Contribution of Na(+)/Ca(2+) exchange to excessive Ca(2+) loading in dendrites and somata of CA1 neurons in acute slice

Multiple Ca(2+) entry routes have been implicated in excitotoxic Ca(2+) loading in neurons and reverse-operation of sodium-calcium exchangers (NCX) has been shown to contribute under conditions where intracellular Na(+) levels are enhanced. We have investigated effects of KB-R7943, an inhibitor of reverse-operation NCX activity, on Ca(2+) elevations in single CA1 neurons in acute hippocampal slices. KB-R7943 had no significant effect on input resistance, action potential waveform, or action potential frequency adaptation, but reduced L-type Ca(2+) entry in somata. Nimodipine was therefore included in subsequent experiments to prevent complication from effects of L-type influx on evaluation of NCX activity. NMDA produced transient primary Ca(2+) increases, followed by propagating secondary Ca(2+) increases that initiated in apical dendrites. KB-R7943 had no significant effect on primary or secondary Ca(2+) increases generated by NMDA. The Na(+)/K(+) ATPase inhibitor ouabain (30 microM) produced degenerative Ca(2+) overload that was initiated in basal dendrites. KB-R7943 significantly reduced initial Ca(2+) increases and delayed the propagation of degenerative Ca(2+) loads triggered by ouabain, raising the possibility that excessive intracellular Na(+) loading can trigger reverse-operation NCX activity. A combination of NMDA and ouabain produced more rapid Ca(2+) overload, that was contributed to by NCX activity. These results suggest that degenerative Ca(2+) signaling can be triggered by NMDA in dendrites, before intracellular Na(+) levels become sufficient to reverse NCX activity. However, since Na(+)/K(+) ATPase inhibition does appear to produce significant reverse-operation NCX activity, this additional Ca(2+) influx pathway may operate in ATP-deprived CA1 neurons and play a role in ischemic neurodegeneration.