Large neurons from layer V in a slice preparation of cat sensorimotor cortex were impaled with microelectrodes containing KCl plus different concentrations of the Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid (BAPTA) or two of its derivatives. Impalement with electrodes containing high BAPTA (200 mM) quickly abolished Ca(2+)-dependent afterhyperpolarizations. Spike parameters were normal, but the usual time- and voltage-dependent rectification of subthreshold membrane potential was absent. Normally, this rectification results from activation of two voltage-gated currents, the persistent sodium current (INaP) and the hyperpolarizing inward rectifier current (Ih). Both of these currents were absent during voltage clamp with high BAPTA microelectrodes. Impalement with electrodes containing low BAPTA (2 mM) or derivatives caused a different effect. Injection of a 1-s current pulse evoked phasic firing instead of the tonic firing seen normally. Both the amplitude and the duration of the Ca(2+)-dependent afterhypolarization that followed repetitive firing were much greater than normal. The effectiveness of BAPTA derivatives in altering afterhyperpolarizations and firing properties were similar to their effectiveness in chelating Ca2+. It is assumed that the BAPTA effects result from reduction of intracellular Ca2+ concentration. Results with high BAPTA suggest that (i) both INaP and Ih require a minimal intracellular calcium concentration for normal expression, and that (ii) these voltage-gated currents may be modulated by changes in intracellular calcium concentration. Results with low BAPTA suggest that a small reduction of intracellular calcium concentration preferentially enhances a slow, Ca(2+)-dependent K+ current which then dominates the firing properties of the cell. The transformed firing properties resemble those of hippocampal pyramidal neurons.(ABSTRACT TRUNCATED AT 250 WORDS)