Simultaneous measurements of intracellular Ca2+-concentration ([Ca2+]1) using Indo-1 and the current generated by electrogenic Na1/Ca2+-exchange (INa??? have been performed on atrial myocytes from hearts of adult guinea-pigs. Whereas the flourescence-measurements provide information on global[Ca2+]i, INa??? which is a linear function of Ca2+-concentration, indicates subsarcolemmal [Ca2+]. Under conditions in which intracellular Ca2+-transients due to Ca2+-release from the sarcoplasmic reticulum (SR) have been artificially slowed, a deviation between the two different Ca2+-signals can be found. During onset of release signals Ca2+-concentration seen by the membrane is higher than global [Ca2+]1. Our results provide evidence that in atrial myocytes, lacking a T-system, Ca2+-release occurs first from a subsarcolemmal compartment of the SR. The resulting Ca2+-transient serves to trigger Ca2+-release from deeper SR-compartments.
