Differentiation of the peripherally mediated from the centrally mediated influences of adenosine in the rat during systemic hypoxia

In two groups of Saffan‐anaesthetized, spontaneously breathing rats we have attempted to identify the peripheral influences of adenosine in mediating the responses evoked by hypoxia by using an adenosine receptor antagonist, 8‐sulphophenyltheophylline (8‐SPT, 20 mg kg‐1 i.v., Group 1) and adenosine deaminase (ADA, 500 units in 0.04 ml infused into the tail artery for 10 min, Group 2); neither of these drugs crosses the blood‐brain barrier. Recordings were made of respiration, heart rate, arterial pressure, blood flow and vascular conductance in the femoral artery, with ankle ligated (FBF and FVC, respectively) and in the carotid artery with all branches except the internal carotid ligated (CBF and CVC, respectively, Group 1 only) in order to indicate responses in skeletal muscle and cerebral vasculature. Hypoxia (breathing 8 or 10% O2 for 10 min) evoked an increase followed by a secondary decrease in respiration, tachycardia followed by secondary bradycardia, a fall in arterial pressure, an increase in FVC and CVC and an increase, followed by a decrease, in CBF. Neither 8‐SPT nor ADA had any significant effect on the secondary decrease in respiration. The secondary bradycardia was unaffected by 8‐SPT, but abolished by ADA. Both drugs reduced the fall in arterial pressure and the increase in FVC; 8‐SPT had no significant effect on the increase in CVC, but CBF no longer fell with arterial pressure. We propose that adenosine contributes to the hypoxia‐induced fall in arterial pressure by causing vasodilatation in skeletal muscle and possibly by causing bradycardia by a direct action on the heart; other evidence suggests that adenosine contributes to the secondary decrease in respiration by acting on central respiratory neurones. The possibility that the fall in arterial pressure and the secondary falls in CBF, respiration and heart rate, can become interdependent in a positive feedback manner is discussed.