SUBCELLULAR PROPERTIES OF TRIGGERED CA2-LOADED GUINEA-PIG ATRIAL MYOCYTES CHARACTERIZED BY RATIOMETRIC CONFOCAL MICROSCOPY( WAVES IN ISOLATED CITRATE)

1. Spatiotemporal aspects of subcellular Ca2+ signalling were studied in cultured adult guineapig atrial myocytes. A mixture of the Ca2+ ind icators fluo-3 and Fura Red in combination with laser-scanning confoca l microscopy was used for [Ca2+](i) measurements while membrane curren ts were recorded simultaneously.2. In citrate-loaded atrial myocytes n ot every Ca2+ current (I-Ca) could trigger Ca2+ release from the sarco plasmic reticulum (SR). Two types of Ca2+ signals could be observed: C a2+ transients resulting from (i) Ca2+ influx alone and (ii) additiona l Ca2+ release. 3. Ca2+ release elicited by voltage steps of 100-150 m s duration was either apparently homogeneous or propagated as Ca2+ wav es through the entire cell. With brief I-Ca (50-75 ms), Ca2+ waves wit h limited subcellular propagation were observed frequently. These wave s always originated from either end of the myocyte. 4. The time course of changes in Na+-Ca2+ exchange current (I-NaCa) depended on the subc ellular properties of the underlying Ca2+ transient and on the particu lar cell geometry. Apparently homogeneous Ca2+ release was accompanied by an inward change of I-NaCa the onset phase of which was fused with I-Ca. Changes in I-NaCa caused by a Ca2+ wave propagating through the entire cell showed a W shape, which could be attributed to difference s of the fractional surface-to-volume ratio in different cell segments during propagation of the Ca2+ wavefront. Those waves with limited sp reading only activated a small component of I-NaCa. 5. The different s ubcellular patterns of Ca2+ release signals can be explained by spatia l inhomogeneities in the positive feedback of the SR. This depends on the local SR Ca2+ loading state under the control of the local Ca2+ in flux during activation of I-Ca. Due to the higher surface-to-volume ra tio at the two ends of the myocyte, SR loading and therefore the posit ive feedback in Ca2+-induced Ca2+ release may be higher at the ends, l ocations where Ca2+ waves are preferentially triggered. 6. We conclude that the individual cell geometry may be an important determinant of subcellular Ca2+ signalling not only in cardiac muscle cells but presu mably also in other types of cells that depend on Ca2+ signalling. In addition, the cell geometry in combination with varying subcellular Ca 2+ release patterns can greatly affect the time course of Ca2+-activat ed membrane currents.