INTRAMEDULLARY SODIUM-CYANIDE INJECTION ON RESPIRATORY AND VASOMOTOR RESPONSES IN CATS

To examine the effect of hypoxia confined to the ventrolateral medulla we microinjected NaCN into the cat medulla (1.0 mm below the ventral surface) unilaterally and investigated cardio-respiratory changes. We studied anesthetized artificially ventilated animals and measured the electrical activity of phrenic and cervical sympathetic nerves and blo od pressure. Histotoxic hypoxia depressed phrenic amplitude and elevat ed sympathetic tone and blood pressure. These responses were obtained predominantly from the region 5.0-8.0 mm caudal to the foramen caecum and 3.0-5.0 mm lateral to the midline (intermediate area). A study wit h C-14-cyanide showed that total and covalently bound cyanide was conf ined within a 1 mm diffusion sphere following microinjection. Isolated areas in both rostral and caudal medulla responded to cyanide with el evated sympathetic tone in the absence of phrenic nerve depression, su ggesting dissociation of respiratory and vasomotor responses to hypoxi a. Thus, the respiratory depression and vasomotor excitation produced by central hypoxia can be reproduced by hypoxia limited to discrete re gions of the ventrolateral medulla.