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

Abstract

1. 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.