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-ATP in the presence of calcium was studied in skinned cardiac fibres from swine. A low-force rigor state was obtained by using butanedione monoxime (BDM) during the induction of rigor. EDM was washed out and following release of ATP in the presence of Ca2+ (pCa 4.3), the muscles contracted 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 release 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 conditions are partially activated by removal of an inhibition of cross-bridge interaction by troponin I. A partial recovery of force was obtained by prolonged incubation in DTT containing solutions possibly reflecting reconstitution with troponin I still present in the fibre bundle. Treatment with a solution containing whole troponin caused almost complete 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 and calcium sensitivity by increasing the cross-bridge attachment rates.