Possible role of the transverse tubules in accumulating calcium released from the terminal cisternae by stimulation and drugs.
The size of the rapidly exchanging and slowly exchanging Ca2+ pools were estimated in frog sartorius muscles. A new technique using Sr2+ to extract the rapidly exchanging pool was used. The method avoids problems of kinetic analysis. The results showed that stimulation causes Ca2+ to be translocated from a compartment which exchanges with a time constant of 800 min to a compartment that can be washed out in 15 min. This is likely a transfer from the terminal cisternae to the transverse tubule. Calculations show that this would represent 0.9% of the Ca2+ released in each twitch. After 300 twitches produced by a 1-Hz stimulation, this accumulation could have increased the Ca concentration in the transverse tubules to 70 mM. A marked increase of Ca2+ concentration of this magnitude in the transverse tubules would raise the mechanical threshold for excitation--contraction coupling and would decrease the efficiency of coupling between contraction and excitation. This could be the explanation of the fatigue observed during this kind of stimulation.
D000818:Animals; D001001:Anura; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D004558:Electric Stimulation; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects; D009119:Muscle Contraction / Q000187:drug effects; D009132:Muscles / Q000187:drug effects* / Q000378:metabolism; D013324:Strontium / Q000494:pharmacology; D013806:Theophylline / Q000494:pharmacology*
D007473:Ion Channels; D002110:Caffeine; D013806:Theophylline; D002118:Calcium; D013324:Strontium