Figure 1. Effects of P_i on net SR Ca²⁺ uptake in the presence of 10 mm CP.

Typical records of fura-2 fluorescence ratio from a preparation comprising two mechanically skinned EDL muscle fibres. A, after 10 min equilibration in the presence of 10 mm CP, stopping perfusion had no effect on fluorescence ratio (left). During continuous perfusion, introduction of 20 mm P_i caused a transient SR Ca²⁺ efflux, followed by a sustained reduction in bathing [Ca²⁺] below control levels. After 3 min exposure to 20 mm P_i the flow was stopped. This resulted in a maintained decrease in bathing [Ca²⁺] due to net SR Ca²⁺ uptake (middle). The preparation was then exposed to a P_i-free solution for over 10 min. Subsequent addition of 2 mm oxalate caused a sustained reduction in bathing [Ca²⁺] below control levels, which was more pronounced when the flow was stopped (right). B, after a control ‘stop flow’ response (left) introduction of 20 mm P_i induced a transient release of Ca²⁺, followed by a small maintained decrease in the resting [Ca²⁺], which became more pronounced when the flow was stopped. When the flow was resumed, [Ca²⁺] returned rapidly to the lower steady-state level. After a further 10 min perfusion, a similar decrease in [Ca²⁺] occurred when the flow was stopped (middle). Following perfusion with a P_i-free solution for 10 min, [Ca²⁺] remained constant when the flow was stopped (right). C, a control response to introduction of P_i and stopping the flow was followed by addition of 20 μm CPA (left). This resulted in a slow transient release of Ca²⁺ from the SR and the [Ca²⁺] returned to the original control level. When the flow was then stopped, a further slow increase in [Ca²⁺] occurred. D, after exposure to Triton X-100 to disrupt the SR membrane, the [Ca²⁺] remained constant when the flow was stopped.