Contribution of adenosine to compensatory dilation in hypoperfused contracting human muscles is independent of nitric oxide

Abstract

We previously demonstrated that nitric oxide (NO) contributes to compensatory vasodilation in the contracting human forearm subjected to acute hypoperfusion. We examined the potential role of an adenosine-NO interaction to this response in 17 male subjects (25 ± 2 yr). In separate protocols subjects performed rhythmic forearm exercise (20% of maximum) while hypoperfusion was evoked by balloon inflation in the brachial artery above the elbow. Each trial included exercise before inflation, exercise with inflation, and exercise after deflation (3 min each). Forearm blood flow (FBF; ultrasound) and local [brachial artery catheter pressure (BAP)] and systemic [mean arterial pressure (MAP); Finometer] arterial pressure were measured. In protocol 1 (n = 10), exercise was repeated during nitric oxide synthase inhibition [N(G)-monomethyl-L-arginine (L-NMMA)] alone and during L-NMMA-aminophylline (adenosine receptor blockade) administration. In protocol 2, exercise was repeated during aminophylline alone and during aminophylline-L-NMMA. Forearm vascular conductance (FVC; ml·min(-1)·100 mmHg(-1)) was calculated from blood flow (ml/min) and BAP (mmHg). Percent recovery in FVC during inflation was calculated as (steady-state inflation + exercise value - nadir)/[steady-state exercise (control) value - nadir]. In protocol 1, percent recovery in FVC was 108 ± 8% during the control (no drug) trial. Percent recovery in FVC was attenuated with inhibition of NO formation alone (78 ± 9%; P < 0.01 vs. control) and was attenuated further with combined inhibition of NO and adenosine (58 ± 9%; P < 0.01 vs. L-NMMA). In protocol 2, percent recovery was reduced with adenosine receptor blockade (74 ± 11% vs. 113 ± 6%, P < 0.01) compared with control drug trials. Percent recovery in FVC was attenuated further with combined inhibition of adenosine and NO (48 ± 11%; P < 0.05 vs. aminophylline). Our data indicate that adenosine contributes to compensatory vasodilation in an NO-independent manner during exercise with acute hypoperfusion.

Fig. 1.

Schematic diagram of experimental protocol. Subjects completed 3 exercise trials. Each exercise trial consisted of baseline, exercise (control), exercise during inflation, exercise after deflation, and recovery measurements (3 min each). Exercise trials were performed during control (no drug), N^G-monomethyl-l-arginine (l-NMMA; protocol 1), adenosine receptor inhibition (aminophylline; protocol 2), and combined l-NMMA-aminophylline infusions (both protocols). Each trial was separated by at least 20 min of rest to allow forearm blood flow (FBF) to return to baseline values. Adenosine (ADO) dose-response infusions were performed at the start of the study under control (no drug) conditions and repeated during l-NMMA (protocol 1), aminophylline (protocol 2), and combined l-NMMA-aminophylline (both protocols). MVC, maximal voluntary contraction.

Fig. 2.

Percent recovery in FBF (A) and forearm vascular conductance (FVC; B) during balloon inflation (protocol 1; n = 9). Nitric oxide synthase inhibition (l-NMMA) reduced the % recovery of FBF and FVC compared with the respective % recovery during the control (no drug) trial. Percent recovery of FBF and FVC was reduced even further with combined l-NMMA-aminophylline compared with the l-NMMA trial. *P < 0.01 vs. control (no drug); †P < 0.01 vs. l-NMMA alone.

Fig. 3.

Percent recovery in FBF (A) and FVC (B) during balloon inflation (protocol 2; n = 7). Adenosine receptor inhibition (aminophylline) reduced the % recovery of FBF and FVC compared with the respective % recovery during the control (no drug) trial. Percent recovery of FBF and FVC was reduced even further with combined aminophylline-l-NMMA compared with the aminophylline trial. *P < 0.01 vs. control (no drug); †P < 0.05 vs. aminophylline alone.

Fig. 4.

Timing of flow restoration. In protocol 1 (A), the time to reach steady-state blood flow during balloon inflation under nitric oxide synthase inhibition (l-NMMA) was increased. The combined inhibition of nitric oxide synthase and adenosine (l-NMMA-aminophylline) did not delay the restoration of flow beyond l-NMMA alone. In protocol 2 (B), the restoration of flow was not delayed during adenosine receptor inhibition (aminophylline) alone. Combined aminophylline-l-NMMA increased the time to reach steady-state blood flow compared with the no-drug and aminophylline-alone trials. *P < 0.05 vs. control (no drug); †P < 0.05 vs. aminophylline alone.

Fig. 5.

Vasodilator responses (ΔFVC) to incremental adenosine administration during saline, l-NMMA, aminophylline, and combined l-NMMA-aminophylline administration. In protocol 1 (A), nitric oxide synthase inhibition (l-NMMA) reduced ΔFVC at all doses of exogenous adenosine (ADO) compared with control (saline). Additionally, there was a significant main effect of drug during combined l-NMMA-aminophylline on the responsiveness to exogenous ADO compared with l-NMMA alone. In protocol 2 (B), adenosine receptor inhibition (aminophylline) reduced ΔFVC at all doses of exogenous ADO compared with control (saline). Combined aminophylline-l-NMMA did not reduce ΔFVC compared with aminophylline alone. *P < 0.01 vs. l-NMMA (protocol 1) or aminophylline (protocol 2) and combined l-NMMA-aminophylline; †P < 0.01 main effect combined l-NMMA-aminophylline vs. l-NMMA alone.