, increased variability of cardiointervals and significant increase of centralization of the heart rhythm regulation due to an increase in the power of low-frequency waves (specifically VLF) were observed. In both mature and young rats, changes of heart rate frequency and variability in response to atenolol administration were more pronounced in females. Phenylephrine administration was followed by a significant heart rate deceleration, increase in cardiointerval variability and centralization of heart rate regulation in mature rats and by a decrease in heart rate variability in all frequency ranges in 6-week-old rats. In mature rats, changes in heart rate frequency and variability produced by phenylephrine administration were more pronounced in males; in young rats, the most strained heart rhythm developed in females.