Efficacy of peak Ca2+ currents (ICa) 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 (ICa) 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 ICa 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 InsP3 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 ICa was ‐0.80 +/‐ 0.17 microA cm‐2. delta[Ca2+] 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 microM Bay K 8644, peak ICa was ‐2.6 +/‐ 1.1 microA 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 microM Bay K 8644) increased peak ICa up to 12.5 microA cm‐2. ICa induced a delta[Ca2+]c of 990 +/‐ 210 nM and was accompanied by a ‘hump’ of IK,Ca. When applied briefly after peak delta[Ca2+]c, caffeine increased [Ca2+]c only moderately. The results suggest that a peak ICa can trigger a synchronized whole‐cell Ca2+ release only if ICa 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 ICa. 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.(ABSTRACT TRUNCATED AT 400 WORDS)