The determination of the active state curve in isolated heart muscle

Abstract

Models of muscle relevant to the study of the active state have been reviewed. Some of the implications of the existing force-velocity equations have been examined preliminary to the presentation of a new method for determining the active state curve and the formulation of a general active state equation. It has been demonstrated, both theoretically and experimentally, that the intensity of the active state at any instant can be determined from a set of isometric myograms recorded rapidly but with different external compliances. The length of the muscle must be kept constant while these myograms are being recorded. From such sets of isometric myograms taken at 25uC and at different resting tensions, regression lines of developed tension against compliance were calculated at 10 msec intervals after the stimulus. The tension, independent of external compliance, was calculated from such regression lines. The extension of the series elastic component at any instant was determined according to the method of MacPherson (1953) while the compliance of the series elastic component was determined by dividing this extension by P t' the intensity of the active state at that instant. Substituting this compliance in the corresponding regression equation, a general equation Pt - Ptpt - a Lt = 0 (where P t is the intensity of the active state, pt is the isometric tension at zero external compliance, a is the regression coefficient and Lt is the extension of the series elastic component at time t after the stimulus) was developed. Pt was obtained by solving this equation. The active state curves determined according to this method rise much more gradually than those previously proposed.