HIGH CYTOPLASMIC CA2-INDUCED CA2+ RELEASE IN SINGLE SMOOTH-MUSCLE CELL AS REPORTED BY A LOW-AFFINITY CA2+ INDICATOR MAG-INDO-1( LEVELS REACHED DURING CA2+)

The low-affinity Ca2+ indicator Mag-Indo-1 was used to measure increme nts of ionised Ca2+ concentration in the cytoplasm of single smooth mu scle cells isolated from guinea-pig urinary bladder. With 3.6 mM [Ca2](0), depolarization steps to 0 mV were associated with a transient in crease of fluorescence ratio (F-410/F-470) only when I-Ca triggered a Ca2+-induced Ca2+ release (CICR). [Ca2+](i) transiently peaked to 3-5 mu m and despite continuous Ca2+ influx the [Ca2+](i) signal fell clos e to the baseline. Rapidly applied caffeine (10 mM) increased [Ca2+](i ) by 16 mu M, the response was completely blocked by intracellular rya nodine (20 mu M). With ryanodine intracellularly, I-Ca produced very s mall [Ca2+](i) signals unless it was augmented by elevation of [Ca2+]( 0) to 10 mM and addition of 1 mu M Bay K8644. Under these conditions, [Ca2+](i) responded with a tonic elevation lasting as long as the depo larizing pulse. It is concluded that the low-affinity indicator Mag-In do-1 reports predominantly Ca2+ release from SR in cytoplasm.