ENHANCED CA2-RETICULUM CA2+ CONTENT AFTER DEPLETION( CURRENT AND DECREASED CA2+ EFFLUX RESTORE SARCOPLASMIC)

[Ca2+](i) was measured using the fluorescent indicator indo 1 in volta ge-clamped ferret and rat ventricular myocytes. The Ca2+ content of th e sarcoplasmic reticulum (SR) was estimated from the integral of the N a+-Ca2+ exchange current activated by caffeine. Refilling of the SR af ter caffeine removal was enhanced by stimulation. As the systolic Ca2 transient recovered, the integral of the L-type Ca2+ current decrease d and that of the Na+-Ca2+ exchange tail current increased. For the ea rly pulses, the gain of Ca2+ via the Ca2+ current is greater than the loss via the exchanger, and during steady state stimulation, the fluxe s are equal. The difference in the integrals gives a measure of the ne t gain of cell Ca2+ with each pulse. When these are summed, the calcul ated gain of cell Ca2+ agrees well with the increase of SR Ca2+ produc ed by stimulation, as measured from the caffeine-evoked currents. Ther e was a nonlinear relationship between SR Ca2+ content and the magnitu de of the systolic Ca2+ transient such that at high SR Ca2+ content a given increase of content had a greater effect on the Ca2+ transient t han did an increase at low SR content. In conclusion, the effects of s ystolic Ca2+ on the Ca2+ current and Na+-Ca2+ exchange current provide a means to regulate SR Ca2+ content and thence the systolic Ca2+ tran sient.