Function of the in-situ papillary muscle in the canine left ventricle.

From strain gauge arches applied directly to the longitudinal surface of the papillary muscle of the canine left ventricle, its force of contraction was recorded simultaneously with that on an epicardial segment of the ventricle immediately overlying it. Left ventricular and atrial pressures, as well as ECG and local electrograms were also recorded in a closed-heart, open-chest preparation. Active participation of the papillary muscle in the contractile processes of the whole heart is indicated, and under positive inotropic drive (stellate stimulation, administration of isoproterenol, norepinephrine) the force of contraction of this muscle is greatly augmented. During volume loading resulting from afterloading procedures (central vagus stimulation, methoxamine, aortic clamping) the papillary muscle participates in augmenting the contractile force, but in lesser proportion than when elicited by a direct positive inotropic procedure. The increase in contractile force during afterload was related to passive changes in length-tension relationships. Direct vagal innervation of the papillary muscle is indicated by distinct inhibition in contractile force during electrical excitation of the peripheral end of the cut vagosympathetic trunk, both in the paced and unpaced preparation. ADDITIONAL