Figure 4. SR Ca²⁺ content at the point of spontaneous Ca²⁺ release.

A, a spontaneous Ca²⁺ release is shown, followed by a release induced by rapid application of caffeine when spontaneous release would otherwise have occurred (left trace). The amplitude of the caffeine-induced Ca²⁺ transient was used as an index of the SR Ca²⁺ content at the point of spontaneous release. Withdrawal of PCr resulted in a decrease in the amplitude of the spontaneous Ca²⁺ transient and the caffeine-induced transient (middle trace). Following reintroduction of PCr, the responses returned to control levels (right trace). B, accumulated data showing the effects of PCr withdrawal and reintroduction on the amplitude of the spontaneous Ca²⁺ transient. C, the amplitude of the caffeine-induced Ca²⁺ transient. D, accumulated data showing the effects of PCr withdrawal and reintroduction on the fractional release (i.e. amplitude of caffeine-induced Ca²⁺ transient/amplitude of spontaneous Ca²⁺ transient). ***P < 0.002, n = 6–15.