Age-related reduction of baroreflex tachycardia without loss of beta-adrenoceptor-mediated tachycardia in Sprague-Dawley rats.

We examined baroreflex function and cardiac responsiveness in young (3-7 months) and old (21-24 months) Sprague-Dawley rats. Nitroprusside was more effective in lowering diastolic blood pressure (DBP) in old than in young conscious animals, but there was no such difference in anaesthetised animals. In conscious animals, the slope of the regression line for the fall in DBP produced by nitroprusside against the resultant tachycardia was significantly steeper for young animals, so that a given fall in DBP produced a greater rise in heart rate in young animals. In anaesthetised or pithed animals, the tachycardia produced by isoprenaline, noradrenaline, and single-pulse electrical stimulation did not differ between young and old animals. Hence, we have evidence that baroreflex sensitivity is reduced in aged rats, but that this is not due to a decreased responsiveness of the heart.