STUDIES ON THE INHIBITION OF MUSCULAR ACTIVITY IN THE PRESENCE OF UREA.

Abstract

The inhibitory effect of urea on frog's muscular activities depends on the work or tension output of the muscles. The inhibition is complete when the muscles are stimulated 12 times/min under a load of 5–10 g or stimulated 12/min under isometric conditions. Some spontaneous recoveries of inhibited contractions (Rottcher and Straub 26) can be observed at a frequency of 2/min under a load of 1 g or at the same frequency under isometric conditions. No reversal of the inhibited contractions are found even at the lower frequency and load when the muscles are treated with 1.0 M urea-Ringer at once (Barany et al.4) Small or no contractures in Ringer's solution containing 1.0 M urea and 0.5% caffeine are exhibited under lighter loads, but no contractures are produced in the same solution when the muscles are loaded with heavier weights. The specific inhibitory effect of urea on muscle activities has been shown by comparing its effect with acetamide. Under identical conditions, acetamide has much less effect on contractions and contractures than urea despite its identical osmotic effect and penetration into the fibre water. The intracellular urea concentration is in the range of 0.3–0.5 M at the time when muscular contractions are inhibited by urea. These urea concentrations inhibit the Mg++-activated actomyosin ATPase considerably, and thus no large external work or tension can be produced by the muscles.