Effects of creatine phosphate and inorganic phosphate on the sarcoplasmic reticulum of saponin-treated rat heart.
1. Ventricular trabeculae from rat heart were permeabilized by treatment with saponin. In the presence of 150 nM Ca2+, application of 20 mM caffeine released Ca2+ from the sarcoplasmic reticulum (SR), resulting in a transient contracture. Ca2+ released from the SR was detected using fura-2 fluorescence. The amplitudes of the caffeine-induced Ca2+ transients were used to assess SR Ca2+ content. 2. In the absence of creatine phosphate (CP), introduction of 5-30 mM inorganic phosphate (Pi) caused a net release of Ca2+ from the SR. Subsequent caffeine-induced Ca2+ and tension transients were smaller in the presence of Pi. Under these conditions, 30 mM Pi decreased the caffeine-induced Ca2+ transients by 45 +/- 3.1% (mean +/- S.D., n = 14). On removal of Pi, the [Ca2+] transiently decreased and the caffeine-induced Ca2+ transients returned to control levels over 4-6 min. 3. In the presence of CP (5-15 mM), the Ca2+ transients were unaffected by the introduction of Pi (5-30 mM) or slightly increased in amplitude. Pi (30 mM) significantly increased the caffeine-induced Ca2+ transients by 7 +/- 8.8% (mean +/- S.D., n = 19, P < 0.05) in the presence of 15 mM CP. The release of Ca2+ on addition of Pi and decrease in [Ca2+] on Pi withdrawal was less pronounced or absent completely in the presence of CP. The inhibitory effects of Pi on caffeine-induced Ca2+ release became apparent as the [CP] was decreased from 5 to 0 mM. 4. In the presence of the creatine phosphokinase inhibitor dinitro-fluorobenzene (DNFB) the effects of Pi (in the presence of CP) were qualitatively similar to the results obtained in the absence of CP, although the decrease in caffeine-induced Ca2+ release was less pronounced. 5. These results suggest that the rise in [Pi]i during ischaemia or anoxia will have little effect on the regulation of Ca2+ by the SR while the [CP]i remains above 5 mM. However, as the [CP] decreases below 5 mM, the accumulation of Pi within the cytosol will progressively reduce the SR Ca2+ content. CP may act in conjunction with endogenous creatine phosphokinase to modify the response of the SR to Pi, and possible mechanisms are considered.
Steele
D S
DS
Institute of Physiology, University of Glasgow, UK.
McAinsh
A M
AM
Smith
G L
GL
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D003402:Creatine Kinase / Q000378:metabolism; D004139:Dinitrofluorobenzene / Q000494:pharmacology; D066298:In Vitro Techniques; D008856:Microscopy, Fluorescence; D009200:Myocardial Contraction; D009206:Myocardium / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism / Q000494:pharmacology*; D010725:Phosphocreatine / Q000378:metabolism / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012503:Saponins / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*
D010710:Phosphates; D012503:Saponins; D010725:Phosphocreatine; D002110:Caffeine; D000255:Adenosine Triphosphate; D004139:Dinitrofluorobenzene; D003402:Creatine Kinase; D002118:Calcium