Halothane does not alter Ca2+ affinity of troponin C

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.