FIGURE 4.

The effects of ISO and higher stimulation frequencies on [Ca²⁺]_i transient parameters in cardiomyocytes isolated from old WT and R21C mice. A, representative [Ca²⁺]_i transients recorded from WT (black) and R21C (gray) cardiomyocytes paced at 1 Hz in the absence (solid lines) and presence (dashed lines) of 100 nmol/liter ISO. In B–E, the following parameters are plotted as a function of stimulation frequency in the absence (solid lines) and presence (dashed lines) of ISO. B, t_½ of [Ca²⁺]_i decay; C, TTP systolic [Ca²⁺]_i; D, diastolic [Ca²⁺]_i; E, Ca²⁺ amplitude. t_½, time it takes the [Ca²⁺]_i transient amplitude to decrease 50%. Data are given as mean ± S.E. Measurements were taken from three WT and three R21C mice; 4–6 cells/heart were recorded and analyzed. The data are summarized in Table 4. *, p < 0.05 versus WT −ISO; §, p < 0.05 versus R21C −ISO; †, p < 0.05 versus WT +ISO; ‡, p < 0.05 versus WT −ISO. Error bars, S.E.