Mechanisms of Na+ and Ca2+ influx into respiratory neurons during hypoxia

Changes in intracellular Na+ and Ca2+ in inspiratory neurons of neonatal mice were examined by using ion-selective fluorescent indicator dyes SBF1 and fura-2, respectively. Both [Na+](i) and [Ca2+], signals showed rhythmic elevations, correlating with the inspiratory motor output. Brief (2-3 min) hypoxia, induced initial potentiation of rhythmic transients followed by their depression. During hypoxia, the basal [Na+](i) and [Ca2+](i) levels slowly increased, reflecting development of an inward current (I-m). By antagonizing specific mechanisms of Na+ and Ca2+ transport we found that increases in [Na+](i), [Ca2+](i) and I-m due to hypoxia are suppressed by CNQX, nifedipine, riluzole and flufenamic acid, indicating contribution of AMPA/kainate receptors, persistent Na+ channels, L-type Ca2+ channels and Ca2+ -sensitive non-selective cationic channels, respectively. The blockers decreased also the amplitude of the inspiratory bursts. Modification of mitochondrial properties with FCCP and cyclosporine A decreased [Ca2+](i) elevations due to hypoxia by about 25%. After depletion of internal Ca2+ stores with thapsigargin, the blockade of NMDA receptors, Na+/K+ pump, Na+/H+ and Na+/Ca2+ exchange, the hypoxic response was not changed. We conclude that slow [Na+](i) and [Ca2+](i) increases in inspiratory neurons during hypoxia are caused by Na+ and Ca2+ entry due to combined activation of persistent Na+ and L-type Ca2+ channels and AMPA/kainate receptors. (c) 2005 Elsevier Ltd. All rights reserved.