Effects of divalent cations on voltage-gated Ca2+ channels and depolarization-induced [Ca2+]i transients of freshly isolated pyramidal cells of the rat dorsal cochlear nucleus

The effects of divalent cations on voltage-activated Ca2+ channels and depolarization-evoked cytoplasmic [Ca2+] elevations were studied in pyramidal neurones isolated from the dorsal cochlear nucleus of the rat. Ca2+ currents were recorded using the whole-cell configuration of the patch-clamp technique. 10 mumol.1(-1) Cd2+ exerted a greater blocking effect on the high-voltage activated (HVA) currents than on the low-voltage activated (LVA) ones (decrease to 26.6 +/- 2.5% and to 87.8 +/- 2.1%, respectively). The blocking effect of 200 mumol.1(-1) Cd2+ was more pronounced and the difference between the effect on the HVA and LVA. currents became smaller (decrease to 11.7 +/- 2.1% and to 32.4 +/- 2.7%, respectively). 200 mumol.1(-1) Ni2+ reduced the LVA component more effectively (to 77.6 +/- 5.4%) than the HVA one (to 86.9 +/- 2.6%). Cytoplasmic [Ca2+] changes were measured applying a fluorimetric technique (Fura-2). 10 mumol.1(-1) Cd2+ decreased the peak values of 50 mmol.1(-1) K+ depolarization-induced [Ca2+], transients to 30.4 +/- 1.4% while 200 mumol.1(-1) Cd2+ caused a drop to 2.5 +/- 0.2%. 200 mumol.1(-1) Ni2+ decreased the peak of the transients to 69.6 +/- 2.9%. Comparison of the blocking effects of divalent cations on Ca2+ currents and [Ca2+], transients supports further the conclusion that the depolarization-induced [Ca2+](i) changes are produced mainly by the activation of the HVA Ca2+ channels.