Use-dependent blockade of sodium channels by local anaesthetics and antiarrhythmic drugs. Effects of chloramine-T and calcium ions.

Voltage clamp studies were carried out of the effects of chloramine-T(CT) and external Ca++ on the blocking interactions of local anaesthetics (LAs) and antiarrhythmic drugs (lidocaine, tetracaine, N-propyl ajmaline, compound KC 3791) with Na+ channels in frog Ranvier nodes. The results obtained provided direct evidence for the notion that: LAs interact preferentially with inactivated Na+ channels and stabilize their inactivated conformation ("drug-induced slow inactivation": SI); and SI underlies the cumulative inhibition of INa during repetitive membrane stimulation. Normal inactivation is not indispensable, but plays an auxiliary role in the mechanism of cumulative inhibition of INa by drugs interacting with open Na+ channels. This block results mainly from accumulation of the channels in the resting blocked state (due to the inability of charged drugs to leave the channel via a "hydrophobic pathway"). The contribution of the blockade-inactivated state to this type of block may depend on some properties of the drug and the holding membrane potential. The problem of the location of the binding site responsible for LA-induced SI requires further investigation in view of the fact that in the myocardium, along with LA, the lipid-insoluble tetrodotoxin (TTX) induces a pronounced SI.