ANOMALOUS HYPOXIC ACIDIFICATION OF MEDULLARY VENTRAL SURFACE

In castrated male goats, two flexible catheters, one open ended for reference and the other ending in a 1-mm-diam glass bulb pH electrode, were advanced ventrally through a left posterior fossa craniotomy into the subarachnoid space between the 9th and 10th cranial nerve roots, passing medially into cerebrospinal fluid (CSF) over the medullary ventral surface (MVS). They were anchored to dura and fascia, tunneled under the scalp, and terminated in connectors on the left horn. After several days for recovery, while the animals were awake, the effects of CO2 and hypoxia on pH of the film of CSF between the pia and arachnoid (pH(MVS)) were recorded along with end-tidal PCO2 and PO2 (mass spectrometer), ventilation (pneumotachometer) through a permanent tracheostomy, and, when possible, ear arterial O2 saturation (SaO2). High PCO2 acidified MVS as expected: DELTA-pH(MVS)/DELTA-logPCO2 = -0.64 +/- 0.14, producing a ventilatory response slope DELTA-VI/DELTA-pH(MVS) = 372 l/min. Hypoxia resulted in acid shifts even when PCO2 was allowed to fall. The development of hypoxic acidosis was related to the location of pH electrodes determined at necropsy. In isocapnic hypoxia, pH over putative chemoreceptor surfaces fell in proportion to desaturation: DELTA-pH(MVS) = 0.0033(Sa(O2)) - 0.34, r = 0.80, S(y.x) = 0.025. With uncontrolled arterial PCO2, similar acidosis occurred when Sa(O2) fell below 85-90%: DELTA-pH(MVS) = 0.0039(Sa(O2)) - 0.34, r = 0.88, S(y.x) = 0.032. With constant hypoxia, pH fell (tau = 3.7 +/- 2.2 min) to a plateau after 10-20 min and showed rapid recovery (tau = 2.0 +/- 1.3 min). CO hypoxia acidified MVS; DELTA-pH(MVS) = 0.0032-DELTA-HbCO where HbCO is carboxy hemoglobin without stimulating ventilation. The pH(MVS) recorded from electrodes found to lie over the pons, pyramidal tracts near midline, or dorsal spinal cord at C1-C2 showed alkaline shifts in hypoxia even with Sa(O2) as low as 20%. We conclude that in hypoxia the MVS rapidly and locally generates more acid (presumably lactic) than adjacent brain surfaces. Because this acid fails to stimulate ventilation, extracellular fluid pH appears not to be the unique signal of these chemoreceptors.