EFFICACY OF PEAK CA2+ CURRENTS (I-CA) AS TRIGGER OF SARCOPLASMIC-RETICULUM CA2+ RELEASE IN MYOCYTES FROM THE GUINEA-PIG CORONARY-ARTERY

1. Increments in cytosolic Ca2+ concentration (Delta[Ca2+](c)) were measured in single smooth muscle cells from guinea-pig coronary artery together with the density of peak Ca2+ currents (I-Ca) in response to clamp steps from -50 to 0 mV. The comparison of depolarization- with caffeine-induced Delta[Ca2+](c) was used to define the efficacy by which I-Ca can trigger Ca2+ release from the sarcoplasmic reticulum (SR). 2. At 2.5 mM extracellular calcium concentration ([Ca2+](o)), depolarization induced a rapid rise of Delta[Ca2+](c) followed by a slow creep. Peak [Ca2+](c) occurred within ca 30 s and could be followed by an undershoot and a second rise in [Ca2+](c). The creep was blocked by ryanodine but was insensitive to block of InsP(3) receptors with heparin. The creep was not observed in Cs+-filled cells. After disappearance of the creep, a tonic Delta[Ca2+](c) became unmasked. 3. At 2.5 mM [Ca2+](o), peak I-Ca was -0.80 +/- 0.17 mu A cm(-2). Delta[Ca2+](c) peaked at the end of the 6 s pulse at 202 +/- 98 nM while caffeine-induced Delta[Ca2+](c) peaked at 1330 +/- 410 nM. The ratio of depolarization- to caffeine-induced Delta[Ca2+](c) was 10 +/- 6%. 4. In media containing 10 mM [Ca2+](o) plus 1 mu M Bay K 8644, peak I-Ca was -2.6 +/- 1.1 mu A cm(-2) and Delta[Ca2+](c) peaked within 2.5 s at 451 +/- 194 nM. Paired measurements yielded the ratio of depolarization- to caffeine-induced Delta[Ca2+](c) as 30 +/- 10%. Depolarization-induced Delta[Ca2+](c) was nearly blocked by caffeine and reduced by ryanodine to 30%, suggesting the contribution of Ca2+ release from caffeine- and ryanodine-sensitive Ca2+ stores. 5. Trypsin (1 mg ml(-1)) in the electrode solution (10 mM [Ca2+](o) plus 1 mu M Bay K 8644) increased peak I-Ca up to 12.5 mu A cm(-2). I-Ca induced a Delta[Ca2+](c) of 990 +/- 210 nM and was accompanied by a 'hump' of I-K,I-Ca. When applied briefly after peak Delta[Ca2+](c), caffeine increased [Ca2+](c) only moderately. The results suggest that a peak I-Ca can trigger a synchronized whole-cell Ca2+ release only if I-Ca is strongly augmented. 6. Amplitude and rate of rise of Delta[Ca2+](c) were graded by test step potentials along a bell-shaped voltage-dependent curve, similar to that of L-type I-Ca. Steps to +80 mV induced no Delta[Ca2+](c) when the electrode solution contained 10 mM Na+. However, with 150 mM intrapipette Na+, pulses to +80 mV induced Delta[Ca2+](c). 7. We conclude that synchronized whole-cell SR Ca2+ release cannot be triggered by peak I-Ca at physiological [Ca2+](o) but by a propagating Ca2+ wave, by augmented peak I-Ca or by caffeine. In contrast to diffusion of caffeine, diffusion of trigger Ca2+ is modified by binding, release and redistribution.