Chemosensitivity of sympathoexcitatory neurones in the rostroventrolateral medulla of the cat

The hypothesis that sympathoexcitatory neurones within the rostroventrolateral medulla (RVLM) may be chemosensitive was tested in chloralose-anaesthetized cats by artificial perfusion of the RVLM via the left vertebral artery. The baroreceptors and peripheral chemoreceptors were denervated by bilaterally dissecting the carotid sinus and vagus nerves. Either white ramus T₃ (WR-T₃) or the renal nerve was recorded to monitor sympathetic activity. Perfusion with saline or Ringer solution bubbled with CO₂ (10%–100%) produced a rapid and pronounced increase in sympathetic activity and blood pressure. Solutions adjusted to the same pH (pH 5.2 for 100% CO₂) with HCl resulted in a much weaker excitation. A linear relationship between PCO₂ and sympathetic activity was demonstrated. During prolonged perfusion (90 s) sympathetic activity returned to the control level after initial excitation and fell below control levels when perfusion ceased. The sympathetic activity response to CO₂-bubbled solutions was unaffected by blockade of synaptic input by microinjection of CoCl₂ into the RVLM, whereas spontaneous sympathetic activity and the supraspinal somato-sympathetic reflex from intercostal nerve T₄ to WR-T₃ were markedly reduced. It is therefore concluded that sympathoexcitatory bulbospinal neurones in the RVLM are directly chemosensitive to changes in arterial PCO₂ and pH.This paper is dedicated to Prof. Dr. H. P. Koepchen on the occasion of his 65th birthday