Role for adenosine A1 and A2 receptors in femoral vasodilatation induced by intra-arterial adenosine in rabbits

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Combined contribution of endothelial relaxing autacoides in the rat femoral artery response to CPCA: an adenosine A2 receptor agonist

We examined the contribution of endothelial relaxing factors and potassium channels in actions of CPCA, potent adenosine A(2) receptor agonist, on isolated intact male rat femoral artery (FA). CPCA produced concentration-dependent relaxation of FA, which was notably, but not completely, reduced after endothelial denudation. DPCPX, A(1) receptor antagonist, had no significant effect, while SCH 58261 (A(2A) receptor antagonist) notably reduced CPCA-evoked effect. Pharmacological inhibition of nitric oxide synthase or cyclooxygenase comparably reduced CPCA-evoked action, still in a lesser degree than after denudation. In the presence of buffer with high K(+) (100  mM), CPCA-produced relaxations were almost abolished. TEA (nonselective K(Ca) blocker), glibenclamide (K(ATP) blocker), Ba(++) (K(IR) blocker), or ouabain (Na(+)/K(+)-ATPase inhibitor) did not change CPCA-induced relaxation. Concentration-response curve for CPCA was significantly shifted to the right after the incubation of apamin (SK channel blocker). CPCA produced concentration-dependent relaxation of FA that was partly dependent on endothelial cells. Endothelium-related portion of CPCA-elicited effect was mediated by combined action of endothelial NO, prostacyclin, and EDHF after activation of endothelial A(2A) receptors. Small conductance K(Ca) channels were involved in this action.

Coronary Blood Flow Responses are Impaired Independent of NO and Endothelial Function in Conscious Dogs with Dilated Cardiomyopathy

BACKGROUND

Dilated cardiomyopathy (DCM) is characterized by nitric oxide (NO) deficiency and endothelial dysfunction. Whether endothelium-independent vasodilation is preserved, particularly in the coronary circulation, remains controversial.

METHODS AND RESULTS

We studied systemic and coronary flow responses to the endothelium-dependent agonist, acetylcholine, the cGMP-dependent NO-donor, nitroglycerin, the predominantly endothelium-independent agonist, adenosine, the beta-adrenergic cAMP-dependent agonist, isoproterenol, and the calcium channel antagonist, nicardipine, in conscious dogs with pacing-induced DCM. Systemic blood flow response was impaired to acetylcholine but preserved to other vasodilators in DCM. In contrast, coronary blood flow response was significantly ( P < .05) depressed to all agonists. (Peak coronary blood flow response, control versus DCM: acetylcholine: 221 +/- 14% versus 156 +/- 11%; nitroglycerin: 220 +/- 17% versus 138 +/- 9%; adenosine: 635 +/- 65% versus 376 +/- 56%; nicardipine: 338 +/- 59% versus 115 +/- 23%; isoproterenol: 219 +/- 18% versus 86 +/- 20%). The attenuation was independent of systemic hemodynamic differences.

CONCLUSION

In contrast to systemic responses, coronary blood flow responses in DCM are impaired dependent or independent of NO or second messenger mechanisms, implying either distal signaling defects or structural abnormalities in the coronary vasculature.

The effect of caffeine on peripheral vascular resistance in isolated perfused guinea pig hind limbs

BACKGROUND

The role of caffeine in cardiovascular disease is controversial. Most of its pharmacologic actions are attributed to its role as an adenosine antagonist. Adenosine is one of the most important endogenous vasodilatative substances and is released under ischemic conditions, for example, in the skeletal muscle of patients with peripheral arterial occlusive disease. We aimed to investigate the influence of caffeine on peripheral vascular resistance and on the beneficial vasodilatory effect of adenosine in isolated perfused guinea pig hind limbs.

MATERIALS AND METHODS

(1) Caffeine was administered at 0.5, 5, and 50 micromol/L under normoxic conditions. (2) The vasculature of the perfused guinea pig hind limb was precontracted with noradrenaline (3 micromol/L), followed by adenosine (10 micromol/L) under normoxic conditions. When vascular resistance (VR) had reached a steady state, caffeine was administered additionally at dosages of 0.5, 5, and 50 micromol/L. (3) This protocol was repeated using iloprost 0.1 micromol/L instead of adenosine as vasodilatory substance. (4) Under hypoxia, caffeine was again administered at the above dosages. (5) Under hypoxia, experiments with adenosine A2-receptor antagonists (alloxazine 10 micromol/L and ZM 241385 100 nmol/L) were done.

