Evidence for roles of the activating function in electric stimulation

The hypothesis that the activating function drives transmembrane voltage changes (/spl Delta/V/sub m/) has been tested in hearts. Optical /spl Delta/V/sub m/ were measured during activating functions produced with nonuniform and uniform transparent electrodes. When a nonuniform electrode was used to produce /spl part//sup 2/V/sub e///spl part/x/sup 2/, the signs of /spl Delta/V/sub m/ and /spl part//sup 2/V/sub e///spl part/x/sup 2/ matched. The extracellular voltage gradients, often assumed important, did not predict /spl Delta/V/sub m/. When a uniform electrode was used to eliminate /spl part//sup 2/V/sub e///spl part/x/sup 2/, the signs of /spl Delta/V/sub m/ matched the signs of (/spl part/g/sub i///spl part/x)(/spl part/V/sub e///spl part/x) estimated from variations in heart width. Demonstration of the activating function as a determinant of stimulation may improve research and therapy that use electric stimulation.