Factors Influencing Movement of Calcium in Vertebrate Striated Muscle

Abstract

Stimulation of isolated frog muscles at a frequency of one shock per second increases at first the rate of exchange of Ca⁴⁰ with Ca⁴⁵ but after 3 hours the total exchange is the same as in the unstimulated control muscles, only about 44% of the calcium being radioactive. Subsequent stimulation of the control muscles does not increase the specific activity. When Ca⁴⁵ is injected i.p. in frogs the muscles exchange 26% of their Ca in 30 minutes and this level is not further increased for 8 hours. When the frog is stimulated at 15 shocks/min. the exchange becomes 100% in 400 minutes. With higher frequency of stimulation the complete exchange is achieved more rapidly. Repeated daily injection of Ca over a period of 6 days results in 80% exchange without stimulation. Stimulation of one leg only results in greater exchange in the stimulated leg. If the Achilles tendon is cut on one side and both sides are stimulated the exchange is more rapid on the operated side. Young rats injected with Ca⁴⁵ exchange all their muscle Ca in 100 minutes without artificial stimulation.