The site and state of myosin in intestinal smooth muscle.

The longitudinal layer of the guinea-pig ileum represents a highly advantageous specimen for the study of vertebrate smooth muscle structure. In this muscle we regularly observed thick filaments, consisting presumably of myosin, in longitudinal as well as in cross-sections, if the samples were fixed at constant length, i.e. standing under mechanical tension. Thick filaments were regularly present also in muscles relaxed by atropine. On the other hand, thick filaments were absent in many cases in slack muscles in K $^+$ contracture. As a consequence, we regard myosin filaments as regular constituents of smooth muscle, independently of the functional state. Their absence in electronmicrographs taken from slack muscles seems an artefact due to processing. We observed the same artefact in bee-wing muscle, i.e. in a striated muscle, too. This fact indicates the importance of mechanical tension and polymer crystallization in the survival of myosin filaments. On the basis of a recent work of Ladik, Biczo and Garamvolgyi we discuss how tension may be exerted on the myosin filaments of the resting muscle. Anyway, the sliding model seems valid also for vertebrate smooth muscle, without any substantial modification.