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.

The reflex ventilatory responses of conscious, newborn rats to alternations of inspiratory oxygen concentration.

We examined the effect of a dynamic, hypoxic stimulus upon the reflex respiratory responses of 15, conscious rat pups on post-natal days 5-7 in order to ascertain the influence of a non-adapting peripheral chemoreceptor discharge upon respiratory control during hypoxia in the newborn. Respiration was measured as integrated airflow into and out of a body plethysmograph. The respiratory response to 6 minutes of a 16-breath cycle (approximately 5 s) in FiO2 between 0.21 and 0.10 (alternating hypoxia) was compared with the response to 6 min of a constant FiO2 of 0.12 (non-alternating hypoxia). Ventilation increased significantly from a control level of 0.12 +/- 0.02 ml/s (mean +/- SEM) to 0.18 +/- 0.02 and 0.17 +/- 0.02 ml/s in non-alternating and alternating hypoxia runs respectively during the first minute (phase 1) of each run, after which ventilation in both run types fell progressively and significantly back towards control levels to reach, by the sixth minute (phase 2), 0.13 +/- 0.01 and 0.12 +/- 0.02 ml/s respectively. No significant difference was found between the levels of ventilation in non-alternating hypoxia and alternating hypoxia during either phase 1 (P greater than 0.10) or phase 2 (P greater than 0.60). No significant alternation was found in any respiratory variable at the frequency of the 16-breath hypoxic cycle during either phase 1 or phase 2 of non-alternating hypoxia. However, a significant alternation, at this frequency, of 37 +/- 6% (P less than 0.05 compared to control) was found in ventilation during phase 1 of alternating hypoxia which was further increased to 62 +/- 8% (P less than 0.05 compared to phase 1) during phase 2. In phase 1 the alternation was due primarily to significant alternation in inspiratory time whilst in phase 2 significant alternation also occurred in tidal volume, expiratory time and mean inspiratory flow. Our results show that the magnitude of hypoxic ventilatory depression (HVD) in the newborn is not affected by an alternating hypoxic stimulus and that, during phase 2, ventilation can still be stimulated by peripheral chemoreceptors. We suggest that peripheral chemoreceptor adaptation is unlikely to be a major cause of HVD in the newborn rat and that the magnitude of HVD is, in part, the result of a competitive interaction between peripheral chemoreceptor stimulation and a centrally-mediated inhibitory action of hypoxia.