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Department of Physiology II, University of Tübingen, Germany.

1. At 36 degrees C and 2 mM [Ca2+]o single guinea-pig ventricular myocytes were voltage clamped with patch electrodes. With a paired-pulse protocol applied at 1 Hz, a first pulse to +5 mV was followed by a second pulse to +50 mV. When paired pulsing had potentiated the contraction to the maximum, the cells were shock-frozen for electron-probe microanalysis (EPMA). Shock-freezing was timed at the end of diastole (-80 mV) or at different times during systole (+5 mV). 2. The same paired-pulse protocol was applied to another group of myocytes from which contraction and [Ca2+]i was estimated by microfluospectroscopy (50 microM-Na5-Indo-1). Potentiation moderately reduced diastolic sarcomere length from 1.85 to 1.82 microns and increased diastolic [Ca2+]i from about 95 to 180 nM. In potentiated cells, during the first pulse, contraction peaked within 128 +/- 25 ms after start of depolarization. [Ca2+]i peaked within 25 ms to 890 +/- 220 nM (mean +/- S.E.M.) and fell within 100 ms to about 450 nM. 3. Sigma Camyo, the total calcium concentration in the overlapping myofilaments (A-band), was measured by EPMA in seventeen potentiated myocytes. During diastole, sigma Camyo was 2.6 +/- 0.4 mmol (kg dry weight (DW]-1 which can be converted to 0.65 mM (mmoles per litre myofibrillar space). Since [Ca2+]i was 180 nM, we estimate that 99.97% of total calcium is bound. 4. A time course for systolic sigma Camyo was determined by shock-freezing thirteen cells at different times after start of depolarization to +5 mV. Sigma Camyo was 5.5 +/- 0.3 mmol (kg DW)-1 (1.4 mM) after 15-25 ms, 4.6 +/- 0.5 mmol (kg DW)-1 (1.1 mM) after 30-45 ms, and 3.1 mmol (kg DW)-1 (0.8 mM) after 60-120 ms. The fast time course of sigma Camyo suggests that calcium binds to and unbinds from troponin C at a fast rate. Hence, it is the slow kinetics of the cross-bridges that determines the 130 ms time-to-peak shortening. 5. Mitochondria of potentiated cells contained during diastole a total calcium concentration, sigma Camito, of 1.3 +/- 0.2 mmol (kg DW)-1 (0.4 mM). During the initial 15-25 ms of systole, sigma Camito did not change, however, during 30-45 ms sigma Camito rose to 3.7 +/- 0.5 mmol (kg DW)-1 (1.2 mM). The data suggest that sigma Camito can follow sigma Camyo with some delay, thereby participating in both slow diastolic and fast systolic changes in total calcium (sigma Ca), at least under the given conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

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