“…An altemative suggestion was that guanethidine depresses NA release by limiting the access of calcium to those sites in the postganglionic sympathetic nerve ending with which calcium interacts to cause the release of NA (Kirpekar, Wakade, Dixon & Prat, 1969). The present investigation was undertaken to test this theory and to determine whether tetraethylammonium (TEA) and 4-aminopyridine (4-AP), which enhance NA output, presumably by increasing calcium entry into the sympathetic nerves (Thoenen, Haefely & Staehelin, 1967;Kirpekar, Prat, Puig & Wakade, 1972;Kirpekar, Kirpekar & Prat, 1976a;Kirpekar, Wakade & Prat, 1976b), reverse the neuronal blockade produced by guanethidine. A preliminary account of some of these findings has been published (Kirpekar, Kirpekar & Prat, 1977).…”