TENSION RESPONSE OF THE CARDIOTONIC AGENT (-EMD-57033 AT THE SINGLE-CELL LEVEL())

The effects of the Ca sensitizer (+)-EMD-57033 were tested on single c hemically skinned cells isolated from rat ventricle. The present study demonstrates that (+)-EMD-57033 (10 mu M) increased maximal force by 20% (at pCa 4.5) and myofilament Ca sensitivity by 0.2 pCa unit. Howev er, the force-length dependency was not affected by the addition of(+) -EMD-57033, since similar Ca-sensitizing effects occurred at different sarcomere lengths. Consequently, the Ca-sensitizing effect of the dru g and of the sarcomere length might be additive. Cross-bridge kinetics were also investigated in the presence of the thiadiazinone derivativ e. (+)-EMD-57033 induced marked increases in the rate of tension redev elopment (k(tr)) after brief slack release/restretch, particularly at low Ca concentrations. These results suggest that the Ca-sensitizing e ffects of (+)-EMD-57033 are due, at least in part, to an increased num ber of attached cross bridges during one cycle. This observation, toge ther with the increase in peak force, is discussed in relation to the reduction in energy cost induced by such Ca-sensitizing agents.