Effect of stimulation frequency on intracellular Na+ activity in rat atrial muscle

Mean intracellular Na+ activity (aNai) was measured in rat left atrial muscle stimulated at increasing frequencies between 0 and 12 Hz. Low-pass filtered signals from conventional and ion-selective microelectrodes were used to determine aNai. Preparations were bathed in a low Ca2+ (0.1 mM) Krebs-Henseleit buffer containing 1.0 mM Mn2+ to abolish contractile motion and permit stable impalements. Under these conditions, aNai increased progressively with frequency from 5.8 +/- 1.5 mM at 0 Hz to a maximum of 12.7 +/- 2.1 mM, which was observed at 10, 11, or 12 Hz. Further increases in frequency exceeded the effective refractory period, and aNai tended to decrease. These data suggest that aNai can be approximately doubled in rat atrial muscle by increasing the depolarization rate from 0 to 10-12 Hz, a range that has been shown to elicit a two- to three-fold elevation in Na+-pump activity in similar preparations.