OXYGEN-SUPPLY AND ION HOMEOSTASIS OF THE RESPIRATORY NETWORK IN THE IN-VITRO PERFUSED BRAIN-STEM OF ADULT-RATS

An in vitro arterially perfused medulla preparation of 3- to 8-week-ol d rats is described in which synchronous rhythmic activity (frequency 4.5+/-1.7 cycles/min, burst duration 3.1+/-1.1 s, n=40) was recorded f rom hypoglossal (XII), vagal (X), or spinal (C1-2) nerves and from dif ferent classes of neurons in the region of the ventral respiratory gro up (VRG). Stimulation of dorsal X nerve rootlets produced a reversible blockade of rhythmic activity. Under steady-state conditions, tissue oxygen (pO(2)) in the VRG (depth of 600-1600 mu m below the ventral su rface) fell from 180 to 40 mmHg. Extracellular K+ activity (aK(e)) in the VRG was about 0.3 mM higher, calcium concentration ([Ca](e)) did n ot differ, and pH (pH(e)) was about 0.27 units lower than in the perfu sion or superfusion solution (with an aK(e) of 2.2 mM, a [Ca](e) of 1. 5 mM and a pH(e) of 7.4). During inspiratory XII nerve discharges, rhy thmic increases of aK(e) by up to 0.8 mM were detected in the VRG. Per fusion of N-2-gassed hypoxic solutions (5-10 min) resulted in a tissue anoxia of the VRG and a reversible cessation of rhythmic activity aft er 2-7 min. Such anoxia was accompanied by a rise of aK(e) by up to 35 mM, whereas pH(e) and [Ca](e) fell (from mean levels of 7.17 and of 1 .5 mM, respectively) by more than 0.2 pH units and 1 mM. Similar obser vations were made during a 2- to 5-min arrest of the perfusion pump to simulate ischaemia, whereas significantly larger changes in aK(e), pH (e) and [Ca](e) were revealed during an ''ischaemia'' period of 10 min . The results indicate that the rhythmic activity is generated by the functionally intact respiratory network of the VRG in which neurons ar e under aerobic conditions and ion homeostasis is not impaired. We con clude that the preparation is an appropriate in vitro model for the an alysis of the cellular mechanisms for generation of respiratory rhythm and of metabolic perturbations like anoxia and ischaemia in the matur e respiratory network.