Quantal mechanisms in cardiac contraction.

SERENDIPITY and technology have combined forces to reveal a phenomenon of striking proportions. What may prove to be the quantum element underlying contraction was discovered by chance, following the development of a high-resolution optical diffractometer designed to follow the dynamics of shortening in isolated cardiac muscle. Records showed a most unusual feature: periods of sarcomere shortening were punctuated by periods of pause during which there was little or no length change, conferring a staircase-like character on the shortening waveform. Could shortening really occur in discrete quanta? To find precedent for quantal behavior one need not look far. Nearby, at the myoneural junction, for example, a detailed understanding of the mechanism of intercellular communication followed the discovery that acetylcholine was discharged in quantal packets. These packets sum to form the end plate potential in much the same way that the steps apparently sum to give the shortening waveform. In the latter case, however, an unfathomable implication has impeded acceptance. The difficulty is that stepwise shortening implies a colossal degree of synchrony. Why so? The optical diffraction pattern reflects the behavior of a sizable fraction of sarcomeres ranging over the field illuminated by the laser. This ensemble could pause in either of two ways. In the complicated way, some sarcomeres would shorten while others lengthened in such a way that the average velocity remained zero for a period; this situation would then recur fortuitously time and again as contraction progressed to give the observed cascade of pauses and steps. In fact, this possibility is now ruled out; even when the sampled volume is diminished extraordinarily, the pauses remain in evidence (see below). The alternative way is simpler. It implies that mem-