Ca2+ 'sparks' and waves in intact ventricular muscle resolved by confocal imaging

The [Ca2+]i transient in heart is now thought to involve the recruitment and summation of discrete and independent "units" of Ca2+ release (Ca2+ "sparks") from the sarcoplasmic reticulum, each of which is controlled locally by single coassociated L-type Ca2+ channels ("local control theory of excitation-contraction coupling"). All prior studies on Ca2+ sparks, however, have been performed in single enzymatically dissociated heart cells under nonphysiological conditions. In order to understand the possible significance of Ca2+ sparks to normal working cardiac muscle, we used confocal microscopy to record Ca2+ sparks, spatially averaged [Ca2+]i transients and Ca2+ waves in individual cells of intact rat right ventricular trabeculae (composed of < 15 cells in cross section) microinjected with the Ca2+ indicator fluo 3 under physiological conditions ([Ca2+]o, 1 mmol/L; temperature, 33 +/- 1 degree C). Twitch force was recorded simultaneously. When stretched to optimal length (sarcomere length, 2.2 microns) and stimulated at 0.2 Hz, the trabeculae generated approximately equal to 700 micrograms of force per cell. Spatially averaged [Ca2+]i transients recorded from individual cells within a trabecula were similar to those recorded previously from single cells. The amplitude distribution of the peak ratio of Ca2+ sparks was bimodal, with maxima at ratios of 1.8 +/- 0.3 and 2.7 +/- 0.2 (mean +/- SD), respectively. The amplitude of the peak of Ca2+ sparks was approximately equal to 170 nmol/L. Ca2+ sparks occurred at a frequency of 12.0 +/- 0.8/s (mean +/- SEM) in line scans covering 94 sarcomeres. Ca2+ waves occurred randomly at a frequency of 0.57 +/- 0.08/s and propagated with a velocity of 29.5 +/- 1.7 microns/s. The extent of Ca2+ wave propagation was 3.9 +/- 0.3 sarcomere lengths (sarcomere length, 2.2 microns). Ca2+ sparks could be identified along the leading edge of the waves at intervals of 1.30 +/- 0.11 sarcomere length. Our observations suggest that (1) Ca2+ sparks, similar to those recorded in single cells, occur in trabeculae under physiological conditions and (2) coupling of Ca2+ spark generation between neighboring sites occurs and may lead to (3) the development of Ca2+ waves, which propagate under physiological conditions at a low velocity over limited distances. The results suggest that concepts of excitation-contraction coupling recently derived from isolated myocytes are applicable to intact cardiac trabeculae [corrected].