Effects of hypoxia and hypercapnia on whole body release and clearance of choline.

We have recently demonstrated an increase in arterial blood choline (Ch) concentration in normocapnic hypoxia and apnea. This could be due to enhanced release of free Ch from tissues, to decreased Ch clearance, or both. The present investigations was undertaken to determine the individual contributions of these processes to the whole body balance of Ch, using an intravenous infusion of tracer quantities of [2H4]Ch to assess the bidirectional flux between the central pool and peripheral pools. Rats were subjected to normocapnic hypoxia or hypercapnia; release and clearance of Ch were calculated using a simple model. Hypoxia caused an increase in Ch production and a decrease in Ch clearance. At severe levels of hypoxia, Ch clearance was essentially zero. Hypoxia was attended by progressive acidosis that was related to the magnitude of the hypoxic challenge. To determine the possible effects of acidosis per se on the variables measured, respiratory acidosis with normoxia was provoked by controlled administration of CO2. Under these conditions, parallel decreases in Ch production and Ch clearance were observed.