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Halothane does not alter Ca2+ affinity of troponin C
T J Blanck1, E Chiancone, G Salviati
1Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University, Baltimore, Maryland.
Insights
Volatile anesthetics like halothane do not affect cardiac troponin C structure or its calcium binding. This suggests troponin C is not the target for halothane's negative inotropic effects.
Area of Science:
- Biochemistry
- Molecular Biology
- Anesthesiology
Background:
- Volatile anesthetics can induce negative inotropic effects.
- Cardiac troponin C (cTnC) is a potential target for these effects.
- Understanding anesthetic interactions with cTnC is crucial for cardiac function research.
Purpose of the Study:
- To investigate the impact of halothane on the structure and Ca2+ binding of cardiac troponin C.
- To assess the effect of halothane on the Ca2+-dependent force generation in muscle fibers.
Main Methods:
- Measured intrinsic tyrosine fluorescence and UV circular dichroism of cTnC with varying Ca2+ concentrations in the presence and absence of halothane.
- Assessed the binding rate of a sulfhydryl reagent to cTnC to evaluate low-affinity Ca2+ sites.
- Determined the pCa/tension relationship in skinned soleus muscle fibers using halothane.
Main Results:
- Halothane did not alter the Ca2+-induced fluorescence enhancement or the Ca2+ concentration for half-maximal effect in cTnC.
- Secondary structure of cTnC, measured by molar ellipticity, showed similar increases with Ca2+ regardless of halothane presence.
- Halothane did not affect the Ca2+ sensitivity or maximal tension in skinned soleus muscle fibers.
Conclusions:
- Halothane does not appear to alter the structure or Ca2+ binding properties of cardiac troponin C.
- The study provides evidence against troponin C being the primary molecular target for halothane's negative inotropic action.
- Further research is needed to identify the specific molecular targets of volatile anesthetics in cardiac muscle.
Abstract:
Troponin C has been suggested as a possible target for the negative inotropic action of volatile anesthetics. This study has examined the effect of halothane on the structure and response of isolated cardiac troponin C to Ca2+ and the response of skinned soleus and cardiac muscle fibers to Ca2+. The high-affinity Ca(2+)-binding sites of cardiac troponin C were assessed by measurement of the change in intrinsic tyrosine fluorescence and ultraviolet circular dichroism in response to Ca2+ in the presence and absence of halothane. Halothane (0.9 mM, 1.4%) did not alter the 45% enhancement in intrinsic tyrosine fluorescence that occurs with saturation of the high-affinity sites or change the Ca2+ concentration at which half-maximal enhancement occurred. The molar ellipticity in the far ultraviolet region, a measure of the secondary structure, increased to a similar extent with addition of 10(-6) M Ca2+ in the absence and presence of 1.0 mM (1.6%) halothane. The binding rate of the sulfhydryl reagent, 5,5'-dithiobis (2-nitrobenzoic acid), to troponin C in response to Ca2+ titration was used as a measure of the integrity of the low-affinity Ca(2+)-binding site in troponin C in the presence and absence of 1.0 mM (1.6%) halothane. The rate of reaction was stimulated twofold, and the half maximal effect was observed at pCa 4.8 +/- 0.2 in both control and halothane-treated samples. Halothane (5 mM; 7.8%) did not change the pCa/tension response of skinned soleus fibers; the data were fit to the Hill equation and yielded dissociation constants of 6.2 x 10(-7) M for control and halothane-treated specimens.(ABSTRACT TRUNCATED AT 250 WORDS)