EFFECTS OF TROPONIN-I EXTRACTION WITH VANADATE AND OF THE CALCIUM SENSITIZER EMD-53998 ON THE RATE OF FORCE GENERATION IN SKINNED CARDIAC-MUSCLE

The rate of tension development following release of ATP from caged-AT P in the presence of calcium was studied in skinned cardiac fibres fro m swine. A low-force rigor state was obtained by using butanedione mon oxime (BDM) during the induction of rigor. EDM was washed out and foll owing release of ATP in the presence of Ca2+ (pCa 4.3), the muscles co ntracted with an apparent rate of about 2 s(-1) at 22 degrees C, After treatment with 10 mM vanadate to extract troponins I and C the fibres contracted independently of calcium, The rate of contraction upon rel ease of ATP was slower than prior to extraction and was independent of [Ca2+], Since treatment with vanadate has been shown to extract about 90% of troponin-I the results suggest that the muscles under these co nditions are partially activated by removal of an inhibition of cross- bridge interaction by troponin I. A partial recovery of force was obta ined by prolonged incubation in DTT containing solutions possibly refl ecting reconstitution with troponin I still present in the fibre bundl e. Treatment with a solution containing whole troponin caused almost c omplete recovery of calcium sensitivity and rate of force development. The calcium sensitizer EMD 53998 increased rates of contraction in a dose dependent manner, suggesting that this compound increases force a nd calcium sensitivity by increasing the cross-bridge attachment rates .