Measurement of SR Ca2+ content in the presence of caffeine in permeabilised rat cardiac trabeculae

 This study was designed to measure the Ca²⁺ content of rat cardiac sarcoplasmic reticulum (SR) after equilibration with normal diastolic levels of Ca²⁺ (100 nM), in the absence and presence of caffeine. Measurements of Ca²⁺] based on Fura-2 fluorescence were made from a limited bath volume (230 nl) containing individual saponin-permeabilised rat cardiac trabeculae. Injection of caffeine (5–40 mM) into this volume caused an initial release of Ca²⁺ from the SR, but within 30 s the SR was able to re-accumulate a significant proportion of the Ca²⁺. Ca²⁺ re-accumulation into the SR could be prevented by removal of ATP to inhibit the SR Ca²⁺ pump. Incubation of the preparation in an ATP-containing solution containing caffeine (5–40 mM) and 100 nM Ca²⁺ indicated that the SR’s ability to retain Ca²⁺ depends inversely on the dose of caffeine. The relative Ca²⁺ content of the SR after preincubation with caffeine was 86.7±3.5% at a caffeine concentration of 5 mM, 62.5±5.1% at 10 mM caffeine, 37.8±8.1% at 20 mM caffeine and 7.1±1.9% at 40 mM caffeine. Measurement of the SR Ca²⁺ release in the presence of different BAPTA concentrations was used to calculate (1) the Ca²⁺-binding capacity of the preparation (equivalent to 245±10 μM BAPTA) and (2) the Ca²⁺ content of the SR accessed by caffeine after equilibration with 100 nM Ca²⁺ (186±11 μmol/l cell volume or 5.6 mmol/l SR volume). Received: 9 June 1998 / Received after revision: 29 July 1998 / Accepted: 31 July 1998