ACTIONS OF VERAPAMIL, DILTIAZEM AND OTHER DIVALENT CATIONS ON THE CALCIUM‐CURRENT OF Helix NEURONES

Abstract

1 Effects of organic verapamil and diltiazem, and cations, Ni2+, Mn2+, Co2+ and La3`on Ca2+ current (ICa) 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 10-4M) increased the threshold, and decreased both the amplitude and rate of rise of the soma Ca2+-spike. Both agents inhibited ICa 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 Ca2+ overcame the inhibitory action of the agents. 3 Divalent cations, Ni2+, Mn2+, Co2+ and the trivalent cation, La3+ inhibited ICa dosedependently, but induced shifts of the I-V relationship to more positive voltages. The order of potency of inhibition of ICa among these cations was as follows; Ni2+ > La3+ > Mn2+ > Co2+. 4 Double reciprocal plots for peak ICa versus external Ca2+ concentrations in the presence or absence of both organic and inorganic Ca2+-antagonists intersect at the ordinate. Results indicate that both organic and inorganic Ca2+-antagonists compete for Ca2+ at the common binding site for Ca2+.5 Internal application of the organic Ca2+-antagonists (10-4M) inhibited ICa in a time-dependent manner to about 40-60% of the control. Ni2+, when applied internally, also depressed ICa* 6 The results provide evidence that organic Ca2+-antagonists occupy the binding site for Ca2+ in a competitive manner at the surface of the soma membrane of the Helix neurone, while divalent and trivalent cations, in addition to inhibiting ICa in a similar manner to the organic Ca2+_antagonists, change the surface charge of the soma membrane.

Accelerating effects of pentobarbitone on the inactivation process of the calcium current in Helix neurones

“…ICa was attained at membrane potentials of + 10 to +20 mV in the control (Akaike et al, 1978;Akaike, Brown, Nishi & Tsuda, 1981 (Figure 2b). The currentvoltage (I-V) relationships for ICa in the presence of pentobarbitone are shown in Figure 2c; the agent induced a dose-dependent depression of the transient peak amplitude of ICa without shifting of the I-V relationship.…”

FURTHER ANALYSIS OF INHIBITORY EFFECTS OF PROPRANOLOL AND LOCAL ANAESTHETICS ON THE CALCIUM CURRENT IN Helix NEURONES

“…Considering the relatively high lipophilicity of propranolol and lignocaine, the membrane may be permeable to these agents and thus the inhibition of ICa would result. Therefore, we propose, that there are at least 2 types of receptor sites for Ca2+-blocking agents in Ca2+-channel; one is located at the outer surface of the membrane as originally suggested by Hagiwara & Takahashi (Hagiwara & Takahashi, 1967;Akaike et al, 1981a) channel (Ritchie, 1979). At the moment, however, further characterization of both Ca2+-channels and effects of Ca2'-blocking agents on the drug-receptor reaction in the Ca2+-channel is required.…”

“…The depressant action of propranolol resembled that of an organic Ca2+-antagonist as follows: (1) propranolol inhibited the ICa dose-dependently over the entire range of the I-V relationship without shifting the threshold and peak potentials of the I-V relationship. (2) An increase in dose of these agents prolonged the time for ICa to reach its peak, and at the same time, slowed the time course of inactivation, and (3) internal application of propranolol depressed the ICa (Akaike et al, 1981a). However, the Lineweaver-Burk plots for ICa and external Ca2" concentration in the presence and absence of organic Ca2"-antagonists intersected on the ordinate scale (Akaike et al, 1981a), while with propranolol it was found that the respective Lineweaver-Burk plots did not intersect on the ordinate.…”

“…(2) An increase in dose of these agents prolonged the time for ICa to reach its peak, and at the same time, slowed the time course of inactivation, and (3) internal application of propranolol depressed the ICa (Akaike et al, 1981a). However, the Lineweaver-Burk plots for ICa and external Ca2" concentration in the presence and absence of organic Ca2"-antagonists intersected on the ordinate scale (Akaike et al, 1981a), while with propranolol it was found that the respective Lineweaver-Burk plots did not intersect on the ordinate. This finding that the P-blocker inhibited the ICa in a non-competitive manner for Ca2+ on the membrane site, suggests that the site and mode of inhibitory action of the P-blocker on ICa is different from that of the organic Ca2+-antagonists.…”

“…In a previous study we showed that intracellularly applied organic Ca2+-antagonists depressed ICa in a dose-and time-dependent manner (Akaike et al, 1981a). To compare the action of propranolol with that of the Ca2+-antagonists, the agent was also applied intracellularly.…”