Contractures in Normal and Denervated Inferior Oblique Muscle of the Rabbit

Isometric contracture responses of normal and denervated inferior oblique muscles (IO) of the rabbit have been investigated in vitro at 35 degrees C. The threshold concentration for eliciting potassium contractures was about 20 mM K+. In normal IU low potassium concentrations up to about 50 mM K+ evoked only sustained contractures, higher concentrations were responded by contractures with an initial transient component. The transient tension development was maximal at about 100 mM K+ the sustained component at 80 mM K+. After denervation the characteristic time course of the contractures was not changed, but the tension output of the preparation was diminished and long-term denervated IO have a somewhat lowered threshold. In normal IO acetylcholine (ACh), succinylcholine (SCh) and choline (Ch) caused also sustained contractures, the threshold doses were about 5 microM for ACh and SCh and 500 microM for Ch. The ACh sensitivity of the preparations was increased by physostigmine and decreased or abolished by d-tubocurarine. Denervation increased the drug sensitivity but the shape of the contractures was hardly influenced. The properties of slow tonic muscle fibres in mammalian extraocular muscles (EOM) probably responsible for sustained contractures and their changes after denervation are discussed.