The effects of aging on cardiovascular regulatory mechanisms were studied in 12 mth old (young) and 24 mth old (old) unanesthetized male Wistar rats. Baroreceptor reactivity was measured by the decrease in heart rate in response to a 55 mm Hg increase in systolic blood pressure induced by continuous phenylephrine infusion. Baroreceptor reactivity was significantly greater in the young than in the old animals. In both age groups the baroreceptor reflex was abolished by cholinergic blockade with atropine. After beta adrenergic blockade with propranolol, baroreceptor reactivity was diminished in both the young and old animals but was still significantly greater in the young than in the old. It is concluded that the diminished response of heart rate in the senescent rat to baroreceptor stimulation is primarily due to decreased parasympathetic activity in the old animal.
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