P(i) inhibits the SR Ca(2+) pump and stimulates pump-mediated Ca(2+) leak in rabbit cardiac myocytes.
Measurements of sarcoplasmic reticulum (SR) Ca(2+) uptake were made from aliquots of dissociated permeabilized ventricular myocytes using fura 2. Equilibration with 10 mM oxalate ensured a reproducible exponential decline of [Ca(2+)] from 600 nM to a steady state of 100-200 nM after addition of Ca(2+). In the presence of 5 microM ruthenium red, which blocks the ryanodine receptor, the time course of the decline of [Ca(2+)] can be modeled by a Ca(2+)-dependent uptake process and a fixed Ca(2+) leak. Partial inhibition of the Ca(2+) pump with 1 microM cyclopiazonic acid or 50 nM thapsigargin reduced the time constant for Ca(2+) uptake but did not affect the SR Ca(2+) leak. Addition of 10 mM inorganic phosphate (P(i)) decreased the rate of Ca(2+) accumulation by the SR and increased the Ca(2+) leak rate. This effect was reversed on addition of 10 mM phosphocreatine. 10 mM P(i) had no effect on Ca(2+) leak from the SR after complete inhibition of the Ca(2+) pump. In conclusion, P(i) decreases the Ca(2+) uptake capacity of cardiac SR via a decrease in pump rate and an increase in Ca(2+) pump-dependent Ca(2+) leak.
Institute of Biomedical and Life Sciences, Glasgow University, Glasgow G12 8QQ, Scotland. g.smith@bio.gla.ac.uk
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D000818:Animals; D000889:Anti-Arrhythmia Agents / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors*; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D006321:Heart / Q000187:drug effects; D007211:Indoles / Q000494:pharmacology; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D010710:Phosphates / Q000494:pharmacology*; D011817:Rabbits; D012430:Ruthenium Red / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel / Q000502:physiology; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D019284:Thapsigargin / Q000494:pharmacology
D000889:Anti-Arrhythmia Agents; D002121:Calcium Channel Blockers; D007211:Indoles; D010710:Phosphates; D019837:Ryanodine Receptor Calcium Release Channel; D012430:Ruthenium Red; D019284:Thapsigargin; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D000252:Calcium-Transporting ATPases; D002118:Calcium; C000543:cyclopiazonic acid