High-voltage-activated calcium currents (HVA) of CA1 neurons are prominently attenuated following a switch from HEPES-buffered solution to one buffered with CO(2)/HCO(3)(-). In the present study we investigated whether bicarbonate ions or the dissolved CO(2) induce this alteration in current characteristic. The study was carried out on freshly isolated CA1 neurons using the whole cell patch-clamp technique. Maximal calcium conductance and the mean peak amplitude of the currents showed a concentration-dependent decrease when cells were consecutively bathed in solutions containing increasing amounts of bicarbonate and CO(2). This decrease is best described by the Hill equation, yielding a maximal attenuation of 69%, a half-maximal concentration (EC(50)) of 7.4 mM HCO(3-), and a Hill coefficient of 1.8. In parallel, the potentials of half-maximal activation (V(h,a)) and inactivation (V(h,i)) were linearly shifted in hyperpolarizing direction with a maximal shift, in the 10% CO(2)/37 mM HCO(3)(-) containing solution of 10 +/- 1 mV for V(h,a) (n = 23) and 17 +/- 1.4 mV for V(h,i) (n = 18). When currents were evoked in solutions containing equal concentrations of bicarbonate but different amounts of CO(2), only nonsignificant changes were observed, while marked alterations of the currents were induced when bicarbonate was changed and CO(2) held stable. The experiments suggest that bicarbonate is the modulating agent and not CO(2). This bicarbonate-induced modulation may be of critical relevance for the excitation level of the CNS under pathological situation with altered concentration of this ion, such as hyperventilation and metabolic acidosis.