Re-setting of the hypoxic sensitivity of aortic chemoreceptors in the new-born lamb.

We tested to see whether the steady-state hypoxic sensitivity of aortic chemoreceptors was re-set during the first 2-3 weeks of post-natal life. Aortic chemo-receptor activity was recorded from the distal end of the cut aortic branch of the cervical vagus in pentobarbitone - anesthetized, new-born lambs. Two groups were studied, the first aged 1-4 days and the second aged 10-19 days. Chemoreceptor discharge increased as hyperbolic function with increasing isocapnic hypoxia in both groups and we quantified the position and the shape of this response curve. It was shifted to the right significantly in the older group of lambs, the mean vertical asymtote increasing from 10.00 to 27.95 torr PO2. No significant difference was found in the horizontal asymotote or in the 'shaping term' between the two groups. The greatest differences between the stimulus-response curves of the two groups of animals with respect to the mean level of discharge and the slope of the curve occurred when PaO2 was below ca. 50 torr. The aortic chemoreceptors of older lambs were unable to maintain a sustained discharge at arterial PO2 values below ca. 30 torr. In contrast, in the younger group PO2 often had to be reduced below this level before discharge increased significantly. We conclude that, like the carotid chemoreceptors, aortic chemoreceptor sensitivity is re-set over the first few weeks of life. The re-setting may contribute to the increase in the ventilatory response to hypoxia which occurs over this period.