Fig. 1.

Comparison of real and virtual spheres.A, Plot of the distance at which the concentration of NO drops below 0.1 μm (the threshold concentration). This concentration is about the same as the equilibrium dissociation constant for soluble guanylyl cyclase (an NO receptor), 0.25 μm (Stone and Marletta, 1996). Thus, this distance defines the area within which NO can have a functional role (Vaughn et al., 1998a). Both virtual and real sources have been set to have the same source strength. The solutions give similar results for “spheres” of radii <5 μm but diverge for greater radii.B, Concentration of NO against distance from the center of virtual and real spheres of radius 20 μm measured at the end of a 10 msec burst of synthesis. Both sources have been set to have the same strength. Note the very different solutions both inside (where the point source has infinite concentration at the center) and outside the sphere. C, Concentration of NO measured at 35 μm from the center of virtual and real spheres of radius 35 μm for a 10 msec burst of synthesis. As in B, both sources have been set to have the same strength. The difference between the two solutions is clear, with the point-source producing its peak concentration around time t = 70 msec, well after synthesis has finished. The square wave shown beneath the figure represents the strength function.