RESULTS

Under normoxic conditions, 0.5 and 5 micromol/L caffeine had nearly no effect on vascular resistance compared with baseline conditions. A slight, but statistically not significant decrease in VR was achieved with 50 micromol/L caffeine. In the presence of noradrenaline, the vasodilatory effect of adenosine was reduced by 7.6 +/- 1.6% after the addition of 0.5 micromol/L caffeine, and by 37.3 +/- 3.8% at a dosage of 5 micromol/L caffeine. A dosage of 50 micromol/L caffeine completely abolished the vasodilatative effect of adenosine. In the presence of iloprost, only a slight but statistically insignificant inhibitory influence (0.9%) of caffeine at a dosage of 50 micromol/L could be seen. Hypoxia significantly reduced VR. Caffeine at 0.5 micromol/L diminished this effect by about 53.2 +/- 4.6% and abolished it at 5 and 50 micromol/L. The hypoxia-induced adenosine-mediated vasodilatation seems to be an adenosine A2A-receptor-mediated effect.

CONCLUSIONS

The observed effect of hypoxia-induced vasodilatation in peripheral arteries may be the result of the vasodilatory effect of elevated endogenous adenosine during hypoxia. For patients with peripheral arterial disease, drinking of caffeine-containing beverages may reduce the beneficial vasodilatory effect of elevated endogenous adenosine levels.

Glucose-induced islet blood flow increase in rats: interaction between nervous and metabolic mediators

This study investigated the mechanisms for glucose-induced islet blood flow increase in rats. The effects of adenosine, adenosine receptor antagonists, and vagotomy on islet blood flow were evaluated with a microsphere technique. Vagotomy prevented the islet blood flow increase expected 3, 10, and 20 min after injection of glucose, whereas theophylline (a nonspecific adenosine receptor antagonist) prevented the islet blood flow increase from occurring 10 and 20 min after glucose administration. Administration of selective adenosine receptor antagonists suggested that the response to theophylline was mediated by A1 receptors. Exogenous administration of adenosine did not affect islet blood flow, but local accumulation of adenosine, induced by the adenosine uptake inhibitor dipyridamole, caused a doubling of islet blood flow. In conclusion, the increased islet blood flow seen 3 min after induction of hyperglycemia is caused by the vagal nerve, whereas the increase in islet blood perfusion seen at 10 and 20 min after glucose administration is caused by both the vagal nerve and adenosine.

Comparison of the Effect of Prostaglandin E(1), Prostacycline and Adenosine on Peripheral Vascular Resistance

Prostacycline, prostaglandin E(1), and adenosine are highly effective vasodilators. These three drugs are widely used in the treatment of peripheral arterial occlusive disease. The aim of the study was to compare the vasodilatory potency of these substances in the isolated perfused guinea pig hind limb. After equilibration with Tyrode's solution and precontraction with noradrenaline 3µM, prostaglandin E(1) and adenosine were administered at dosages of 0.1, 0.3, and 1µM, whereas prostacycline was administered at a dosage of 0.01, 0.03 and 0.1µM. 0.01µM prostacycline, 0.1 µM prostaglandin E(1), and 0.1µM adenosine were the lowest dosages at which a significant vasodilation could be reached for each substance. The reduction of peripheral vascular resistance at comparable dosages of 0.1µM was 11.0 +/- 2.6% (x +/- SEM, n = 5) for adenosine, 12.0 +/- 1.0% (n = 5) for prostaglandin E(1), but 28.0 +/- 9.3% (n = 5) for prostacycline (p < 0.05 versus adenosine and prostaglandin E(1)). Even at a dosage of 0.01 µM prostacycline, a comparable reduction in peripheral vascular resistance (16.0 +/- 2.8%) could be reached, compared to a ten-fold higher dosage of prostaglandin E(1) and adenosine. At the highest concentration of 1 µM, the vasodilatory effect of adenosine was significantly less expressed, compared to that of prostaglandin E(1) (18.0 +/- 3.4% versus 33.0 +/- 4.7%). In summary, prostacycline, at a ten-fold lower concentration, showed comparable vasodilatory effects to adenosine and prostaglandin E(1). The rank order at the vasodilatory potency is prostacycline > prostaglandin E(1) > adenosine. </hea