Chlorpromazine produced a slowly developing, irreversible contracture of isolated frog rectus abdominis muscle. The contracture was unaffected by tubocurarine, hexamethonium, atropine, physostigmine, procaine, cocaine, ouabain, quinine, and sparteine. Bathing the muscles in calcium-deficient Ringer, Ringer-containing oxalate or choline-Ringer did not affect action of chlorpromazine, which was strongly reduced in potassium-depolarized muscle or by calcium-deficient Ringer containing EDTA. Nitrate-Ringer facilitated the contracture. Chlorpromazine seems to act not through a cholinergic mechanism or membrane depolarization but by releasing calcium, probably from transverse tubular sites or some other superficial site.
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