Calcium gradients during excitation‐contraction coupling in cat atrial myocytes.

Confocal microscopy in combination with the calcium-sensitive fluorescent probe fluo-3 was used to study spatial aspects of intracellular Ca2+ signals during excitation-contraction coupling in isolated atrial myocytes from cat heart. 2. Imaging of Ca2+i transients evoked by electrical stimulation revealed that Ca2+ release started at the periphery and subsequently spread towards the centre of the myocyte. 3. Blocking sarcoplasmic reticulum (SR) Ca2+ release with 50 microM ryanodine unmasked spatial inhomogeneities in the Ca2+i was higher in the periphery than in central regions of the myocyte. 4. Positive (or negative) staircase or postrest potentiation of the 'whole-cell' Ca2+ transients were paralleled by characteristic changes in the spatial profile of the Ca2+i signal. With low SR Ca2+ load Ca2+i transients in the subsarcolemmal space were small and no Ca2+ release in the centre of the cell was observed. Loading of the SR increased subsarcolemmal Ca2+i transient amplitude and subsequently triggered further release in more central regions of the cell. 5. Spontaneous Ca2+ release from functional SR units, i.e. Ca2+ sparks, occurred at higher frequency in the subsarcolemmal space than in more central regions of the myocyte. 6. Visualization of the surface membrane using the membrane-selective dye Di-8-ANEPPS demonstrated that transverse tubules (t-tubules) were absent in atrial cells. 7. It is concluded that in atrial myocytes voltage-dependent Ca2+ entry triggers Ca2+ release from peripheral coupling SR that subsequently induces further Ca2+ release from stores in more central regions of the myocyte. Spreading of Ca2+ release from the cell periphery to the centre accounts for Ca2+i gradients underlying the whole-cell Ca2+i transient. The finding that cat atrial myocytes lack t-tubules demonstrates the functional importance of Ca2+ release from extended junctional (corbular) SR in these cells.