Regional medullary blood flow during isocapnic hyperpnea in anesthetized cats

This study was performed to test the hypothesis that an increase in the activity of medullary respiratory neurons, which is not associated with change in arterial blood gas tensions, results in an increase in the blood flow within the medulla. We studied 12 anesthetized cats during isocapnic hyperpnea induced by either electrical stimulation of hindlimb muscle groups (HS) (change in minute ventilation equaled 220%) or by carotid sinus nerve stimulation (CNS) (change in peak phrenic activity·min^-1 equaled 180%). Changes in total brain blood flow (BBF) and in ventral and dorsal medullary blood flow were measured using the radioactive microsphere (15 μm) technique. In each cat measurements were made both before and after isocapnic hyperpnea had been sustained for 4 min. Although HS did not produce any changes in either total BBF or dorsal medullary blood flow, there was a significant (P < 0.05) increase in ventral medullary blood flow (+ 22%). CNS stimulation produced small increases (P < 0.05) in total BBF (+ 12%) and dorsal medullary blood flow (+ 6%), and larger increases in ventral medullary blood flow (19%). The increase in ventral medullary blood flow was significantly (P < 0.05) greater than the increase in the dorsal medullary or total BBF. The data indicate that stimulation of respiratory motor neurons without change in arterial blood composition enhances perfusion of the ventral medulla. The findings suggest that motor neuron stimulation is accompanied by a substantial increase in metabolic rate of the ventral medulla with release of metabolic products which mediate the vasodilatation. These metabolic products could also act as positive feedback stimuli for the central chemoreceptor.