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Am J Physiol Regul Integr Comp Physiol 279: R1707-R1715, 2000;
0363-6119/00 $5.00
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Vol. 279, Issue 5, R1707-R1715, November 2000

Ca2+ binding to cardiac troponin C: effects of temperature and pH on mammalian and salmonid isoforms

Todd E. Gillis1,2, Christian R. Marshall2,3, Xiao-Hua Xue2, Thor J. Borgford3, and Glen F. Tibbits1,2,3

Departments of 1 Biological Sciences, 3 Molecular Biology and Biochemistry, and the 2 Cardiac Membrane Research Laboratory, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada

A reduction in temperature lowers the Ca2+ sensitivity of skinned cardiac myofilaments but this effect is attenuated when native cardiac troponin C (cTnC) is replaced with skeletal TnC. This suggests that conformational differences between the two isoforms mediate the influence of temperature on contractility. To investigate this phenomenon, the functional characteristics of bovine cTnC (BcTnC) and that from rainbow trout, Oncorhynchus mykiss, a cold water salmonid (ScTnC), have been compared. Rainbow trout maintain cardiac function at temperatures cardioplegic to mammals. To determine whether ScTnC is more sensitive to Ca2+ than BcTnC, F27W mutants were used to measure changes in fluorescence with in vitro Ca2+ titrations of site II, the activation site. When measured under identical conditions, ScTnC was more sensitive to Ca2+ than BcTnC. At 21°C, pH 7.0, as indicated by K1/2 (-log[Ca] at half-maximal fluorescence, where [Ca] is calcium concentration), ScTnC was 2.29-fold more sensitive to Ca2+ than BcTnC. When pH was kept constant (7.0) and temperature was lowered from 37.0 to 21.0°C and then to 7.0°C, the K1/2 of BcTnC decreased by 0.13 and 0.32, respectively, whereas the K1/2 of ScTnC decreased by 0.76 and 0.42, respectively. Increasing pH from 7.0 to 7.3 at 21.0°C increased the K1/2 of both BcTnC and ScTnC by 0.14, whereas the K1/2 of both isoforms was increased by 1.35 when pH was raised from 7.0 to 7.6 at 7.0°C.

calcium affinity; fluorescence; contractility


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