Potassium outward current dependent on extracellular calcium in snail neuronal membrane

In experiments on isolated intracellularly dialysed neurons of the snail Helix pomatia a component of delayed inactivating potassium outward current depending on the presence of Ca2+ ions in the extracellular medium has been distinguished, which differs from the already known potassium current sensitive to intracellular calcium ions. This component decreases with a decrease in extracellular calcium (in the range of 10(-2) - 10(-5) M); it is not affected or even increased by intracellular introduction of ethyleneglycolbis(aminoethylether)tetra-acetate (10 mM) or fluoride ions (77 mM) and can be blocked by addition of 1.5 mM cobalt ions to the extracellular solution. Contrary to the slow rising potassium current dependent on intracellular calcium, this current has a fast rising phase (several milliseconds) and time-dependent inactivation. The inactivation depends on extracellular potassium ions: it slowed down when [K+]out is increased in the range of 1-10 mM. Extracellular application of calmodulin blockers calmidazolium (6.5 X 10(-7) M) and chlorpromazine (2.5 X 10(-6) M) selectively inhibits the potassium current dependent on intracellular calcium but does not affect that dependent on external calcium. Tetraethylammonium (10 mM) depresses the latter current on both intra- and extracellular application, the former being more effective. The existence of a special type of potassium channel sensitive to extracellular calcium ions is postulated.