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. Ln order to understand the possible significance of Ca2+ sparks to normal working cardiac muscle, we used confocal microscopy to record Ca2+ sparks, which are 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 flue 3 under physiological conditions ([Ca2+](upsilon), 1 mmol/L; temperature, 33+/-1 degrees C). Twitch force was recorded simultaneously. When stretched to optimal length (sarcomere length, 2.2 mu m) and stimulated at 0.2 Hz, the trabeculae generated approximate to 700 mu g 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 Ca:(2+) sparks was approximate 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 mu m/s. The extent of Ca2+ wave propagation was 3.9+/-0.3 sarcomere lengths (sarcomere length, 2.2 mu m). 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 these 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.