Recently we demonstrated that basal whole-limb (femoral) blood flow is reduced in older compared with young healthy men due to a corresponding reduction in vascular conductance (Dinenno et al. 1999, 2001). We also found that this tonically elevated vasoconstrictor state with age was positively related to basal leg muscle sympathetic nerve activity (Dinenno et al. 1999). This suggested that the age-associated reductions in limb blood flow and vascular conductance might be mediated, at least in part, by chronically augmented α-adrenergic vasoconstriction. However, such a conclusion concerning cause and effect could not be made solely from these correlational observations.
Although basal muscle sympathetic nerve activity increases markedly with advancing age in humans (Sundlof & Wallin, 1978; Davy et al. 1998), it is not known whether this causes tonically augmented limb vasoconstriction. Several processes involved, from the discharge of the postganglionic sympathetic nerve fibres to vascular smooth muscle cell contraction, may be affected by ageing and could serve to uncouple this increased sympathetic drive from the predicted end-organ response. For example, age-related changes in noradrenaline release per unit nerve discharge, the binding of noradrenaline to postsynaptic α-adrenergic receptors, α-receptor density, and/or post-receptor intracellular signalling may occur. Although the effects of ageing on these specific functions are unclear, we (Davy et al. 1998) and others (Hogikyan & Supiano, 1994) have reported that limb vasoconstrictor responsiveness to sympathetic α-adrenergic stimulation may be attenuated with age. Thus, it is possible that the higher muscle sympathetic nerve activity with age is negated by reduced responsiveness and does not result in augmented limb vasoconstrictor tone. To determine this, it is necessary to measure limb haemodynamics before and after removal of α-adrenergic vasoconstrictor tone.
Accordingly, in the present investigation we sought to determine whether the lower basal whole-limb blood flow and vascular conductance with human ageing are mediated, at least in part, by tonic increases in sympathetic α-adrenergic vasoconstriction. To do so, we studied young and older healthy men before and after removing local α-adrenergic vasoconstrictor tone using intra-femoral artery infusion of phentolamine.
- Top of page
The key new finding from the present study is that the age-related reductions in basal whole-limb blood flow and vascular conductance in healthy men are mediated primarily by augmented sympathetic α-adrenergic vasoconstrictor tone. The experimental evidence supporting this conclusion is that the vasodilator responses to local α-adrenergic receptor blockade with phentolamine were significantly greater in the older compared with the young men. As a consequence, the significant age-associated differences in basal levels of femoral blood flow, vascular conductance and vascular resistance at baseline no longer were evident after local α-adrenergic receptor blockade.
Previously we reported that basal whole-leg blood flow was ∼25 % lower in older compared with young healthy men, that this was due to a lower vascular conductance (increased vascular resistance), and that the decline was progressive (linear) over the adult age range in this population (Dinenno et al. 1999, 2001). The results of the present study in which basal femoral blood flow and vascular conductance were ∼30 % lower and vascular resistance ∼50 % higher in the older men are consistent with these previous observations.
In our initial investigations (Dinenno et al. 1999, 2001), we could only speculate on the mechanism(s) mediating the age-associated reductions in basal whole-leg blood flow and vascular conductance. The results of these previous studies indicated that cardiac output was not significantly related to age in this population and was only weakly related to femoral blood flow, suggesting that reductions in systemic arterial blood flow do not contribute importantly to the decline in whole-leg blood flow with age, at least in healthy men. In contrast, we found a moderately strong inverse relationship between basal femoral blood flow (and vascular conductance) and muscle sympathetic nerve activity (Dinenno et al. 1999). These correlational data suggested that age-associated increases in sympathetic outflow to the limb arterial circulation might result in a greater tonic α-adrenergic vasoconstrictor state in older men. However, given the potential age-related reduction in vascular responsiveness to sympathetic stimulation (Davy et al. 1998), we could not conclude this from our original observations. Thus, the findings of the present study significantly extend our earlier observations by providing the first direct experimental support for this hypothesis. Additionally, our results indicate that any reduction in α-adrenergic responsiveness with age does not negate the tonic increase in sympathetic vasoconstrictor drive to the leg.
Previously Hogikyan & Supiano (1994) reported no differences in the forearm vasodilator responses to intra-brachial administration of phentolamine in young and older men. However, interpretation of the results of Hogikyan & Supiano (1994) is limited by several factors. First, complete α-adrenergic receptor blockade was not documented. Second, β-adrenergic receptors were not blocked before phentolamine administration. Under conditions of α-adrenergic receptor blockade, the facilitation of noradrenaline release by phentolamine (due to blockade of presynaptic α2-receptors) can stimulate β-adrenergic receptors (Frewin & Whelin, 1968; Saeed et al. 1982), which are less responsive with age (van Brummelen et al. 1981; Pan et al. 1986). Third, arterial blood pressure was significantly reduced by phentolamine in the older, but not young men participating in their study. This could have elicited baroreflex-mediated increases in sympathetic vasoconstrictor outflow in their older men. In the absence of documentation of complete α-adrenergic receptor blockade, it cannot be established that such a counter-regulatory vasoconstrictor effect did not influence their results. Our study design either eliminated or minimized these limitations. Additionally, it is possible that age-related changes in sympathetic α-adrenergic vasoconstrictor tone and responsiveness differ in the vascular beds of the arms compared with the legs.
