Ks. Mcdonald et al., LENGTH DEPENDENCE OF CA2-C( SENSITIVITY OF TENSION IN MOUSE CARDIAC MYOCYTES EXPRESSING SKELETAL TROPONIN), Journal of physiology, 483(1), 1995, pp. 131-139
1. Beat-to-beat performance of myocardium is highly dependent on sarco
mere length. The physiological basis for this effect is not well under
stood but presumably includes alterations in the extent of overlap bet
ween thick and thin filaments. Sarcomere length dependence of activati
on also appears to be involved since length-tension relationships in c
ardiac muscle are usually steeper than those in skeletal muscle. 2. An
explanation recently proposed to account for the difference between l
ength-tension relationships is that the cardiac isoform of troponin C
(cTnC) has intrinsic properties that confer greater length-dependent c
hanges in the Ca2+ sensitivity of tension than does skeletal troponin
C (sTnC), presumably due to greater length-dependent changes in the Ca
2+-binding affinity of cTnC. To test this hypothesis, transgenic mice
were developed in which fast sTnC was expressed ectopically in the hea
rt. This allowed a comparison of the length dependence of the Ca2+ sen
sitivity of tension between myocytes having thin filaments that contai
ned either endogenous cTnC or primarily sTnC. 3. In myocytes from both
transgenic and normal mice, the Ca2+ sensitivity of tension increased
similarly when mean sarcomere length was increased from approximately
1.83 to 2.23 mu m. In both cases, the mid-point (pCa(50)) of the tens
ion-pCa (i.e. -log[Ca2+]) relationship shifted 0.12 +/- 0.01 pCa units
(mean +/- S.E.M.) in the direction of lower Ca2+. 4. We conclude that
the Ca2+ sensitivity of tension in myocytes changes as a function of
sarcomere length but is independent of the isoform of troponin C prese
nt in the thin filaments.