Quinine skeletal muscle

Quinidine Quinidex) was one of the first clinically used antiarrhythmic agents. Because of the high incidence of ventricular proarrhythmia associated with its use and numerous other equally efficacious agents, quinidine is now used sparingly. Quinidine shares all of the pharmacological properties of quinine, including an-timalarial, antipyretic, oxytocic, and skeletal muscle relaxant actions. 

The exact mechanism of toxicity is unknown. Quinine acts on all body muscle groups, most notably cardiac, uterine, and skeletal muscles. 

Action on Skeletal Muscle Quinine increases the tension response to a single maximal stimulus delivered to muscle, but it also increases the refractory period of muscle so that the response to tetanic stimulation is diminished. The excitability of the motor end-plate region decreases so that responses to repetitive stimulation and to acetylcholine are reduced. Thus, quinine can antagonize the actions of physostigmine on skeletal muscle. Quinine also may produce respiratory distress and dysphagia in patients with myasthenia gravis. 

The liver has been strongly implicated as the major site of cinchona alkaloid degradation. Thus Kelsey and Oldham (74) found that the liver of the rabbit was far more active than lung, kidney, spleen, adrenals, skeletal muscle, uterus, and intestine. Other investigators have found that excretion of unchanged quinine is increased by liver injury or partial hepa-tectomy (85). Knox (86) has studied the liver oxidase system responsible for quinine degradation and has concluded that the liver enzyme is a flavo-protein with functions like those of xanthine oxidase. 

Quinine is used as the basis for anti-malarial drugs, as a skeletal muscle relaxant (treatment of nocturnal cramps) and traditionally as a febrifuge for influenza and other acute infectious disorders. Quinine is very bitter and is the main flavour component of tonic water. 

Quinine has a skeletal mnscle-relaxant effect, increasing the refractory period by direct action on the muscle fiber. 

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