LENGTH-DEPENDENT ACTIVATION IN INTACT FERRET HEARTS - STUDY OF STEADY-STATE CA2-STRESS-STRAIN INTERRELATIONS()

We examined the steady-state stress-strain relationships and the stead y-state stress-intracellular calcium concentration ([Ca2+](i)) relatio nship in intact ferret hearts and compared these to previously publish ed analogous relationships in skinned and intact muscle. Langendorff-p erfused ferret hearts were treated with ryanodine and tetanized by rap id stimulation to create steady-state conditions. [Ca2+](i) was measur ed concurrently by macroinjected aequorin. Over a range of volumes cor responding to strains between 1.0 and 0.75, steady-state stress decrea sed by 33% when saturating levels of perfusate calcium were used, indi cating the degree to which physical factors contribute to the Frank-St arling relationship. The steady-state stress-[Ca2+](i) relationship wa s sigmoidal with a mean Hill coefficient (n(H)) of 4.91 +/- 0.29 at a strain of 1.0, and the [Ca2+](i) required for half-maximal activation (K-1/2) was 0.41 +/- 0.03 mu M. K-1/2 increased and n(H) decreased wit h decreasing strains. These results are similar to those observed in i ntact muscle but differ quantitatively from results obtained in isolat ed, skinned preparations. Based on these results, we suggest that whol e heart function can be related to average sarcomere function without the need for complex models of ventricular structure.