Simultaneous recording of Indo‐1 flourescence and Na+/Ca2+ exchange current reveals two components of Ca2+‐release from sarcoplasmic reticulum of cardiac atrial myocytes

Abstract

Simultaneous measurements of intracellular Ca²⁺‐concentration ([Ca²⁺]₁) using Indo‐1 and the current generated by electrogenic Na¹/Ca²⁺‐exchange (I_Na??? have been performed on atrial myocytes from hearts of adult guinea‐pigs. Whereas the flourescence‐measurements provide information on global[Ca²⁺]_i, I_Na??? 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.