Smooth muscle contractile ability can be limited in several circumstances, including ischemia, shortening, or agonist overdose. Of these, the cellular mechanism is most clear for ischemia, i.e., the reduction in energy supply limits the contractile process. In preliminary studies, we showed that the rabbit bladder can recover rapidly from extreme states of high-energy phosphate deprivation. This indicates that cells will sacrifice specialized function, in this case contractile ability, in order to maintain their ionic gradients. We are able to study the metabolic, ionic, and mechanical properties regulating contraction by combining 31P-nuclear magnetic resonance (NMR) with force-velocity mechanics. The rabbit bladder is a particularly appropriate tissue for these studies, as it has sufficient mass to produce adequate NMR signals in several minutes and also has historically produced many studies of strip mechanics. We will use these methods to investigate why smooth muscle loses its contractile ability. With the isolated perfused rabbit bladder, NMR experiments can assess the metabolite levels of ATP and PCr, the tissue pH, the extracellular volume and pH using phenylphosphonic acid, flux through the creatine kinase reaction, tissue tension and blood pressure. From mechanical studies, the multiple regulatory system of smooth muscle can be assessed under conditions of different pH, anoxia, length or agonist overdose. The studies will provide the first quantitative study of the metabolic and ionic limitations of smooth muscle contraction, the state of the creatine kinase in smooth muscle, and the behavior of latchbridges (mechanically stable slowly-cycling crossbridges) under conditions of mechanical deficit. The application of 31P-NMR is a particular technical advance in measuring physiological function at the cellular level. When combined with mechanical studies, it will facilitate our knowledge of the regulation of smooth muscle contraction.
