Changes in extracellular [K+] and [Ca2+] induced by anoxia in neonatal rabbit medulla

Ion-selective microelectrodes were used to record levels of extracellular K+ and Ca2+ concentrations ([K+]o and [Ca2+]o, respectively) and the changes evoked by anoxia in the medulla of anesthetized ventilated newborn rabbits, 1-14 days of age. Resting levels of [K+]o showed significant decline with increasing age. The mean initial level of [K+]o was 6.5 +/- 0.3 mM for the 1st neonatal wk and 5.1 +/- 0.4 mM for the 2nd wk. Brief periods of N2 inhalation (1-3 min) induced reversible increases in [K+]o that were smaller with progression of maturity. The duration of the initial increase (phase I) in [K+]o was twofold greater during the 1st wk than the 2nd wk. At the maximal steady-state level of change (phase III), [K+]o increased by 1.8 +/- 0.2 mM during wk 1 and by 0.9 +/- 0.1 mM during wk 2. Large transient bursts of increase were frequently observed during both early and recovery phases of the sustained [K+]o accumulation. Resting levels of [Ca2+]o also showed age dependence. The mean values during early neonatal days 1-3, compared with days 7-10, were 3.7 +/- 0.3 and 2.4 +/- 0.6 mM, respectively. Anoxia evoked multiphasic decreases in [Ca2+]o that were largest in the youngest pups. Early transient falls of 0.5 +/- 0.1 mM occurred at times similar to those for the earliest increases of [K+]o and negative focal potentials; larger decreases (0.9 +/- 0.2 mM) were recorded after reoxygenation.(ABSTRACT TRUNCATED AT 250 WORDS)