Mechanical characteristics of the cat pylorus

In vitro force-length curves of rings from the gastroduodenal junction were obtained in standard tyrode solution, in tyrode with 140 mM KCl, and in calcium-free tyrode with 5 mM sodium EDTA, to determine basal, total, and passive forces, respectively. Active force was obtained as a difference between total and passive forces. Basal, total, passive, and active forces were higher for pyloric rings than for duodenal and antral rings. Furthermore, in the pylorus the force-length curves were shifted to the left along the length axis due to a narrowing at the gastroduodenal junction. Pressure-diameter relationships calculated from the in vitro force-length data and compared with previously determined in vivo pressure-diameter data, demonstrated good correlation between the in vivo and in vitro data. These data indicate that the higher forces and pressures observed in the pylorus are due to a combination of higher active and passive components. At low levels of stretch, the active forces are higher, while at high levels of stretch the passive forces become prevalent. These data suggest that, because of its passive properties the pylorus may act as a stricture and resist opening to large diameters, thus impeding the passage of large boluses. Near closure, however, any pressure gradient at the gastroduodenal junction would be due to active contraction of the pyloric circular muscles, and thus be susceptible to modulation through neural and/or myogenic factors.