Potentiation of contraction as related to changes in free and total intracellular calcium

In voltage-clamped guinea-pig ventricular myocytes, we studied the potentiation of contraction in dependence on the concentration of intracellular calcium; ionized calcium [Ca2+]c was measured by Indo-1 microfluospectroscopy and total calcium (sigma Ca) by electronprobe microanalysis (EPMA). After a 15 min rest period, [Ca2+]c was approx. 90 nM and sigma Ca was below the detection limit (80 microM) in myoplasm (sigma Ca(myo)), junctional sarcoplasmic reticulum (sigma CaSR) and mitochondria (sigma Ca(Mito)). Post rest, repetitive clamp steps (1 Hz) potentiated extent and rate of shortening by 300%. In the literature, post-rest potentiation is attributed to the replenishment of SR with releasable calcium; by EPMA the postulated increase in sigma CaSR was measured directly. Post-rest, the peaks of systolic [Ca2+]c transients increased, however only by 40%. In addition, a moderate increase of end-diastolic [Ca2+]c was measured. In an other series of experiments, contraction was potentiated by 800% increase by means of paired voltage-clamp pulses (1 Hz, 36 degrees C, 2 mM [Ca2+]o). In the potentiated state, end-diastolic [Ca2+]c was 180 nM and sigma Ca(myo) was 0.65 mM. During systole, [Ca2+]c peaked within 20 ms to 950 nM. sigma Ca(myo) rose within 20 ms to 1.4 mM and fell within 40 ms to 1.1 and within 90 ms to 0.8 mM. In contrast, the time course of contraction was slow and peaked at a time (130 ms) when the [Ca2+]c and sigma Ca(myo) transients were finished. We suggest that Ca2+ bound to troponin C (TnC) controls only the onset but not the time course of myofilament interaction. From [Ca2+]c and sigma Ca(myo) we estimated a Ca2+ buffering capacitance of 1.5 mmol sigma Ca(myo) per pCa change, only a fraction of which can be attributed to Ca2+ binding sites on TnC. A model explaining the results requires the assumption of 0.6 mM additional slow, high affinity Ca2+ sites and 2 mM fast, low affinity Ca2+ sites. We discuss that end-diastolic Ca2+ binding to these sites contributes to the potentiation of contraction. Junctional SR. At the end of diastole sigma CaSR was 2.4 mM which is 4 times larger than sigma Ca(myo). This difference disappeared 20 ms after depolarization (sigma CaSR 1.1 mM), within another 20 ms it largely recovered (sigma CaSR 2.0 mM). These properties suggest that the junctional SR is a compartment suitable not only for Ca2+ release but also for rapid Ca2+ reuptake. Mitochondria. Paired-pulse potentiation increased end-diastolic sigma Ca(Mito) significantly (0.4 mM).(ABSTRACT TRUNCATED AT 400 WORDS)