Alterations in the cardiovascular control by the chemoreflex and the baroreflex in old rats.

The aging process is associated with alterations in the autonomic control of cardiovascular function. In the present study, we observed that reflex bradycardia and tachycardia produced by pressor and depressor agents were attenuated in old (24 mo) rats [70 and 56% of responses in young rats (4 mo), respectively]. In contrast, the bradycardia induced by chemoreflex activation with increasing doses of KCN (60, 100, 140, and 180 micrograms/kg) was greater in old [-30 +/- 19, -155 +/- 32, -198 +/- 15, and -204 +/- 24 beats/ min (bpm)] than in young rats [-13 +/- 6, -75 +/- 20 (P < 0.05), -99 +/- 26 (P < 0.05), and -103 +/- 20 (P < 0.05) bpm]. The chemoreflex-pressor responses tended to be greater in old (12 +/- 6, 47 +/- 6, 54 +/- 7, and 55 +/- 5 mmHg) than in young rats (4 +/- 2, 32 +/- 8, 42 +/- 6, and 44 +/- 4 mmHg), but the differences between both groups were not statistically significant. However, pressor responses were higher in old rats after the chemoreflex-activated bradycardia was attenuated by atropine (4 mg/kg iv) [old (17 +/- 9(1) P < 0.05; 57 +/- 4, P < 0.05; 61 +/- 4; and 64 +/- 5 mmHg) vs. young (3 +/- 2, 29 +/- 9(1) 50 +/- 5, and 58 +/- 7 mmHg)]. Administration of the alpha 1-blocker prazosin (1 mg/kg iv) blunted pressor responses but did not change the bradycardia induced by chemoreflex in both groups. In conclusion, our data indicate that the bradycardia and pressor responses to chemoreflex activation were exaggerated, whereas the heart rate responses (bradycardia and tachycardia) to baroreflex were depressed in old rats.