Fig. 5.

Mean Ca²⁺–force relations and distributions of pCa₅₀ values in isolated cardiomyocytes. a Ca²⁺–force relations of isolated cardiomyocytes obtained from the pacing and opposite sites of the HF animals before and after in vitro PKA treatments. The leftward position of the Ca²⁺–force relations at the HF pacing site before PKA treatment illustrated an increased Ca²⁺-sensitivity of force production in cardiomyocytes of this region relative to all others. After PKA treatment, a rightward shift in the Ca²⁺–force relation was observed. b Small differences between the Ca²⁺–force relationships of the control pacing and control opposite groups could be detected before and after in vitro PKA treatment. c Means of pCa₅₀ values in the four groups of cardiomyocytes before and after in vitro PKA treatment. Mean pCa₅₀ at the HF pacing site was significantly higher than in all other groups under basal conditions, and this difference could be eliminated by PKA (*P < 0.05). d PKA-evoked changes in pCa₅₀ values in the four experimental groups. The effect of PKA on mean pCa₅₀ was the highest at the pacing site of HF animals (*P < 0.05). The Hill coefficient did not differ significantly among the four experimental groups before and after PKA exposures (n _Hill ~2.6). e–h Distributions of pCa₅₀ values in cardiomyocytes from the four experimental groups before and after in vitro PKA treatment. pCa₅₀ values of the HF pacing group were dispersed on a wider range than in all other groups. After PKA treatment, a narrower distribution for the pCa₅₀ values could be measured in HF cardiomyocytes at the pacing site