Inorganic phosphate decreases the Ca2+ content of the sarcoplasmic reticulum in saponin‐treated rat cardiac trabeculae.

1. Measurements of [Ca2+] were made in saponin‐permeabilized rat ventricular trabeculae using the fluorescent indicator Indo‐1. Application of caffeine (20 mM) caused a transient rise in [Ca2+] within the preparation as a result of Ca2+ release from the sarcoplasmic reticulum (SR). The size of the caffeine‐induced Ca2+ transient was related to the amount of Ca2+ accumulated by the SR prior to addition of caffeine. Caffeine‐induced Ca2+ release was abolished by ryanodine (10 microM), an inhibitor of SR Ca2+ release. 2. At a bathing [Ca2+] of 0.2 microM, the amount of Ca2+ released from the SR on addition of caffeine was sufficient to generate a tension transient. Ca2+ and tension responses were stabilized by application of caffeine at regular intervals (2 min). Addition of 10 mM inorganic phosphate (Pi) induced a transient increase in [Ca2+] within the preparation due to a net release of Ca2+ from the SR. The amplitude of subsequent caffeine‐induced Ca2+ transients were reduced to 65 +/‐ 7.5% (mean +/‐ S.D., n = 13) of control. In addition, the accompanying tension transient fell to 45 +/‐ 6.9% of control. Removal of Pi caused a transient decrease in the [Ca2+] within the preparation consistent with a net increase in Ca2+ uptake by the SR. Subsequent caffeine‐induced Ca2+ and tension transients returned to control levels. 3. Inclusion of Pi (2‐30 mM) in the perfusing solution decreased the size of caffeine‐induced Ca2+ and tension transients in a dose‐dependent manner. 4. Addition of 10 mM ADP caused a transient increase in [Ca2+] and depressed subsequent caffeine‐induced Ca2+ transients to a greater extent than 10 mM Pi. Despite the reduction in Ca2+ release from the SR, tension responses were larger in the presence of 10 mM ADP than under control conditions. This is a consequence of an increase in Ca(2+)‐activated force by ADP. 5. A decrease in the amplitude of caffeine‐induced Ca2+ transients also occurred on changing from a solution containing 1 mM ADP and 10 mM Pi to a solution with 10 mM ADP and 1 mM Pi. This confirms the previous observation that ADP is more effective than Pi at reducing caffeine‐induced Ca2+ released from the SR. 6. Spontaneous oscillations of [Ca2+] and tension occurred in the presence of 0.5 microM Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)