Electrogenic Na-K pump at the basal face of the sensory epithelium in thePlotosus electroreceptor

Abstract

1. The electrogenic Na-K pump activity was studied in thePlotosus electroreceptor. Single ampullae (sensory epithelium) were dissected free, and mounted over an air gap at the ampullary duct, to electrically isolate the receptor activity for current- and voltage-clamp experiments. 2. After equilibration in K-free saline, the bathing medium around the ampulla was exchanged to test saline. Under current-clamp, 4 mM K saline induced reversible hyperpolarization of about 13 mV. Under voltage-clamp, the 4 mM saline induced an outward current of about 10 nA. 3. Both responses were suppressed by ouabain, with a half suppression at 1.5×10−7M for current, which suggested involvement of ionic pumps. In the K-free saline, ouabain alone induced no response. 4. The current response to high K saline was always outward, and was dependent on K concentration. In the presence of ouabain, high K saline as high as 40 mM, ten times the standard K, induced no inward current, which suggested little contribution of K conductance to the responses. 5. The outward current responses were induced, also dose-dependently, by various alkali metal cations. The apparent dissociation constants were 6.8, 1.2, 21.7, and 85.1 mM for K, Rb, Cs, and Li, suggesting their potency in the order of Rb>K> Cs>Li. 6. Thus, the hyperpolarization and outward current responses were due to an electrogenic NaK pump in the basal face of the sensory epithelium. The Na-K pump seems to be responsible, to a certain extent, for maintenance of the negative DC potential in the standard 4 mM K saline, which supplies an outward current to bias the sensory epithelium in situ.