AJP - Regulatory, Integrative and Comparative Physiology, Vol 242, Issue 3 195-R198, Copyright © 1982 by American Physiological Society
Ventral medullary extracellular fluid pH and blood flow during hypoxia
W. F. Nolan, P. C. Houck, J. L. Thomas and D. G. Davies
Vascular responses of the ventral medulla and total brain to 30-60 min of
isocapnic hypoxia (PaO2 = 32 +/- 2 Torr) were examined using radioactive
microspheres in anesthetized, paralyzed, and artificially ventilated cats.
Ventral medullary extracellular fluid (ECF) pH was measured using pH
microelectrodes with tip diameters of 1-2 micrometers. Total brain blood
flow (Q) increased significantly from a control value of 53 +/- 8 (mean +/-
SE) to 160 +/- 42 ml.100 g-1.min-1 following 30-60 min of hypoxia. Ventral
medullary Q increased from 28 +/- 5 to 97 +/- 20 ml.100 g-1.min-1 and ECF
pH decreased by 0.15 +/- 0.06 pH U. Q responses are attributable to
decreased vascular resistance as arterial pressure remained constant. The
sensitivity of the ventral medullary vasculature to isocapnic hypoxia did
not differ from that of the brain as a whole. The results show that under
the conditions of our experiment, the ventral medullary vascular response
to hypoxia is not sufficient to stabilize local ECF pH. The observation of
simultaneously reduced pH and increased Q is consistent with a role for ECF
H+ in mediating the cerebrovascular response to hypoxia.
