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Isoform specific interactions of troponin I and troponin C determine pH sensitivity of myofibrillar Ca2+ activation
K L Ball1, M D Johnson, R J Solaro
1Department of Physiology and Biophysics, University of Illinois College of Medicine, Chicago 60612-7342.
Abstract:
We investigated whether differences in isoforms of troponin I (TnI) and troponin C (TnC) can account for the greater inhibition of Ca(2+)-dependent MgATPase activity by acidic pH in cardiac (c) than in fast skeletal (fs) myofilaments. We studied fast skeletal myofibrils from which whole Tn was extracted by displacement with excess fsTnT (the tropomyosin binding subunit of Tn) followed by reconstitution with TnC-TnI. Exchange of fsTnI with cTnI did not alter the effect of a drop in pH from 7.0 to 6.5 on the relation between pCa (-log[Ca2+]) and MgATPase activity of fast skeletal myofibrils. Exchange of fsTnC with cTnC did, however, induce an increase in the effect of this same pH change on Ca2+ activation. Yet, the pH sensitivity of Ca2+ activation of fast skeletal myofibrils containing cTnC was not as great as that of native cardiac myofibrils. However, when both fsTnC and fsTnI of fast skeletal myofibrils were replaced by cTnC-cTnI, there was a pH-induced shift in Ca2+ sensitivity similar to that of cardiac myofibrils. In studies using fluorescent probes, both pure fsTnC and pure cTnC showed decreased Ca2+ binding as pH was lowered. This decrease was potentiated in the fsTnC-fsTnI and cTnC-cTnI complexes. However, the effect of acidic pH was the same in fsTnC and the hybrid complex, fsTnC-cTnI, and in cTnC and the hybrid complex, cTnC-fsTnI. Thus, isoform specific interactions between TnI and TnC appear important in the differential response of skeletal and cardiac myofilaments to acidosis.
Insights
Differences in cardiac troponin I (TnI) and troponin C (TnC) isoforms explain why acidic pH inhibits cardiac myofilaments more than skeletal. Isoform interactions are key to acidosis response in muscle contraction.
Area of Science:
- Muscle Physiology
- Biochemistry
- Cardiovascular Research
Background:
- Acidosis, a common condition in cardiac stress, affects muscle contraction.
- Troponin complexes (TnI, TnC, TnT) regulate Ca(2+)-dependent muscle activity.
- Differential sensitivity of cardiac and skeletal muscle to pH changes is not fully understood.
Purpose of the Study:
- To investigate if troponin I (TnI) and troponin C (TnC) isoform differences explain varying pH inhibition in cardiac versus fast skeletal myofilaments.
- To elucidate the role of specific TnI and TnC isoforms in the pH-dependent modulation of Ca(2+)-activated MgATPase activity.
Main Methods:
- Fast skeletal myofibrils were reconstituted with different combinations of cardiac (c) and fast skeletal (fs) troponin isoforms (TnC, TnI).
- Ca(2+)-dependent MgATPase activity was measured across a pH range (7.0 to 6.5).
- Fluorescent probes assessed Ca(2+) binding to TnC under varying pH conditions.
Main Results:
- Replacing fast skeletal TnI with cardiac TnI did not change pH effects on MgATPase activity.
- Replacing fast skeletal TnC with cardiac TnC increased the pH effect on Ca(2+) activation, but sensitivity remained lower than native cardiac myofilaments.
- Complete exchange with cardiac TnC-TnI isoforms mimicked the pH sensitivity of native cardiac myofilaments, indicating isoform-specific interactions are crucial.
Conclusions:
- Isoform differences in troponin C (TnC) and troponin I (TnI) significantly contribute to the differential response of cardiac and skeletal myofilaments to acidic pH.
- Specific interactions between TnI and TnC isoforms are critical for modulating Ca(2+) sensitivity during acidosis.
- Understanding these molecular mechanisms is vital for addressing muscle dysfunction in conditions like heart failure.