Small event Ca²⁺ release: a probable precursor of Ca²⁺ sparks in frog skeletal muscle

Abstract

Fluo‐3 fluorescence associated with Ca2+ release was recorded with confocal microscopy in single muscle fibres. Clamp depolarization to −65 or −60 mV elicited Ca2+ sparks with amplitudes and spatial widths distributed approximately normally, with mean values of 0.79 of resting fluorescence and 0.8 μm (S.D., 0.17 and 0.2 μm; n= 193), respectively. Given these distributions, events of amplitude less than 0.45 or width less than 0.4 μm are unlikely to be sparks. Low voltage depolarization (−72 mV) elicited only one spark per triad every 6 s, but generated a relative increase in fluorescence at triads of 0.05. This increase must therefore have been due to events smaller than sparks. The variance/mean ratio of triadic fluorescence gradients averaged 0.11 at low voltages and increased severalfold at the higher voltages at which sparks appeared, indicating the existence of at least two event amplitudes. Tetracaine (200 μm) reversibly abolished sparks and the early peak of Ca2+ release at all voltages. In its presence, discrete events were smaller than the spark criterion, and triadic gradients had a variance/mean ratio of 0.11. The phenylalkylamine D600 (2 μm) reduced release at all voltages, abolishing sparks and the peak of Ca2+ release at low but not at high voltages. The parallel abolition of all sparks and the peak of Ca2+ release indicates that both phenomena are activated by Ca2+. The restoration of sparks by voltage in D600 suggests that release in small events provides the trigger Ca2+ for activation of sparks.

Elemental propagation of calcium signals in response-specific patterns determined by environmental stimulus strength

“…In pollen tubes, discrepancy between the position of major vacuoles and the location of Ca 2ϩ signals, elicited by photorelease of caged Ins(1,4,5)P 3 , also suggested the involvement of nonvacuolar Ca 2ϩ stores (30). The present study provides evidence that Ca 2ϩ elevations propagate as unitary increases that have not been reported in nonanimal cells and which meet the criteria for elemental Ca 2ϩ release from single channels or groups of channels in the ER (31)(32)(33)(34)(35)(36)(37). Moreover, the demarcation of osmotic Ca 2ϩ signals to specific domains according to the strength of the stimulus indicates that Ca 2ϩ release may be regulated differently in apical and perinuclear regions.…”

Spatiotemporal Characteristics of Junctional and Nonjunctional Focal Ca 2+ Release in Rat Atrial Myocytes

“…The focal Ca 2ϩ releases that had one stationary center for their growth and decay were then subjected to Gaussian approximations, 32 which allowed detailed morphometry and routine measurements of the amplitude, width, and equivalent area of sparks originating from the peripheral and central regions. Individual focal Ca 2ϩ release signal in single frames were approximated in a restricted area (30ϫ30 pixels) by a Gaussian function defined in space 32,37 by its center (x o ,y o ), the central increase in fluorescence (⌬F), and its standard deviation (), giving a total fluorescence:…”

Calcium-Induced Calcium Release in Skeletal Muscle

“…The presence and absence of small event Ca 2ϩ release, however, may not be the difference between cut fibers and intact fibers, because small event Ca 2ϩ release was not observed in other cut-fiber experiments (129, 130). In any case, Ca 2ϩ sparks in cut fibers occurring over the small event Ca 2ϩ releases also have a stereotypic morphology (84,234), which again indicates that coordination has an extremely high success rate.…”