Calcium antagonism of the block in excitation-contraction coupling produced by a urea exposure-removal treatment.

Exposing frog's toe muscles to Ringer's solution made hypertonic with 400 mM urea for 60 min followed by placing the muscles back in Ringer's (urea-removal treatment) completely blocks the twitch without disrupting the surface openings of the T-tubules. The urea-removal treatment also increased the triadic junction width. Placing the muscles in Ringer's with an elevated calcium concentration (5 mM) following exposure to the hyertonic solution prevented the block of the twitch response but not the increase in the triadic junction width. Exposing untreated muscles to Ringer's with 5 mM calcium either had no effect on the twitch on reduced it by 30 percent or less. These results suggest the possibility that increasing the width of the triadic junction decreases the amount of calcium ions reaching the terminal cisternae during an action potential thereby blocking the twitch. Elevating the calcium concentration in the T-tubules would increase the amount of calcium which enters the triadic junction during an action potential and thus antagonize the above effects.