Simultaneous recording of Indo-1 fluorescence and Na+/Ca2+ exchange current reveals two components of Ca2(+)-release from sarcoplasmic reticulum of cardiac atrial myocytes.

Simultaneous measurements of intracellular Ca2(+)-concentration ([Ca2+]i) using Indo-1 and the current generated by electrogenic Na+/Ca2(+)-exchange (INaCa) have been performed on atrial myocytes from hearts of adult guinea-pigs. Whereas the fluorescence-measurements provide information on global [Ca2+]i, InaCa 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+]i. Our results provide evidence that in atrial myocytes, lacking a T-system, Ca2(+)-induced Ca2(+)-release occurs first from a subsarcolemmal compartment of the SR. The resulting Ca2(+)-transient serves to trigger Ca2(+)-release from deeper SR-compartments.