Mechanical properties of developing swine myocardium.

ABSTRACT

Mechanical responses of myocardium from 16 piglets were studied from 18 hr to 12 days after birth. Tension, time and velocity parameters of contraction and relaxation were determined for every contraction cycle. Increasing the frequency of stimulation in step-changes induced negative inotropy in some muscles regardless of age. Doubling extracellular calcium ion concentration induced a positive force-frequency response in all muscles. Epinephrine increased tension and velocities without affecting contraction time. The ultrastructure was immature even on the 12th postnatal day. We concluded that in newborn piglet hearts, the mechanisms for calcium delivery are not fully developed. Thus, the heart undergoes a transient phase during which at least a principal portion of calcium for the myofibers is supplied by the extracellular fluid. While receptors for catecholamines are present, the time course for their response is immature.