STEADY-STATE TWITCH CA2+ FLUXES AND CYTOSOLIC CA2+ BUFFERING IN RABBIT VENTRICULAR MYOCYTES

Intracellular Ca2+ ([Ca2+](i)) transients and transsarcolemmal Ca2+ cu rrents were measured in indo 1-loaded isolated rabbit ventricular myoc ytes during whole cell voltage clamp to quantitate the components of c ytosolic Ca2+ influx and to describe the dynamic aspects of cytosolic Ca2+ buffering during steady-state contraction (0.5 Hz, 22 degrees C). Sarcolemmal Ca2+ influx was directly measured from the integrated Ca2 + current (I-Ca) recorded during the clamp (158 +/- 10 attomoles; amol ). Sarcoplasmic reticulum (SR) Ca2+ content was determined from the in tegrated electrogenic Na+/Ca2+ exchange current (I-X) induced during r apid application and sustained exposure of cells to caffeine to elicit the release of the SR Ca2+ load (1,208 +/- 170 amol). The mean steady -state SR Ca2+ load was calculated to be 87 +/- 131 mu M (pmol/l nonmi tochondrial cytosolic volume). Ca2+ influx via I-Ca represented simila r to 14% of the stored SR Ca2+ and 23% of the total cytosolic Ca2+ flu x during a twitch (47 +/- 6 mu M) Comparison of electrophysiologically measured Ca2+ fluxes with Ca2+ transients yields apparent buffering v alues of 60 for caffeine contractures and 110 for twitches (Delta Ca2 total/Delta Ca2+ free). This is consistent with the occurrence of ''a ctive'' buffering of cytosolic Ca2+ by SR Ca2+ uptake during the twitc h.