title: Troponin I isoforms and differential effects of acidic pH on soleus and cardiac myofilaments.
abstract: 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 Ca(2+)-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 (cTnCIA) labeled with a fluorescent probe, 2-(4'-iodoacetamidoanilino)-naphthalene-6-sulfonic acid. Under acidic conditions, cTnCIA demonstrated a decrease in Ca(2+)-affinity. This decrease was amplified both in the binary complex cTnCIA-cTnI and in the complex cTnCIA-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 cTnCIA 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.
authors.affiliation: Department of Physiology and Biophysics, College of Medicine, University of Illinois, Chicago 60612-7342.
authors.forename: J
authors.initials: J
authors.lastname: Wattanapermpool
authors.forename: P J
authors.initials: PJ
authors.lastname: Reiser
authors.forename: R J
authors.initials: RJ
authors.lastname: Solaro
mesh_terms: D000143:Acids / Q000378:metabolism*; D008841:Actin Cytoskeleton / Q000378:metabolism* / Q000648:ultrastructure; D000818:Animals; D002118:Calcium / Q000378:metabolism; D005453:Fluorescence; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D008297:Male; D009132:Muscles / Q000378:metabolism* / Q000648:ultrastructure; D009206:Myocardium / Q000378:metabolism* / Q000648:ultrastructure; D051381:Rats; D014336:Troponin / Q000737:chemistry / Q000378:metabolism*; D019209:Troponin C; D019210:Troponin I; D020107:Troponin T
chemical_list: D000143:Acids; D014336:Troponin; D019209:Troponin C; D019210:Troponin I; D020107:Troponin T; D002118:Calcium