IONOPHORETICALLY APPLIED ACETYLCHOLINE AND VAGAL-STIMULATION IN THE ARRESTED SINUS VENOSUS OF THE TOAD, BUFO-MARINUS

1. The effects of acetylcholine (ACh), applied by ionophoresis, on the isolated arrested sinus venosus of the toad, Bufo marinus, were exami ned. 2. At each position where ACh was applied across the surface of s inus venosus preparations, a hyperpolarization was produced. These res ponses were abolished by hyoscine, indicating that muscarinic cholinoc eptors are widely distributed over the surface of these muscle cells. 3. Vagal stimulation produced hyperpolarizations which were mimicked, to some extent, by ionophoretically applied ACh. 4. The responses to i onophoretically applied ACh were abolished by adding barium ions to th e perfusion fluid, whereas responses to vagal stimulation persisted. 5 . The responses to ionophoretically applied ACh were consistently slow er than those to vagal stimulation. It is argued that the pathways act ivated by neural and applied ACh have different kinetics of activation .