Physiological loading of isolated mammalian cardiac muscle.

Cat papillary muscles were subjected to a complex loading function resulting from an analysis of the heart as a pump. The papillary muscle was assumed to be a hypothetical bundle of circumferential muscle fibers in the wall of a simplified cylindrical ventricle. The loading included inertial, resistive, and capacitive components of the cardiovascular system. Changes of ventricular dimensions were taken into account by application of a Laplace relationship. When this complex dynamic loading function was imposed on a shortening muscle by means of an electromagnetic feedback system, the developed force continuously changed with time. The time course of this changing force corresponded to the time course of calculated stress in the intact ejecting heart. Directly displayed force-velocity loops also were similar to loops obtained for the intact heart. Loads proportional to velocity of shortening (damping), acceleration of shortening (inertia), and to the square of shortening velocity (Bernoulli) were investigated separately. Cardiac muscle appeared rather insensitive to inertial loads, and the contribution of inertial loads in the early phase of a contraction under physiological pump loading was minimal. Moreover, during all these dynamic loadings, as long as loading was dynamically increasing or decreasing, velocity of shortening was respectively lower or higher at any muscle length and total load, when compared to velocity at the same length and load under static (constant preload and afterload only) loading.