External ions and direct membrane effects of enkephalins on Amoeba proteus

We have shown that amphiphilic hormones like vasopressins and endorphins increase water permeability and activate the contractile vacuole (CV) in Amoeba by direct action on the plasma membrane. Using our standard CV assay, the effects of nine opioids, morphine, naloxone and 7 enkephalin derivatives, have been compared in normal, ion-containing growth medium (Chalkley's) and in glass-distilled water. While the absence of external ions does not affect the activity of molecules with net positive charges, opioids with no net charge are devoid of action in glass-distilled water. This shows that, in addition to amphiphilicity, which permits insertion into the lipid core of the membrane, electrostatic interactions with ions and with negative charges of phospholipids membrane and glycocalyx, the thick glycoproteic cell coat of Amoeba, are important in the direct action of these compounds.