FUNCTIONAL ADAPTATIONS OF INTESTINAL SMOOTH MUSCLE
The overall objective of this research is to gain an understanding of the cellular basis of adaptations that occur in the contractile process of smooth muscle in response to changes in functional demand. We will begin by analyzing an interesting congenital disorder of intestinal smooth muscle, Hirschsprung's disease, in which two very different adaptive changes have occurred. The rectum is hypoganglionic, and specifically lacks non-adrenergic, non-cholinergic inhibitory enteric nerves. As a result, the rectum is tonically constricted which leads to marked distension and muscular hypertrophy of the proximal colon, or "megacolon". Thus, smooth muscles of both regions undergo functional change: the rectum assumes a state of tonic contraction and the colon undergoes hypertrophy. A well-defined murine model of Hirschsprung's disease and one in which megacolon is induced in normal mice by experimental stenosis will be used in order to address the question: to what extent are adaptations to new functional demands expressed through structural remodelling, to modification of the mechanochemical transduction process or to its regulation? We will study the basic mechanical, chemical and ultrastructural changes that occur in the smooth muscle, determining the (1) mechanical properties, such as the ability to generate force and shorten and do mechanical work, through the length-tension and force-velocity characteristics, respectively; (2) fundamental relationships between high-energy phosphate usage and mechanical output, in order to determine the economy of force production and efficiency of work output.
Time-resolved, direct measurements of high-energy phosphate usage associated with various mechanical states in smooth muscle, and standard methods for the measurement of steady- state oxygen consumption and lactate production will be used; (3) mechanisms whereby contraction is regulated by calcium, through phosphorylation of the 20,000-dalton light chains of myosin and other calcium-dependent processes; (4) ultrastructural studies will allow us to determine force production/cell, cellular dimensions for a definition of hypertrophy and/or hyperplasia, and changes in contractile filament and cytoskeletal elements which bear on force transmission. In all of these studies, comparisons will be made among muscle segments from animals with experimentally- induced and congenital megacolon, the congenitally constricted rectum and the corresponding segments from their normal littermates.