Figure 8. Effects of CP and P_i on net SR Ca²⁺ uptake.

A, simultaneous records of fluorescence ratio (upper panel) and force (lower panel) from a muscle preparation comprising two mechanically skinned fibres. The preparation was initially perfused with a solution containing 5 mm ATP and 10 mm CP. Introduction of 20 mm caffeine into the perfusate resulted in Ca²⁺ release from the SR and an associated force response. SR Ca²⁺ uptake was then abolished by withdrawal of ATP. This resulted in the development of a rigor tension response and a further, transient SR Ca²⁺ efflux. Perfusion was stopped and the bath solution replaced rapidly with one containing 5 mm ATP. This resulted in relaxation from rigor and a maintained reduction in [Ca²⁺] due to net SR Ca²⁺ uptake. B, superimposed fluorescence ratio records of ATP-induced SR Ca²⁺ uptake under control conditions, or in the presence of 20 mm P_i (left). In the presence of CP, 20 mm P_i had little effect on SR Ca²⁺ uptake. However, when CP was absent from the perfusate, Ca²⁺ reuptake was reduced. When 20 mm P_i was present (but CP absent), Ca²⁺ uptake was further reduced and a slow increase in [Ca²⁺] followed the initial rapid decline (right). C, in the presence of 10 mm CP and 20 mm P_i, Ca²⁺ reuptake was markedly reduced following inhibition of CK with 50 μm DNFB. Responses shown in A and B were from the same preparation.