Inactivation of contraction as a determinant of the length-active tension relation in heart muscle of the cat

In isometrically contracting isolated cat papillary muscles, the mechanism determining the reduction in active tension (P) at lengths (L) below the optimal length (L_max) for maximal tension development (P_max) was studied. Increasing /_–^–Ca^2+– /_–o^max from 1.25 to 5 mM, paired electrical stimulation and caffeine augmented P_max by 77,61 and 23% respectively, as well as the tension developed at any given length on the ascending limb of the lengthactive tension relation. To see whether below L_max the degree of activation is dependent on length, these length-active tension relations were expressed as P/P_max relative to L/L_max where P_max and L_max in each inotropic state was 100%. These normalized Iength-active tension relations were not altered by different /_–^–Ca_–^2+–/_o, but shifted upward and to the left by caffeine and paired stimulation. The effects of elevating /_–^–Ca_–^2+–/_o on the length-active tension relation suggests that restoring forces and fixed internal loads are not an important factor in decreasing active tension at short lenghts. The effects of caffeine and paired stimulation support the view that in cardiac muscle inactivitation of contraction is a major factor determining the fall in active tension at decreasing length.