Figure 2. Method for direct measurement of ATP utilization associated with Ca²⁺ pumping.

A fibre bundle was first exposed to pre-activating solution with a low [Ca²⁺] and there was very little utilization of ATP. The solution was then exchanged for one with pCa ≈ 5.75, resulting in the utilization of ATP at a moderate rate (ATPase_pCa=5.75; 4.6 pmol s⁻¹) but no force generation. To prove that the ATPase_pCa=5.75 does not include ATP utilized by crossbridges, 20 μm TBQ and 20 μm CPA were administered, causing the ATP utilization rate to drop to the baseline. This also shows that TBQ and CPA are effective in poisoning the SR-Ca²⁺ pumps.