[Effect of low-molecular nonelectrolytes on caffeine skeletal muscle contracture in the rat].

Contractures induced in rat fast (EDL) and slow (SOL) skeletal muscles by 0.03--3 mM of caffeine in conjunction with rapid cooling of muscle from 30 to 0 degrees C (rapid cooling contructures, RCC) were studied. Uprising speed and tension of RCC were dependent on caffeine concentration and cooling gradient. The minimal necessary temperature, below which contractures still developed, was +6 degrees. The initial temperature did not play any important role. Optimal conditions for RCC (when its tension reached 80--200% of twitch) were: cooling from 30 to 0 degrees, and concentrations of caffeine being 5 mM for SOL, and 6--7 mM for EDL. Disruption of T tubules caused by the removal of glycerol and urea (400--600 mM) from muscle fibers did not influence the RCC tension. During the first hour of the removal, relaxation rate of RCC was lowered. In the presence of 400 mM of urea and 600 mM of 1.3-dimethylurea (the latter did not disrupt the T-system), RCC was depressed by 90%, and the rate of tension development was greatly lowered, while twitches remained unchanged. This effects could be reversed during non-electrolyte removal. This may suggest that Ca2+ release is inhibited selectively by urea and by dimethylurea.