The effect of adenosine on the reduced heart rate response to exercise in the elderly

In a previous study we demonstrated that excessive endogenous adenosine production and/or response was responsible for the blunted tachycardia to bolus intravenous doses of isoproterenol in the elderly. In this study, we tested the hypothesis that excessive endogenous adenosine may also be responsible for the diminished maximal tachycardia with aging.

Twelve young (mean age 27.3 +/- .61 yr) and 12 older (mean age 66.8 +/- .9 yr) healthy men were asked to perform maximum exercise tests in the presence of placebo or theophylline at plasma concentrations between 15-20 micrograms/ml. Heart rate, oxygen consumption, and respiratory gas exchange ratio were continuously monitored and recorded during the exercise. In addition, plasma lactate, glycerol, renin activity, and catecholamines were measured before and after maximal exercise.

Maximum heart rate to exercise was higher in the young (190 +/- 3 bpm in the young, 157 +/- 2 bpm in the old) and increased by 4.5 +/- 1.2 bpm in the young and 9.8 +/- 2.6 bpm in the old with the administration of theophylline that resulted in an equivalent serum concentration in the two age groups. This age difference in the increase was not significant but approached significance at a p value of .07. Maximum VO2 was also greater in the young group and this was unaffected in both groups by theophylline administration. The increase in serum lactate and plasma renin activity (PRA) to exercise was higher in the young group both in the presence and absence of theophylline.

Our data could not identify excessive adenosine in the older group as the cause of diminished maximal exercise heart rate with aging. It is likely that the diminished chronotropic response to exercise in the older humans is due to a mechanism intrinsic to the cardiac excitatory tissue.