CALCIUM ANTAGONISM OF THE BLOCK IN EXCITATION-CONTRACTION COUPLING PRODUCED BY A UREA EXPOSUREREMOVAL TREATMENT

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Abstract
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 blocked the twitch without disrupting the surface openings of the T[transverse]-tubules. The urea-removal treatment also increased the triadic junction width. Placing the muscles in Ringer''s with an elevated Ca concentration (5 mM) following exposure to the hypertonic 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 Ca either had no effect on the twitch or reduced it by 30% or less. Increasing the width of the triadic junction may decrease the amount of Ca reaching the terminal cisternae during an action potential thereby blocking the twitch. Elevating the Ca concentration in the T-tubules may increase the amount of Ca which enters the triadic junction during an action potential and thus antagonize the above effects.