Adenosine receptor subtypes and vasodilatation in rat skeletal muscle during systemic hypoxia: a role for A1 receptors

Abstract

1. In anaesthetized rats we tested responses evoked by systemic hypoxia (breathing 8% O2 for 5 min) and adenosine (i.a. infusion for 5 min) before and after administration of a selective adenosine A1 receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine), or a selective adenosine A2A receptor antagonist ZM 241385. Arterial blood pressure, (ABP), heart rate (HR), femoral blood flow (FBF) and femoral vascular conductance (FVC: FBF/ABP) were recorded together with the K+ concentration in arterial blood ([K+]a) and in venous blood of hindlimb muscle ([K+]v) before and at the 5th minute of hypoxia or agonist infusion. 2. In 12 rats, DPCPX reversed the fall in ABP and HR and the increase in FVC evoked by the selective A1 agonist CCPA (2-chloro-N6-cyclopentyladenosine; i.a. infusion for 5 min). DPCPX also reduced both the increase in FVC induced by hypoxia and that induced by adenosine; the control responses to these stimuli were comparable in magnitude and both were reduced by approximately 50%. 3. In 11 rats, ZM 241385 reversed the fall in ABP and increase in FVC evoked by the selective A2A agonist CGS 21680 (2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadeno sin e hydrochloride; i.a. infusion for 5 min). ZM 241385 also reduced the increase in FVC induced by adenosine by approximately 50 %, but had no effect on the increase in FVC induced by hypoxia. 4. In these same studies, before administration of DPCPX, or ZM 241385, hypoxia had no effect on the venous-arterial difference for K+ ([K+]v-a), whereas after administration of either antagonist, hypoxia significantly reduced [K+]v-a suggesting an increase in hypoxia-induced K+ uptake, or a reduction in K+ efflux. 5. These results indicate that both A1 and A2A receptors are present in hindlimb muscle and can mediate vasodilatation and that A1 and A2A receptors contribute equally to dilatation induced by infused adenosine. However, they suggest that endogenous adenosine released during systemic hypoxia induces dilatation only by acting on A1 receptors. Given previous evidence that adenosine can stimulate receptors on skeletal muscle fibres that are coupled to ATP-sensitive K+ (KATP) channels so promoting K+ efflux, our results allow the proposal that KATP channels may be coupled to both A1 and to A2A receptors and may be stimulated to open by adenosine released during hypoxia, but indicate that, during systemic hypoxia, K+ efflux caused by either receptor subtype makes a very minor contribution to the muscle vasodilatation.

Figure 1. Cardiovascular responses evoked by systemic hypoxia (8 % O₂) for 5 min before and after administration of the A₁ receptor antagonist DPCPX

Means ±s.e.m. recorded at the times indicated; hypoxia began at time 0. ▾, control values; •, values after administration of DPCPX (0.1 mg kg⁻¹i.v.). Abbreviations: ABP; arterial blood pressure; HR, heart rate; FBF, femoral blood flow; FVC, femoral vascular conductance. ††P < 0.01, †P < 0.05, significant difference between values recorded before and after DPCPX.

Figure 2. Cardiovascular responses evoked by infusion of adenosine (1.2 mg kg⁻¹ min⁻¹i.a.) before and after administration of the A₁ receptor antagonist DPCPX

Means ±s.e.m. recorded before (0) and at the 5th minute of infusion, as indicated. □, control; , after DPCPX (0.1 mg kg⁻¹i.v.). ***P < 0.001, **P < 0.01, significant change from baseline (0 min). †††P < 0.001, ††P < 0.01, significant difference between change recorded before and after DPCPX. Abbreviations as in Fig. 1.

Figure 3. Original traces showing cardiovascular changes evoked by infusion of the A₁ receptor agonist CCPA and the effect of the A₁ receptor antagonist DPCPX

Infusion of CCPA (0.35 μg kg⁻¹ min⁻¹i.a.) began at the time indicated by the arrow and continued for 5 min. DPCPX (0.1 mg kg⁻¹i.v.) was given ~2 min after the end of the CCPA infusion as indicated by the arrow. Abbreviations as in Fig. 1.

Figure 4. Cardiovascular changes evoked by the A₁ receptor agonist CCPA and by subsequent administration of the A₁ receptor antagonist DPCPX

Means ±s.e.m.□, control values; ▪, values recorded at the 5th minute of CCPA infusion (0.35 μg kg⁻¹ min⁻¹i.a.); , values recorded after DPCPX (0.1 mg kg⁻¹i.v.). ***P < 0.001, **P < 0.01. Abbreviations as in Fig. 1.

Figure 5. Effects of systemic hypoxia (8 % O₂), and infusion of the A₁ and A_2A receptor agonists CCPA and CGS 21680, respectively, on [K⁺]_a, [K⁺]_v and [K⁺]_v-a, before and after administration of the A₁ or A_2A receptor antagonists DPCPX and ZM 241385, respectively

Means ±s.e.m. CCPA, 0.35 μg kg⁻¹ min⁻¹i.a.; CGS 21680, 1.2 μg kg⁻¹ min⁻¹i.a.; DPCPX, 0.1 mg kg⁻¹i.v.; ZM 241385, 0.05 mg kg⁻¹i.v. In A and C: □, [K⁺]_a; ▪, [K⁺]_v. In B and D, all columns indicate [K⁺]_v-a. Samples were taken before and at the 5th minute of hypoxia or agonist infusion as indicated. **P < 0.01, *P < 0.05, significant difference between [K⁺]_a or [K⁺]_v values recorded at 0 and 5 min. ²²P < 0.01, ²P < 0.05, significant difference between [K⁺]_v-a values at 0 and 5 min.

Figure 6. Cardiovascular responses evoked by systemic hypoxia (8 % O₂ for 5 min) before and after administration of the the A_2A receptor antagonist ZM 241385

Means ±s.e.m. recorded at the times indicated; hypoxia began at time 0. ▾, control values; •, values after ZM 241385 (0.05 mg kg⁻¹i.v.). Abbreviations as in Fig. 1.

Figure 7. Cardiovascular responses evoked by infusion of adenosine (1.2 mg kg⁻¹ min⁻¹i.a.) before and after administration of the A_2A receptor antagonist ZM 241385

Means ±s.e.m. recorded before (0) and at the 5th minute of infusion as indicated. □, control; , after ZM 241385 (0.05 mg kg⁻¹i.v.). ***P < 0.001, **P < 0.01, *P < 0.05, significant change from baseline (0 min). ††P < 0.01, significant difference between change recorded before and after ZM 241385. Abbreviations as in Fig. 1.

Figure 8. Cardiovascular changes evoked by the A_2A agonist CGS 21680 and by subsequent administration of the A_2A receptor antagonist ZM 241385

Means ±s.e.m.□, control values; ▪, values recorded at the 5th minute of CGS 21680 infusion (1.2 μg kg min⁻¹i.a.); , values recorded after ZM 241385 (0.05 mg kg⁻¹i.v.). ***P < 0.001, *P < 0.05. Abbreviations as in Fig. 1.