Nyberg M, Piil P, Egelund J, Sprague RS, Mortensen SP, Hellsten Y. Effect of PDE5 inhibition on the modulation of sympathetic ␣-adrenergic vasoconstriction in contracting skeletal muscle of young and older recreationally active humans. Am J Physiol Heart Circ Physiol 309: H1867-H1875, 2015. First published October 2, 2015; doi:10.1152/ajpheart.00653.2015.-Aging is associated with an altered regulation of blood flow to contracting skeletal muscle; however, the precise mechanisms remain unclear. We recently demonstrated that inhibition of cGMP-binding phosphodiesterase 5 (PDE5) increased blood flow to contracting skeletal muscle of older but not young human subjects. Here we examined whether this effect of PDE5 inhibition was related to an improved ability to blunt ␣-adrenergic vasoconstriction (functional sympatholysis) and/or improved efficacy of local vasodilator pathways. A group of young (23 Ϯ 1 yr) and a group of older (72 Ϯ 1 yr) male subjects performed kneeextensor exercise in a control setting and following intake of the highly selective PDE5 inhibitor sildenafil. During both conditions, exercise was performed without and with arterial tyramine infusion to evoke endogenous norepinephrine release and consequently stimulation of ␣1-and ␣2-adrenergic receptors. The level of the sympatholytic compound ATP was measured in venous plasma by use of the microdialysis technique. Sildenafil increased (P Ͻ 0.05) vascular conductance during exercise in the older group, but tyramine infusion reduced (P Ͻ 0.05) this effect by 38 Ϯ 9%. Similarly, tyramine reduced (P Ͻ 0.05) the vasodilation induced by arterial infusion of a nitric oxide (NO) donor by 54 Ϯ 9% in the older group, and this effect was not altered by sildenafil. Venous plasma [ATP] did not change with PDE5 inhibition in the older subjects during exercise. Collectively, PDE5 inhibition in older humans was not associated with an improved ability for functional sympatholysis. An improved efficacy of the NO system may be one mechanism underlying the effect of PDE5 inhibition on exercise hyperemia in aging.sildenafil; exercise; ATP; nitric oxide; cGMP
NEW & NOTEWORTHY
We recently demonstrated that inhibition of cGMP-binding phosphodiesterase 5 (PDE5) increased blood flow to contracting skeletal muscle of older but not young human subjects. Here we demonstrate that this effect of PDE5 inhibition on exercise hyperemia was related to an improved efficacy of local nonsympatholytic vasodilator pathways.CONTRACTILE ACTIVITY markedly increases the O 2 demand in skeletal muscle, thus requiring an increase in blood flow and O 2 delivery to the tissue. This increase in blood flow to the exercising muscle is the result of a local attenuation of the vasoconstrictor effect of sympathetic nervous activity, termed functional sympatholysis, and vasodilation induced by local vasoactive mechanisms (6,15). Since the first report on the hemodynamic response to exercise in older individuals in 1974 (42), evidence has accumulated for an insufficient blood flow to contracting skeletal m...