Effects of ageing on sympathetically‐mediated cerebrovascular tone and autoregulation

Abstract

The role of sympathetic fibres in cerebral circulation is unclear. Evidence suggests sympathetic activity exerts a tonic constrictor influence on cerebral blood flow (CBF) and protects against increases in arterial blood pressure (ABP) at the upper limit (UL) of autoregulation. Ageing impairs sympathetic functioning, but how this affects cerebral circulation is unknown.In anaesthetised, young male Wistar rats (Y: 6‐8 weeks, n=7), we isolated and stimulated the sympathetic superior cervical ganglion (SNS) at 1‐50Hz for 5s. CBF recorded from common carotid artery after external carotid ligation, was 2.5±0.2ml/min and fell in a frequency‐dependent way to 1.1±0.2ml/min; a decrease of 58±7%. In 4 Y rats, phentolamine (10mg/kg i.v.) decreased basal ABP to near the lower limit, and CBF from 2.3±0.2 to 1.4±0.1ml/min and attenuated the response to SNS by 72±3%. In n=9 Y rats, sympathectomy increased basal CBF by 0.6±0.1ml/min. Phenylephrine was infused (0.1–200µg/kg/min i.v.) to raise ABP, and the autoregulatory UL determined by dual‐line linear regression analysis of CBF vs ABP. The UL was higher than intact Y rats (178±3* vs 168±2mmHg; P<0.05).In old rats (O: 52‐58 weeks, n=6), basal CBF was higher than Y rats; 3.3±0.2ml/min*, but was similarly decreased by SNS (to 1.6±0.2ml/min; P=0.06 vs Y: a 51±6% decrease). Phentolamine decreased basal CBF from 3.1±0.2, to a lower level than in Y rats (1.4±0.2ml/min*), but the response to SNS was similar (77±7% decrease).We propose that sympathetic fibres have similar capacity to induce vasoconstriction in O as Y rats and a similar proportion is mediated by α‐adrenoceptors. Further, sympathetic vasoconstriction limits the autoregulatory UL, at least in Y rats.British Heart Foundation funding is gratefully acknowledged.