The role of troponin C in modulating the Ca2+ sensitivity of mammalian skinned cardiac and skeletal muscle fibres.

Abstract

1. We investigated the effects of acidosis, inorganic phosphate (Pi) and caffeine on the Ca2" affinity of isolated fast-twitch skeletal and cardiac troponin C (TnC), labelled with fluorescent probes to report Ca2" binding to the regulatory sites. We also measured the effects of these interventions on the maximum force development and the Ca2" sensitivity of skinned fibres from fast-twitch skeletal muscle and cardiac muscle, as has been done previously. The two types of experiment were carried out under similar solution conditions, so that we could assess the contribution of any direct actions on TnC to the modulation of Ca2" sensitivity in the skinned muscle fibres.2. In skinned fibres, acidosis (decreasing pH from 7 0 to 6 2) and Pi (20 mM) suppressed maximum force to the same extent within a given muscle type, but had greater effects on cardiac fibres compared with skeletal fibres. Caffeine (20 mM) depressed maximum force equally in cardiac and skeletal muscle. Thus, the fall of force induced by acidosis or P1 may involve a different mechanism from that induced by caffeine. 3. Skinned skeletal fibres were more Ca2" sensitive than cardiac fibres by 0-29 pCa units (pCa = -log1O[Ca2"]). Isolated skeletal TnC also had a greater Ca2" affinity than cardiac TnC, by 0-20 pCa units. These results suggest that the Ca2+ sensitivity of skinned fibres is at least partly determined by the type of TnC present.4. Acidosis reduced the Ca2+ sensitivity of force in skinned fibres profoundly and had a 2-fold greater effect in cardiac muscle than skeletal muscle (falls in pCa for 50% activation, pCa50, were 1-09 and 0 55, respectively). Acidosis also reduced the Ca2+ affinity of TnC, again having double the effect on the pCa50 for cardiac TnC (0 58) as on that for skeletal TnC (0 28). The greater effect of acidosis on cardiac skinned fibres, compared with skeletal, may be partly explained, therefore, by the type of TnC present, and one-half of the effect on fibres may be attributed to the direct effect of H+ on TnC.5. Pi reduced the Ca2`sensitivity of force in skeletal and cardiac skinned fibres by 0 30 and 0X19 pCa units, respectively. However, the Ca2+ affinity of isolated cardiac and skeletal TnC was unaffected by Pi, indicating that the decrease in muscle Ca2+ sensitivity is not mediated by a direct action of Pi on TnC.6. Caffeine increased the Ca2+ sensitivity of cardiac skinned fibres by 0f31 pCa units, which was 3 times greater than for the skeletal fibres (0 09 pCa units). Caffeine had no effect on Ca2+ binding to isolated TnC, so its Ca2+ sensitizing action on skinned fibres is unlikely to be due to a direct effect on TnC. 7. We conclude that the different Ca2+ and pH sensitivities of cardiac and skeletal fibres are partly accounted for by the properties of the TnC present. However, the actions of caffeine and Pi on skinned fibres are not explained by any direct actions on TnC. Some of the Pi-induced decrease in Ca2`sensitivity probably results from a reduced Ca2+ affinity of TnC in the thin filament as a result of a decr...

Altered myocardial force generation in end‐stage human heart failure

“…These findings are consistent with the notion that the pH has a major impact on the Ca 2+ -binding function of troponin C but has little direct effect on the transitions between the weak and strong cross-bridge states. 17 10 mM P i reduced F o more than acidosis, but it induced a smaller rightward shift in the calcium-force relations and did not change the Hill coefficient either in the failing or in the non-failing myocytes. These alterations, together with the large increase seen in k tr , are in agreement with the view that P i , via reducing the overall free energy of MgATP hydrolysis, affects the cross-bridges directly 19,24,25 and has no direct effect on the Ca 2+ -binding function of troponin C. 17 It is now generally accepted from animal studies that P i reverses the P i -release step of the cross-bridge cycle by mass action, thereby decreasing the proportion of crossbridges in the high-force conformation.…”

“…19,[24][25][26] Furthermore, the P i -induced changes in pCa 50 are considered to be a consequence of the altered distribution of actin-myosin states within the cross-bridge cycle. 17 An elevation in the rate of the cross-bridge transition from the non-force-generating state to the forcegenerating state (f app ) increases pCa 50. 18 Under control conditions, the higher k tr (f app + g app ) values of the failing myocardial cells are thus logically related with the observed increase in pCa 50 .…”

Muscle Fatigue from the Perspective of a Single Crossbridge

“…64 Importantly, the shift in the force-calcium relationship is more pronounced at 30°C compared with 15°C, suggesting that the decrements in force from these ions may be more important than previously indicated by studies at saturating Ca 2+ . Furthermore, pH 6.2 and 30 mM Pi conditions reduce peak power in rat and rabbit fibers by 55%-63% at 30°C 43,58 (Fig.…”

“…For example, acidosis also decreases the sensitivity of the myofilaments to Ca 2+ due, at least in part, to H + competitively inhibiting the binding of Ca 2+ to troponin C. 64,65 The reduced myofibrillar Ca 2+ sensitivity manifests as a rightward shift in the force-calcium relationship and considerably larger reductions in peak isometric force in rat fibers contracting in pH 6.2 and submaximal Ca 2+ conditions. 59 In addition, acidosis depresses the rate of tension development (ktr) in skinned fibers under submaximal Ca 2+ conditions as reflected by a reduced ktr after a slack reextension maneuver at 15°C.…”

“…64 In addition, it has been suggested that Pi induces an unconventional power stroke where myosin detaches from actin early in the high-force state of the crossbridge cycle before the release of ADP and Pi.…”