Adaptation to hypercapnia vs. intracellular pH in cat carotid body: Responses in vitro

The hypotheses that the chemosensory discharge rate parallels the intracellular pH (pH(i)) during hypercapnia and that the initial change in pH(i) (Delta pH(i)) is always more than the steady-state Delta pH(i) were studied by using cat carotid bodies in vitro at 36.5 degrees C in the absence and presence of methazolamide (30-100 mg/l). Incremental acidic hypercapnia was followed by an incremental initial peak response and a greater adaptation. A given acidic hypercapnia elicited a rapid initial response followed by a slower adaptation; isohydric hypercapnia produced an equally rapid initial response but of smaller magnitude that returned to near-baseline level; alkaline hypercapnia induced a similar rapid initial response but one of still smaller magnitude that decreased rapidly to below the baseline. Methazolamide eliminated the initial overshoot, which also suggested involvment of the initial rapid pH(i) in the overshoot. These results show that the initial Delta pH(i) is always greater than the steady-state Delta pH(i) and that the glomus can effectively regulate pH(i) during hypercapnia. Also, the steady-state chemoreceptor activity varied Linearly with the extracellular pH, indicating a Linear relationship between extracellular pH and pH(i).