Calcium-induced inactivation of calcium release from the sarcoplasmic reticulum of skeletal muscle.
The ability of myofilament space Ca2+ to modulate Ca2+ release from the sarcoplasmic reticulum (SR) of skeletal muscle was investigated. Single fibers of the frog Rana pipiens belindieri were manually skinned (sarcolemma removed). Following a standard load and pre-incubation in varying myoplasmic Ca2+ concentrations, SR Ca2+ release was initiated by caffeine. Ca2+ release rates were calculated from the changes in absorbance of a Ca2+ sensitive dye, antipyrylazo III. An apparent dissociation constant (Kd) for dye-Ca2+ binding of 8000 microM 2 was determined by comparing the buffering action of the dye with that of ethylenebis(oxonitrilo)tetraacetate (EGTA) using the contractile proteins of the skinned fiber as a measure of free Ca2+. This value for Kd was used in the calculation of Ca2+ release rates. As the myoplasmic space Ca2+ was increased from pCa 7.4, Ca2+ release rates declined sharply such that at pCa 6.9 the calculated release rate was 72 +/- 3% (mean +/- SEM) of control (pCa 8.4). Further increases in myoplasmic Ca2+ from pCa 6.9 to pCa 6.1 did not result in a further decline in release rate. The effect of a decreased driving force on Ca2+ ions was investigated to determine whether it could account for the change in release rates observed. At pCa 6.9, where the greatest degree of inactivation occurred, the measured effects of a change in driving force could account for at most 40% of the observed inactivation. Varying concentrations of Ba2+ and Sr2+ in the myofilament space had no inactivating effect on the SR Ca2+ release rates. The ability of myofilament Ca2+ to inhibit SR Ca2+ release at concentrations normally encountered during muscle activation suggests a role for released Ca2+ as a modulator of the SR Ca2+ channel.
Kwok
W M
WM
Department of Physiology and Biophysics, University of Illinois, Urbana 61801.
Best
P M
PM
D000818:Animals; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D004396:Coloring Agents; D009132:Muscles / Q000378:metabolism* / Q000648:ultrastructure; D009282:Naphthalenesulfonates; D011894:Rana pipiens; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D012996:Solutions; D013324:Strontium / Q000494:pharmacology
D004396:Coloring Agents; D009282:Naphthalenesulfonates; D012996:Solutions; D001464:Barium; C016238:antipyrylazo III; D002118:Calcium; D013324:Strontium