Effect of 'autonomic blockade' on cardiac beta-adrenergic chronotropic responsiveness in healthy young, healthy elderly and endurance-trained elderly subjects.

1. Cardiac chronotropic responses to isoprenaline are reduced with ageing in man. It is unclear whether this is due to reduced cardiac beta-adrenergic sensitivity or to age-associated differences in reflex cardiovascular responses to the vasodilatory effects of isoprenaline. Age-associated changes in physical activity are also reported to influence beta-adrenergic sensitivity. 2. The aim of the present study was to determine the contribution of alterations in reflex changes in para-sympathetic and sympathetic influences and physical fitness to the age-associated reduction in cardiac chronotropic responses to beta-adrenergic agonists. 3. The effect of 'autonomic blockade' with atropine (40 micrograms/kg intravenously) and clonidine (4 micrograms/kg intravenously) on blood pressure, heart rate and chronotropic responses to intravenous bolus isoprenaline doses was determined in eight healthy young (mean age 21 years), nine healthy elderly (72 years) and 10 endurance-trained elderly (69 years) subjects. 4. Elderly subjects had a reduced increase in heart rate after atropine (young, 49 +/- 9 beats/min; elderly, 36 +/- 5 beats/min; endurance-trained elderly, 34 +/- 12 beats/min; P < 0.01) and did not demonstrate the transient increase in systolic blood pressure after clonidine observed in young subjects (young, 11 +/- 10 mmHg; elderly, -12 +/- 16 mmHg; endurance-trained elderly, -18 +/- 11 mmHg; P < 0.01). 5. Cardiac chronotropic sensitivity to isoprenaline after 'autonomic blockade' increased in the young but decreased in the elderly subjects.(ABSTRACT TRUNCATED AT 250 WORDS)