Actions of verapamil, diltiazem and other divalent cations on the calcium-current of Helix neurones

Abstract

1 Effects of organic Ca(2+)-antagonists, verapamil and diltiazem, and cations, Ni(2+), Mn(2+), Co(2+) and La(3+) on Ca(2+) current (I(Ca)) separated from other ionic currents in a Helix neurone were studied. A suction pipette technique which allows internal perfusion of the cell body and voltage clamp was used.2 Verapamil and diltiazem (10(-6)-10(-4) M) increased the threshold, and decreased both the amplitude and rate of rise of the soma Ca(2+)-spike. Both agents inhibited I(Ca) over the entire range of the current-voltage (I-V) relationship dose-dependently, without shifting the threshold of the I-V relationship. Increases in external Ca(2+) overcame the inhibitory action of the agents.3 Divalent cations, Ni(2+), Mn(2+), Co(2+) and the trivalent cation, La(3+) inhibited I(Ca) dose-dependently, but induced shifts of the I-V relationship to more positive voltages. The order of potency of inhibition of I(Ca) among these cations was as follows; Ni(2+) > La(3+) > Mn(2+) > Co(2+).4 Double reciprocal plots for peak I(Ca) versus external Ca(2+) concentrations in the presence or absence of both organic and inorganic Ca(2+)-antagonists intersect at the ordinate. Results indicate that both organic and inorganic Ca(2+)-antagonists compete for Ca(2+) at the common binding site for Ca(2+).5 Internal application of the organic Ca(2+)-antagonists (10(-4) M) inhibited I(Ca) in a time-dependent manner to about 40-60% of the control. Ni(2+), when applied internally, also depressed I(Ca).6 The results provide evidence that organic Ca(2+)-antagonists occupy the binding site for Ca(2+) in a competitive manner at the surface of the soma membrane of the Helix neurone, while divalent and trivalent cations, in addition to inhibiting I(Ca) in a similar manner to the organic Ca(2+)-antagonists, change the surface charge of the soma membrane.