In the context of the present study, we do not believe that the reduction in mean arterial pressure in the older subjects during local phentolamine administration confounds the interpretation of our age group comparisons for at least three reasons. First, the absence of significant changes in control limb vascular conductance suggests that the effects of any reflex increases in sympathetic vasoconstrictor nerve activity on femoral artery haemodynamics were minimal. Second, the cold pressor test data demonstrate complete α-adrenergic receptor blockade in the experimental limb of both young and older men. Therefore, any reflex increases in sympathetic nerve activity could not exert additional vasoconstriction in the experimental limb and, thus, could not explain the large age group differences in the femoral artery vasodilatory response to α-adrenergic receptor blockade. Finally, the calculation of femoral vascular conductance takes into account any changes in arterial perfusion pressure, providing an appropriate index of vasomotor tone under conditions of changing local blood flow as in the present study (Lautt, 1989).
Although removal of α-adrenergic vasoconstrictor tone abolished (statistically) the significant age group differences in limb haemodynamics at baseline in the present study, other factors may have played a role. For example the difference in femoral vascular conductance was 1.19 U at baseline, but only 0.31 U after phentolamine. This suggests that elevations in α-adrenergic vasoconstrictor tone were responsible for ∼75 % of the reduction in basal femoral blood flow and vascular conductance with age. Thus, ∼25 % of this reduction remains unexplained. Likely candidates include age-related reductions in tonic nitric oxide-mediated vasodilatation (Taddei et al. 2000), increases in endothelin-1-mediated vasoconstriction (Cardillo et al. 1999), and/or changes in arterial structure (Dinenno et al. 2000). Nevertheless, the present results are consistent with the idea that augmented sympathetic α-adrenergic vasoconstrictor tone is the primary mechanism involved.
We also should emphasize that local infusion of propranolol did not affect femoral artery blood flow or vascular conductance in either age group in the present study (Table 2). To the best of our knowledge, these are the first data concerning the possible effects of age on the tonic β-adrenergic contribution to basal whole-limb blood flow in humans. Our results are consistent with previous findings in young adults in which propranolol infused into the brachial artery did not affect baseline blood flow or vascular conductance in the forearm (Eklund & Kaijser, 1976). Collectively, these observations indicate that tonic stimulation of postsynaptic β-adrenergic receptors plays no obvious role in the regulation of leg vascular tone in either young or older healthy adults under resting conditions and, therefore, does not appear to be mechanistically involved in the age-related reductions in basal leg blood flow and vascular conductance.
Phentolamine is a non-selective α-adrenergic antagonist (Doxey et al. 1977). Therefore, the relative contribution of the α1- and α2-receptor subtypes to basal leg vascular tone and whether this is affected by ageing could not be determined in the present study. Additionally, data from animal studies suggest that stimulation of endothelial α2-receptors can evoke a nitric oxide-mediated vasodilatation (Angus et al. 1986). However, it is not known whether endothelial α2-receptors are involved in tonic (basal) nitric oxide synthesis and release in humans. Further, whether this is affected by ageing and, therefore, could have affected our results during α-adrenergic receptor blockade is unknown.
Limb blood flow and vascular conductance have important implications for both disease risk and physical function in humans. A provocative hypothesis, initially proposed by Julius and colleagues (Julius et al. 1992) and later by Lind & Lithell (1993), proposes a connection between heightened sympathetic nervous system activity and the metabolic syndrome (hypertension, hyperlipidaemia and hyperinsulinaemia). In the context of the present findings, it appears plausible to speculate that elevations in limb α-adrenergic vasoconstrictor tone may contribute to the worsening of these cardiovascular risk factors with age.
With regard to physical function, evidence is accumulating that older adults demonstrate impaired hyperaemic responses to a variety of stimuli including acute hyperinsulinaemia (Hausberg et al. 1997), large-muscle dynamic exercise (Proctor et al. 1998) and ambient heat stress (Kenney, 1997). Elevations in basal α-adrenergic limb vasoconstrictor tone may impose limitations on the ability to vasodilate in response to these stimuli (Sinoway et al. 1988) and, therefore, could contribute to such impaired hyperaemic responses in older adults.
The results from the present study provide experimental support for the hypothesis that the age-related reductions in basal whole-leg blood flow and vascular conductance are mediated primarily by tonically augmented sympathetic α-adrenergic vasoconstriction. This may have important physiological and pathophysiological implications for cardiovascular function and disease in the ageing human.