K+ induced depolarization increased the frequency of Ca2+ sparks. (A) Original recordings of Ca2+ sparks in the absence (top trace), presence (middle trace) and washout (bottom trace) of 37.5 mM K+. Numbers beside transients indicate sparks. (B) Elevation of extracellular K+ from 3.6 to 37.5 mM increased the frequency of Ca2+ sparks but had no effect on amplitude, rise and half decay times of Ca2+ sparks. Recordings in the presence of 37.5 mM K+ were made at 10, 40 and 70s after K+ application and recordings during washout were made at 10, 40 and 70s after termination of K+ application. Time control experiments are replotted from Figure 3.