High cytoplasmic Ca2+ levels reached during Ca(2+)-induced Ca2+ release in single smooth muscle cell as reported by a...

The low-affinity Ca2+ indicator Mag-Indo-1 was used to measure increments of ionised Ca2+ concentration in the cytoplasm of single smooth muscle cells isolated from guinea-pig urinary bladder. With 3.6 mM [Ca2+]o, depolarization steps to 0 mV were associated with a transient increase of fluorescence ratio (F410/F470) only when Ica triggered a Ca(2+)-induced Ca2+ release (CICR). [Ca2+]i transiently peaked to 3-5 microM and despite continuous Ca2+ influx the [Ca2+]i signal fell close to the baseline. Rapidly applied caffeine (10 mM) increased [Ca2+]l by 16 microM, the response was completely blocked by intracellular ryanodine (20 microM). With ryanodine intracellularly, Ica produced very small [Ca2+]i signals unless it was augmented by elevation of [Ca2+]0 to 10 mM and addition of 1 microM Bay K8644. Under these conditions, [Ca2+]l responded with a tonic elevation lasting as long as the depolarizing pulse. It is concluded that the low-affinity indicator Mag-Indo-1 reports predominantly Ca2+ release from SR in cytoplasm.