Proceedings of The Physiological Society

Kings College London (2005) J Physiol 565P, PC164


Cat chemosensory and ventilatory responses to hypoxia are potentiated by exposure to intermittent hypoxia

Rey, Sergio ; Del Rio, Rodrigo ; Alcayaga, Julio ; Iturriaga, Rodrigo ;

1. Fisiologia, P. Universidad Catolica de Chile, Santiago, Chile. 2. Biologia, Universidad de Chile, Santiago, Chile.

It has been proposed that carotid body (CB) chemoreceptors play a main role on the enhanced hypoxic ventilatory response induced by chronic intermittent hypoxia (CIH). Therefore, we studied the effects of cyclic hypoxic episodes of short duration on cat cardiorespiratory reflexes, heart rate variability, and CB chemosensory activity. Cats were exposed to cyclic hypoxic episodes (PO2 ∽ 75 Torr) repeated during 8 hours for 4 days. Acute experiments were performed the morning of the day after the end of hypoxic exposures. Cats were anaesthetized with sodium pentobarbitone (40 mg Kg-1 I.P., followed by 8-12 mg I.V.), and ventilatory and cardiovascular responses to several isocapnic levels of oxygen (PO2 ∽ 20 to 740 Torr) were studied. After studying of the reflex responses, we recorded the CB chemosensory responses to hypoxia. The experimental protocol was approved by the Ethical Committee of the Facultad de Ciencias Biologicas of the Universidad Catolica de Chile and met the guidelines of the Chilean National Fund for Scientific and Technological Development (FONDECYT). All results expressed as mean ± S.E.M. Results showed that exposure of cats to CIH enhanced the ventilatory responses (VT and VI) to acute hypoxia. The curves for the relationships between PO2 and VI and VT were significantly different in 4 CIH cats as compared with 6 control cats (P < 0.001, two-way ANOVA) and the Bonferroni test showed that VI and VT were statistically higher (P > 0.05) at a PO2 of about 20 Torr. Similarly, exposure to CIH increased basal CB discharges (63.2 ± 7.3 vs. 36.3 ±5.1 Hz, P < 0.05, Student T test, n = 6 CIH and control cats, respectively) and the responses to acute hypoxia (PO2 < 100 Torr, P < 0.05 two-way ANOVA followed by Bonferroni test, n = 6). Exposure to CIH did not increase basal arterial pressure, heart rate, or their changes induced by acute hypoxia. However, we found that the low/high frequency ratio of heart rate variability was significantly higher (P < 0.001, Student T Test) in 6 CIH cats (2.4 ± 0.1) as compared to 6 control cats (0.6 ± 0.1). Thus, the enhanced CB reactivity to hypoxia may contribute to the augmented ventilatory response to hypoxia, as well as to modified heart rate variability due to early changes in autonomic activity.

Where applicable, experiments conform with Society ethical requirements