Kellawan, J. Mikhail

Background Nitric oxide (NO) contributes ~30% to the rate of vasodilation during dynamic exercise in healthy humans. This NO‐mediated dilation occurs via increases in cyclic guanosine monophosphate (cGMP), which is catabolized by phosphodiesterase‐5 (PDE‐5). Therefore, we sought to determine the nature of NO‐cGMP signalling to the exercise vasodilator response in healthy humans. We hypothesized the time‐course of exercise vasodilation would be: 1) accelerated by sildenafil citrate (SDF, PDE‐5 inhibitor) 2) slowed by the NO synthase inhibitor L‐N G ‐monomethyl arginine (L‐NMMA) 3) similarly slowed by the combination of SDF+L‐NMMA. Methods Data from 29 healthy individuals were included in the present study. All subjects completed 5‐min of dynamic forearm exercise at 15% of maximal voluntary contraction under control conditions (CON, arterial saline infusion), followed by an identical exercise protocol during: 1) PDE‐5 inhibition via SDF (100mg oral dose, n = 8), 2) NOS inhibition via intra‐arterial infusion of L‐NMMA (50 mg loading dose and 1 mg/min maintenance dose, n = 15), or 3) combined SDF+L‐NMMA (n = 6). Forearm blood flow (FBF; Doppler ultrasound) and mean brachial artery blood pressure (MAP, transducer) were measured continuously. To determine time to steady‐state vasodilation, forearm vascular conductance (FVC = FBF/100mmHg/dL FAV) data were curve fit with a monoexponential model and mean response time (MRT, time to achieve 63% of steady state response) was calculated. In addition, the increase in relative forearm vascular conductance from rest to steady state exercise (average of the last 30s of exercise, ΔFVC = FVC exercise − FVC rest ) was used to assess the total vasodilatory response. Results Data are reported as mean ± SE (FVC = mL/dL/100mmHg/min, MRT = s) . In contrast to our hypothesis, SDF alone had no effect on exercise vasodilation (Δ FVC CON = 20±3 vs. SDF = 21±3, p = 0.50) or time to steady‐state vasodilation (MRT: CON = 32± 7 vs SDF= 33±4, p = 0.90) in young healthy adults. NOS inhibition with L‐NMMA reduced ΔFVC (CON = 16±2 vs. L‐NNMA = 14±1, p = 0.049) and increased the time to reach steady‐state vasodilation (MRT: CON = 22±5 vs. L‐NMMA = 40±6, p = 0.01). Surprisingly, when subjects were given L‐NMMA combined with SDF, reductions in Δ FVC or increases in MRT were no longer observed (Δ FVC: CON = 23±4 vs. SDF+L‐NMMA = 23±6, p = 0.90; MRT: CON = 45±8 vs. SDF+L‐NMMA = 49±11, p = 0.80). Conclusion These data indicate inhibiting PDE‐5 during moderate exercise does not improve the magnitude or time‐course of vasodilation. Whereas, NOS inhibition limits the magnitude and time‐course of the vasodilatory response to dynamic handgrip exercise in healthy adults, combined inhibition of PDE‐5 restores the response to control levels. These data suggest PDE‐5 inhibition does not potentiate the NO‐cGMP pathway during moderate exercise. However, PDE‐5 inhibition may have therapeutic use in pathologies where limited NO bioavailability may contribute to impaired blood flow responses to exercise. Support or Funding Information American Heart Association (15POST23100020) ‐ JMKNational Institutes of Health (HL46493) ‐ MJJ National Institutes of Health (HL078019) ‐ MJJ National Institutes of Health (HL105820) ‐ WGS National Institutes of Health (RR17520) ‐ N/A National Institutes of Health (UL1 TR000135) ‐ N/A

Nitric oxide (NO)-mediated vasodilation contributes to the rapid rise in muscle blood flow at exercise onset. This occurs via increased cyclic guanosine monophosphate (cGMP), which is catabolized by phosphodiesterase-5 (PDE-5). Whether PDE-5 limits exercise vasodilation onset kinetics is unknown. We hypothesized the time course of exercise vasodilation would be 1) accelerated during PDE-5 inhibition (sildenafil citrate, SDF) and 2) decelerated during NO synthase inhibition ( NG-monomethyl-l-arginine, l-NMMA), and 3) the effect of SDF on vasodilation onset kinetics would be attenuated with concurrent l-NMMA. Data from 29 healthy adults were analyzed. Individuals completed 5 min of moderate-intensity forearm exercise under control conditions and during 1) oral SDF ( n = 8), 2) intra-arterial l-NMMA ( n = 15), or 3) combined SDF + l-NMMA ( n = 6). Forearm blood flow (FBF; Doppler ultrasound of the brachial artery) and mean brachial artery blood pressure (MAP) were measured continuously. Forearm vascular conductance (FVC, FBF ÷ MAP) was curve-fit with a monoexponential model, and vasodilation onset kinetics were assessed by mean response time (MRT, time to achieve 63% of steady state). SDF had no effect on MRT ( P = 0.90). NOS inhibition increased MRT ( P = 0.01). MRT during SDF+l-NMMA was not different from control exercise ( P = 0.76). PDE-5 inhibition alone has no effect on rapid-onset vasodilation. Whereas NOS inhibition decelerates vasodilator kinetics, when combined with SDF, vasodilator kinetics do not differ from control. These data suggest NO-independent activation of cGMP occurs at exercise onset; thus PDE-5 inhibition may improve vasodilation in pathologies where NO bioavailability is impaired. NEW & NOTEWORTHY We show that when NO bioavailability is reduced, PDE-5 inhibition can restore vasodilation onset kinetics of exercise-mediated vasodilation via NO-independent cGMP pathways. These data suggest PDE-5 inhibition may improve exercise vasodilation onset kinetics in pathologies where NO bioavailability is impaired.