Intracellular pH buffering shapes activity-dependent Ca2+ dynamics in dendrites of CA1 interneurons

Voltage-gated calcium (Ca) channels are highly sensitive to cytosolic H+, and Ca2+ influx through these channels triggers an activity-dependent fall in intracellular pH (pHi). In principle, this acidosis could act as a negative feedback signal that restricts excessive Ca2+ influx. To examine this possibility, whole cell current-clamp recordings were taken from rat hippocampal interneurons, and dendritic Ca2+ transients were monitored fluorometrically during spike trains evoked by brief depolarizing pulses. In cells dialyzed with elevated internal pH buffering (high beta), trains of >15 action potentials (Aps) provoked a significantly larger Ca2+ transient. Voltage-clamp analysis of whole cell Ca currents revealed that differences in cytosolic pH buffering per se did not alter baseline Ca channel function, although deliberate internal acidification by 0.3 pH units blunted Ca currents by approximately 20%. APs always broadened during a spike train, yet this broadening was significantly greater in high beta cells during rapid but not slow firing rates. This effect of internal beta on spike repolarization could be blocked by cadmium. High beta also 1) enhanced the slow afterhyperpolarization (sAHP) seen after a spike train and 2) accelerated the decay of an early component of the sAHP that closely matched a sAHP conductance that could be blocked by apamin. Both of these effects on the sAHP could be detected at high but not low firing rates. These data suggest that activity-dependent pHi shifts can blunt voltage-gated Ca2+ influx and retard submembrane Ca2+ clearance, suggesting a novel feedback mechanism by which Ca2+ signals are shaped and coupled to the level of cell activity.