EFFECTS OF EXTRACELLULAR-SODIUM, QUINIDINE, AND RYANODINE ON SLOW RESPONSE EXCITABILITY IN RABBIT ATRIAL TRABECULAE

1. We investigated Na+-Ca2+ exchange and the involvement of the sarcop lasmic reticulum in frequency-dependent slow response excitability enh ancement in rabbit atrial trabeculae. 2. Slow responses were induced i n a modified Tyrode solution containing high K+ and Ba2+ and conventio nal electrophysiological techniques were used for stimulating and reco rding membrane potentials. 3. Under these conditions, the frequency-de pendence of slow response excitability can be demonstrated with excita bility enhancement as stimulation frequency is increased (0.25 to 1.0 Hz). 4. The frequency-dependent excitability enhancement depends on ex ternal Na+, increasing in high-[Na+](o) (173.8 mM) and decreasing in l ow-[Na+](o) (103.8 mM) media. 5. Quinidine (10 mu M) and ryanodine (10 mu M) decrease frequency-dependent slow response excitability enhance ment. 6. These results indicate that the Na+-Ca2+ exchange might have an important role in frequency-dependent excitability enhancement of s low responses. Moreover, we suggest that the control of internal Ca2by the sarcoplasmic reticulum might have an additional role in regulat ing the excitability enhancement process in depolarized atrial trabecu lae.