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Department of Physiology, Medical School, University of Birmingham, UK.

1. The effect of chronic hypoxaemia upon in vitro carotid body chemosensitivity was observed in eight rats > 5 weeks of age born and reared in 12% oxygen. Comparisons were made with eight age-matched normoxic rats. 2. Single exponential functions with offset were fitted to the normalized (percentage of maximum) discharge responses to ramp decreases in PO2 at three steady levels of PCO2. CO2 sensitivity was derived from these functions. 3. Increasing hypercapnia increased the horizontal asymptote of the exponential functions in the normoxic (0.15 +/- 0.03% discharge per mmHg PCO2; P < 0.001) and chronically hypoxic (0.13 +/- 0.04% discharge per mmHg PCO2; P < 0.005) animals but was without effect upon the rate constants in both groups (-0.04 +/- 0.18 mmHg PO2 per mmHg PCO2, P > 0.50 and 0.63 +/- 0.48 mmHg PO2 per mmHg PCO2, P > 0.20, respectively). Rate constants were greater in the chronically hypoxic animals (P < 0.05) compared with the normoxic animals. 4. CO2 chemosensitivity increased with decreasing PO2 in normoxic (P < 0.05) but not in chronically hypoxic (P > 0.50) rats. 5. Our results show that chronic hypoxaemia from birth attenuates the maturation of CO2-O2 interaction at the carotid body.

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