Elucidation in the rat of the role of adenosine and A2A-receptors in the hyperaemia of twitch and tetanic contractions

Abstract

Adenosine is implicated in playing a role in blood flow responses to situations where O(2) delivery is reduced (hypoxia) or O(2) consumption is increased (exercise). Strong isometric contractions have been shown to limit vasodilatation, potentially leading to a greater mismatch between and than during twitch contractions. Thus, we hypothesized that adenosine makes a greater contribution to the hyperaemia associated with isometric tetanic than isometric twitch contractions and aimed to elucidate the adenosine-receptor subtypes involved in the response. In four groups of anaesthetized rats, arterial blood pressure (ABP), femoral blood flow (FBF) and tension in the extensor digitorum longus muscle were recorded; isometric twitch and tetanic contractions were evoked by stimulation of the sciatic nerve for 5 min at 4 Hz and 40 Hz, respectively. Groups 1 (twitch) and 3 (tetanic) were time controls for Groups 2 and 4, which received the selective A(2A)-receptor antagonist ZM241385 before the third and 8-sulphophenyltheophylline (8-SPT; a non-selective adenosine receptor antagonist) before the fourth contraction. Time controls showed consistent tension and hyperaemic responses: twitch and tetanic contractions were associated with a 3-fold and 2.5-fold increase in femoral vascular conductance (FVC, FBF/ABP) from baseline, respectively. ZM241385 reduced these responses by 14% and as much as 25%, respectively; 8-SPT had no further effect. We propose that, while twitch contractions produce a larger hyperaemia, adenosine acting via A(2A)-receptors plays a greater role in the hyperaemia associated with tetanic contraction. These results are considered in relation to the A(1)-receptor-mediated muscle dilatation evoked by systemic hypoxia.

Figure 2. Isometric twitch contractions: time controls

A–D show mean (±s.e.m.) EDL tension, FBF, ABP and FVC for time control stimulation 1 (×, dashed and dotted lines), 2 (▪, continuous lines), 3 (○, dashed lines) and 4 (▴, dotted lines) in the 1 min before (baseline), 5 min of (S1–5) and 7 min after (R1–7) sciatic nerve stimulation (4 Hz). *P < 0.05 time control 1 S1–5 and/or R1–7, vs time control 2–4. There was no significant difference in tension, FBF, ABP and FVC at any time point for time controls 2–4. n= 7.

Figure 1. Isometric twitch contractions

Representative original trace showing ABP (A), EDL tension (B), mean FBF (C) and mean FVC (D), before, during and after 5 min isometric twitch contractions.

Figure 7. Comparison of change in integrated tension (ΔInt tension; tension time index (TTI)) and the change in integrated FVC (ΔInt FVC) for isometric twitch and tetanic contractions

A and B show mean (±s.e.m.) ΔInt tension/TTI (S1–5), while C and D show mean (±s.e.m.) ΔInt FVC (S1–5 + R1–7) for time control isometric twitch (Group 1) and tetanic (Group 3) contractions (*P < 0.05 vs time control 2). E and F show ΔInt tension/TTI (S1–5) while G and H show ΔInt FVC (S1–5 + R1–7) for 5 min isometric twitch (Group 2) and tetanic (Group 4) contraction before and after ZM241385 and after ZM241385 + 8-SPT. P values shown in G are vs control 2, §P < 0.05 vs control 2. There is no significant difference between ΔInt FVC for ZM241385 and ZM241385 + 8-SPT in G or H. †P < 0.05 isometric twitch vs tetanic for equivalent periods of contraction.

Figure 3. Involvement of adenosine in the response to isometric twitch contractions

A–D show mean (±s.e.m.) EDL tension, FBF, ABP and FVC for control stimulation 2 (▪, continuous lines), stimulation after ZM241385 (○, dashed lines) and stimulation after ZM241385 + 8-SPT (▴, dotted lines) in the 1 min before (baseline), 5 min of (S1–5) and 7 min after (R1–7) sciatic nerve stimulation (4 Hz). §P < 0.05 control 2 S1–5 and/or R1–7, vs stimulation after ZM241385 and stimulation after ZM241385 + 8-SPT. There was no significant difference in tension, FBF, ABP and FVC at any time point between stimulation after ZM241385 and stimulation after ZM241385 + 8-SPT. n= 10.

Figure 4. Isometric tetanic contraction

Representative original trace showing ABP (A), mean FBF (B), EDL tension (C) and mean FVC (D), before, during and after a 5 min isometric tetanic contraction. There is a fall in mean FBF in the first 20 s of sciatic nerve stimulation corresponding with the peak tension developed by the EDL and an increase in mean FBF for 10 s immediately after the cessation of muscle contraction. E shows a 2 s trace of FBF from the plateau phase of isometric tetanic contraction, illustrating pulsatile blood flow.

Figure 5. Isometric tetanic contraction: time controls

A–D show mean (±s.e.m.) EDL tension, FBF, ABP and FVC for time control stimulation 1 (×, dashed and dotted lines), 2 (▪, continuous lines), 3 (○, dashed lines) and 4 (▴, dotted lines) in the 1 min before (baseline), 5 min of (S1–5) and 7 min after (R1–7) sciatic nerve stimulation (40 Hz). *P < 0.05 time control 1 S1–5 and/or R1–7, vs time control 2–4. There was no significant difference in tension, FBF, ABP and FVC at any time point for time controls 2–4. n= 7.

Figure 6. Involvement of adenosine in the response to isometric tetanic contraction

A–D show mean (±s.e.m.) EDL tension, FBF, ABP and FVC for control stimulation 2 (▪, continuous lines), stimulation after ZM241385 (○, dashed lines) and stimulation after ZM241385 + 8-SPT (▴, dotted lines) in the 1 min before (baseline), 5 min of (S1–5) and 7 min after (R1–7) sciatic nerve stimulation (40 Hz). §P < 0.05 time control 2 S1–5 and/or R1–7, vs stimulation after ZM241385 and stimulation after ZM241385 + 8-SPT. There is no significant difference in tension, FBF, ABP and FVC at any time point between stimulation after ZM241385 and stimulation after ZM241385. n= 10.