Expression of slow skeletal troponin I in adult transgenic mouse heart muscle reduces the force decline observed during acidic conditions

1. Acidosis in cardiac muscle is associated with a decrease in developed fo rce. We hypothesized that slow skeletal troponin I (ssTnI), which is expres sed in neonatal hearts, is responsible for the observed decreased response to acidic conditions. To test this hypothesis directly, we used adult trans genic (TG) mice that express ssTnI in the heart. Cardiac TnI (cTnI) was com pletely replaced by ssTnI either with a FLAG epitope introduced into the N- terminus (TG-ssTnI*) or without the epitope (TG-ssTnI) in these mice. TG mi ce that express cTnI were also generated as a control TG line (TG-cTnI). No n-transgenic (NTG) littermates were used as controls. 2. We measured the force-calcium relationship in all four groups at pH7.0 a nd pH6.5 in detergent-extracted fibre bundles prepared from left ventricula r papillary muscles. The force-calcium relationship was identical in fibre bundles from NTG and TG-cTnI mouse hearts, therefore NTG mice served as con trols for TG-ssTnI* and TG-ssTnI mice. Compared to NTG controls, the force generated by fibre bundles from TG mice expressing ssTnI was more sensitive to Ca2+. The shift in EC50 (the concentration of Ca2+ at which half-maxima l force is generated) caused by acidic pH was significantly smaller in fibr e bundles isolated from TG hearts compared to those from NTG hearts. Howeve r, there was no difference in the force-calcium relationship between hearts from the TG-ssTnI* and TG-ssTnI groups. 3. We also isolated papillary muscles from the right ventricle of NTG and T G mouse hearts expressing ssTnI and measured isometric force at extracellul ar pH 7.33 and pH 6.75. At acidic pH, after an initial decline, twitch forc e recovered to 60 +/- 3 % (n = 7) in NTG papillary muscles, 98 +/- 2% (n = 5) in muscles from TG-ssTnI* and 96 +/- 3 % (n = 7) in muscles from TG-ssTn I hearts. Our results indicate that TnI isoform composition plays a crucial role in the determination of myocardial force sensitivity to acidosis.