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

A reduction in temperature lowers the Ca2+ sensitivity of skinned cardiac m yofilaments but this effect is attenuated when native cardiac troponin C (c TnC) is replaced with skeletal TnC. This suggests that conformational diffe rences between the two isoforms mediate the influence of temperature on con tractility. 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 car diac function at temperatures cardioplegic to mammals. To determine whether ScTnC is more sensitive to Ca2+ than BcTnC, F27W mutants were used to meas ure changes in fluorescence with in vitro Ca2+ titrations of site II, the a ctivation site. When measured under identical conditions, ScTnC was more se nsitive to Ca2+ than BcTnC. At 21 degreesC, pH 7.0, as indicated by K-1/2 ( -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 c onstant (7.0) and temperature was lowered from 37.0 to 21.0 degreesC and th en to 7.0 degreesC, the K-1/2 of BcTnC decreased by 0.13 and 0.32, respecti vely, whereas the K-1/2 of ScTnC decreased by 0.76 and 0.42, respectively. Increasing pH from 7.0 to 7.3 at 21.0 degreesC increased the K-1/2 of both BcTnC and ScTnC by 0.14, whereas the K-1/2 of both isoforms was increased b y 1.35 when pH was raised from 7.0 to 7.6 at 7.0 degreesC.