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)(i) 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 pCa₅₀ (-log half-maximally activating molar Ca²⁺ concentration) when pH was decreased from 7.0 to 6.2. The pH-induced shift in pCa₅₀ 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.