Decline of myoplasmic Ca2+, recovery of calcium release and sarcoplasmic Ca2+ pump properties in frog skeletal muscle.
1. The two calcium indicators Antipyrylazo III (AP III) and Fura-2 were used simultaneously to monitor free myoplasmic [Ca2+] in voltage-clamped cut segments of frog skeletal muscle fibres (8-10 degrees C). Antipyrylazo III was used for the relatively large [Ca2+] transients during 100-200 ms depolarizing pulses to -20 to 0 mV and for the rapid decline of [Ca2+] during the 200 ms after the pulses. Fura-2 was used to follow the slow decline of the small remaining elevation of [Ca2+] during the following 16 s (slow recovery period) and to monitor resting [Ca2+]. 2. From 1 to 16 s of the slow recovery period [Ca2+] declined with two exponential components, having time constants of 1.9 +/- 0.3 and 13.5 +/- 1.5 s (these and all other values are means +/- S.E.M. of eleven runs from seven fibres). At 1.2 s after the end of the pulses the amplitudes of the fast and slow exponential components of decline of [Ca2+] were 34 +/- 7 and 31 +/- 4 nM, respectively. The resting [Ca2+] in these runs was 40 +/- 4 nM. 3. The time course of calcium bound to parvalbumin [( Ca-Parv]) was calculated from the [Ca2+] records using literature values for the parvalbumin kinetic constants. From 1 to 16 s of the slow recovery period the total calcium [Ca]T outside the sarcoplasmic reticulum (SR) was assumed to equal [Ca-Parv] + [Ca-Fura]. During this period [Ca]T declined with two exponential components having time constants of 1.7 +/- 0.2 and 14.2 +/- 1.4 s, the same as those for [Ca2+]. Assuming the total concentration of parvalbumin cation binding sites to be 1000 microM, the fast and slow components of [Ca]T had amplitudes of 117 +/- 21 and 147 +/- 16 microM, respectively, at 1.2 s after the pulses. 4. The rate of decline of [Ca]T, -d[Ca]T/dt, was used as a measure of the net rate of removal of calcium from the myoplasm by the SR. From 3 to 16 s of the slow recovery period and in the resting fibre -d[Ca]T/dt varied with [Ca2+] according to A[Ca2+]n-L. The term A[Ca2+]n represents the pump rate and L represents a constant rate of calcium leak from the SR. 5. For 40 nM less than or equal to [Ca2+] less than or equal to 80 nM, the power n for the [Ca2+] dependence of pump rate was 3.9 +/- 0.6.(ABSTRACT TRUNCATED AT 400 WORDS)
Klein
M G
MG
Department of Biological Chemistry, University of Maryland School of Medicine, Baltimore 21201.
Kovacs
L
L
Simon
B J
BJ
Schneider
M F
MF
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000502:physiology*; D016257:Fura-2; D066298:In Vitro Techniques; D007202:Indicators and Reagents; D007700:Kinetics; D008433:Mathematics; D009132:Muscles / Q000378:metabolism*; D009282:Naphthalenesulfonates; D010320:Parvalbumins / Q000378:metabolism; D011485:Protein Binding; D011894:Rana pipiens; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D013997:Time Factors
D007202:Indicators and Reagents; D009282:Naphthalenesulfonates; D010320:Parvalbumins; D000252:Calcium-Transporting ATPases; C016238:antipyrylazo III; D002118:Calcium; D016257:Fura-2