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

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Abstract
1 The effects of propranolol and local anaesthetics on Ca2+ current (ICa), individually separated from other ionic currents, in Helix neurones were studied under voltage clamp, using a suction pipette technique. 2 Increases in external Ca2+ concentrations overcame the inhibitory action of propranolol on lea-Double reciprocal plots for peak ICa versus external Ca2+ concentrations in the presence or absence of propranolol did not intersect at the ordinate. 3 Internal application of propranolol (10−4m) inhibited ICa to about 40–60% of the control in a time-dependent manner. 4 Lignocaine and procaine at concentrations of 10−3–10−2m inhibited ICa without shifting the threshold in the I-V relationships. Internal application of lignocaine (10−3–10−2 m) also inhibited ICa: the ratio of depression of the ICa was almost equivalent to that of the agent applied externally. 5 The results provide evidence that propranolol inhibits ICa in a noncompetitive manner with Ca2+ at the cell membrane, and suggest that the agents may occupy the receptor site in the Ca2+ -channel somewhere between the outer surface and inner phase of the membrane.