TROPONIN-I ISOFORMS AND DIFFERENTIAL-EFFECTS OF ACIDIC PH ON SOLEUS AND CARDIAC MYOFILAMENTS

Differences in pH sensitivity of tension generation between developing and adult cardiac myofilaments, which contain the same isoform of troponin C (TnC), have been proposed to be due to troponin I (TnI) isoform switching from the slow skeletal (ss) to cardiac (c) TnI isoforms (21). We investigated the effects of acidic pH on Ca2+-activation of force in chemically skinned preparations of adult rat trabeculae and single soleus fibers that also share the same TnC isoform. Compared with the soleus fibers, trabeculae demonstrated a greater suppression of tension and a rightward shift in pCa(50), (-log half-maximally activating molar Ca2+ concentration) when pH was decreased from 7.0 to 6.2. The pH-induced shift in pCa(50) in soleus fibers did not change with sarcomere length. Troponin subunit interactions were also investigated, using cardiac troponin C (cTnC(IA)) labeled with a fluorescent probe, 2-(4'-iodoacetamidoanilino)-naphthalene-6-sulfonic acid. Under acidic conditions, cTnC(IA) demonstrated a decrease in Ca2+-affinity. This decrease was amplified both in the binary complex cTnC(IA)-cTnI and in the complex cTnC(IA)-cTnI-cTnT-tropomyosin to the same extent. In contrast, substitution of ssTnI for cTnI in these complexes produced the same decrease in Ca2+ affinity in response to acidic pH as cTnC(IA) alone. These results support our hypothesis that differential effects of pH on tension generation and Ca2+ sensitivity between soleus fibers and trabeculae are due to the presence of different isoforms of TnI.