Biophys J, January 2001, p. 169-183, Vol. 80, No. 1

A Preferred Amplitude of Calcium Sparks in Skeletal Muscle

E. Ríos,^* N. Shirokova,^* W. G. Kirsch,^* G. Pizarro, M. D. Stern, H. Cheng, and A. González^*

 ^*Department of Molecular Biophysics and Physiology, Rush University, 1750 W. Harrison Street, Suite 1279 JS, Chicago, Illinois 60612 USA;  Departamento de Biofísica, Facultad de Medicina and Facultad de Ciencias, Montevideo, Uruguay; and  Laboratory of Cardiovascular Science, Gerontology Research Center, National Institute of Aging, National Institutes of Health, Baltimore, Maryland 21224 USA

In skeletal and cardiac muscle, calcium release from the sarcoplasmic reticulum, leading to contraction, often results in calcium sparks. Because sparks are recorded by confocal microscopy in line-scanning mode, their measured amplitude depends on their true amplitude and the position of the spark relative to the scanned line. We present a method to derive from measured amplitude histograms the actual distribution of spark amplitudes. The method worked well when tested on simulated distributions of experimental sparks. Applied to massive numbers of sparks imaged in frog skeletal muscle under voltage clamp in reference conditions, the method yielded either a decaying amplitude distribution (6 cells) or one with a central mode (5 cells). Caffeine at 0.5 or 1 mM reversibly enhanced this mode (5 cells) or induced its appearance (4 cells). The occurrence of a mode in the amplitude distribution was highly correlated with the presence of a mode in the distribution of spark rise times or in the joint distribution of rise times and spatial widths. If sparks were produced by individual Markovian release channels evolving reversibly, they should not have a preferred rise time or amplitude. Channel groups, instead, could cooperate allosterically or through their calcium sensitivity, and give rise to a stereotyped amplitude in their collective spark.

Biophys J, January 2001, p. 169-183, Vol. 80, No. 1
© 2001 by the Biophysical Society   0006-3495/01/01/169/15  $2.00

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