Comparative effects of inorganic phosphate and oxalate on uptake and release of Ca2+ by the sarcoplasmic reticulum in saponin skinned rat cardiac trabeculae.

1. Ventricular trabeculae from the right ventricle of rat heart were suspended in a 6 microliters bath and "skinned' with saponin (50 mg ml-1). Preparations were perfused with solutions mimicking the intracellular milieu and the [Ca2+] within the bath was monitored continuously using fura-2. 2. Application of 20 mM caffeine released Ca2+ from the sarcoplasmic reticulum (SR), resulting in a transient increase in the fura-2 fluorescence ratio. Caffeine-induced Ca2+ transients were smaller in the presence of 30 or 60 mM inorganic phosphate (Pi). This depressive effect of Pi on SR function was reversed by 10 mM creatine phosphate (CP). Caffeine-induced Ca2+ transients were also reduced in the presence of 10 mM oxalate, although this effect was not reversed by CP. 3. When perfusion was stopped in the presence of 30 or 60 mM Pi, the [Ca2+] within the bath remained constant. However, when the flow was stopped in the presence of 60 mM Pi and 10 mM CP, a prolonged decrease in [Ca2+] occurred, consistent with precipitation of calcium phosphate within the SR. A similar decrease in [Ca2+] was observed when perfusion was stopped in the presence of 2 or 20 mM oxalate, in the absence or presence of CP. 4. The SR was Ca2+ depleted by withdrawal of ATP and exposure to 20 mM caffeine. Perfusion was then stopped and ATP reapplied, resulting in a maintained decrease in [Ca2+] within the bath, due to SR Ca2+ uptake. Net Ca2+ uptake was markedly reduced in the presence of 30 mM Pi. In contrast, 20 mM oxalate increased Ca2+ uptake and the [Ca2+] within the bath continued to fall over 2-3 min. 5. Introduction of Pi released Ca2+ from the SR. Ryanodine (100 microM) abolished caffeine-induced Ca2+ release while Pi-induced Ca2+ release was unaffected. Pi-induced Ca2+ release was reduced in the constant presence of 20 mM caffeine or 10 mM CP and was abolished completely by disruption of the SR membrane with Triton X-100. Pi-induced Ca2+ release occurred after abolition of SR Ca2+ uptake by ATP withdrawal. 6. These results suggest that the Pi-induced decrease in releasable Ca2+ does not result from precipitation of calcium phosphate within the SR lumen. Pi inhibits net SR Ca2+ uptake, but this appears to result from activation of a ryanodine-insensitive Ca2+ efflux pathway rather then inhibition of Ca2+ uptake. Possible mechanisms are considered, including reversal of the SR Ca2+ pump.