1. The effects of strontium ions, Sr2+, on Ca2+-dependent feedback mechanisms during excitation-contraction coupling were examined in voltage-clamped rat ventricular myocytes in which intracellular [Ca2+] and [Sr2+] were monitored with the fluorescent indicator, indo-1. 2. Voltage clamp depolarizations and caffeine applications during superfusion in Ca2+-free, Sr2+-containing solutions were employed to exchange intracellular Ca2+ with Sr2+. Myocytes were loaded with Sr2+ by applying voltage clamp depolarizations during superfusion in Na+-free, Sr2+-containing solutions. 3. Caffeine applications produced large fluorescence transients in Sr2+-loaded cells. Thus, Sr2+ could be sequestered and released from the sarcoplasmic reticulum. 4. Ca2+ influx, but not Sr2+ influx, via sarcolemmal Ca2+ channels evoked ryanodine-sensitive fluorescence transients in Sr2+-loaded cells. These results demonstrated that Ca2+ influx-induced Sr2+ release (CISR) from the sarcoplasmic reticulum occurred in these experiments, even though Sr2+ influx-induced Sr2+ release was not observed. 5. The amplitude of the Ca2+ influx-induced fluorescence transient was 17 ± 1% of the caffeine-induced transient (n = 5 cells), an indication that fractional utilization of Sr2+ sequestered in the sarcoplasmic reticulum during CISR was low 6. With increased Sr2+ loading, the amplitude of Ca2+ influx- and caffeine-induced fluorescence transients increased, but fractional utilization of sarcoplasmic reticulum divalent cation stores was independent of the degree of Sr2+ loading. These data suggest that Ca2+ influx directly activated the release of divalent cations from the sarcoplasmic reticulum, but mechanisms promoting positive feedback of Sr2+ release were minimal during CISR. 7. By comparison, in Ca2+-loaded myocytes, Ca2+ influx-induced Ca2+ release (CICR) utilized a greater fraction of caffeine-releasable stores than CISR. Fractional utilization of Ca2+ stores during CICR increased with the degree of Ca2+ loading. 8. Taken together, these results suggest that Ca2+-dependent feedback mechanisms play a major role in determining the extent of sarcoplasmic reticulum Ca2+ release during cardiac excitation-contraction coupling under a wide range of conditions.
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