PARVALBUMIN RELAXES FROG SKELETAL-MUSCLE WHEN SARCOPLASMIC-RETICULUM CA2-ATPASE IS INHIBITED()

Inhibition of sarcoplasmic reticulum (SR) Ca2+-adenosinetriphosphatase (ATPase) with 2,5-di-(tert-butyl)-1,4-benzohydroquinone (TBQ) in frog skeletal muscle fibers at 10 degrees C prolonged the half time of the fall of the Ca2+ transient by 62% and twitch force by 100% and increa sed peak force by 120% without increasing the amplitude of the Ca2+ si gnal. In the presence of TBQ the rate of relaxation and the rate of fa ll of Ca2+ became progressively slower in a series of twitches until r elaxation failed. Relaxation rate decreased with a time course (simila r to 2 s(-1)) similar to the Mg2+ off rate from purified parvalbumin ( PA; 3.6 s(-1)). TBQ slowed the rate of fall of Ca2+ (5-fold) and force (8-fold) in a 0.3-s tetanus so that the rate of fall of Ca2+ (similar to 2.5 s(-1)) was similar to the Mg2+ off rate from PA. TBQ caused a near total failure of both Ca2(+) sequestration and relaxation in a 1. 1-s tetanus, during which PA would be saturated with Ca2+ and could no t contribute to relaxation. Thus, when the SR Ca2+ ATPase is inhibited , Mg2+-PA can sequester Ca2+ and produce relaxation at a rate that is defined by the Mg2+ off rate from PA.