Selective inhibition of the Na+/H+ exchanger type 3 activates CO2/H+-sensitive medullary neurones

Hypercapnia as well as lowered intracellular pH (pH(i)) increase the bioelectric activity of CO(2)/H(+)-sensitive neurones (VLNcs) of the ventrolateral medulla oblongata. Here we describe that immunoreactive Na(+)/H(+) exchanger (NHE3) is present in ventrolateral neurones from medullary organotypic cultures (obex level). To test whether VLNcs can be acidified and thereby activated by inhibition of NHE3, we used the novel high-affinity NHE3-inhibitors S1611 and S3226. Both drugs raised the firing rates of VLNcs to at least 150% of the control values, and depolarized membrane potential by up to 15 mV at concentrations (0.5-1 mu mol/l) suitable for selective inhibition of NHE3. The changes in bioelectric activity strongly resembled the responses to hypercapnia (PCO(2): 60-100 mmHg). In BCECF-AM-loaded cultures a subfraction of ventrolateral VLNcs was found to be intracellularly acidified by 0.05-0.1 pH units following treatment with S1611; the time course of this acidification was similar to that evoked by hypercapnia. All drug effects were sustained and readily reversible upon washing, Non-CO(2)/H(+)-responsive medullary neurones as well as hippocampal CA3 neurones were unaffected by up to 20 mu mol/l S1611. It is concluded that the selective inhibition of NHE3 acidifies and activates CO(2)/H(+)-sensitive neurones within the ventrolateral medulla oblongata.