Ionic Requirements of Transepithelial Potentials in Isolated Cell Layers of Hydra

Abstract

Because hydra is one of the few fresh water coelenterates, it requires a unique system to maintain ionic and osmotic equilibrium. Several studies in the past have begun to elucidate the mechanism of ionic and osmotic regulation in hydra (Josephson and Macklin, 1969; Macklin, 1967; Macklin and Josephson, 1971; Marshall, 1969). These studies have demonstrated that there is a maintained electrical potential across the hydra epithelium with the gut positive relative to the external medium and it was shown by Macklin and Josephson (1971) that this sustained positive potential was related to an active sodium transport mechanism. Further, superimposed on this sustained positive potential, there are negative going spikes termed “contraction pulses” (CPs) which had been shown to relate to contraction of the body column (Josephson, 1967). Various studies of the osmotic properties of hydra cells and tissue (Benos and Prusch, 1972; Koblick and Yu-Tu, 1967; Lilley, 1955; Marshall, 1969; Steinbach, 1963), have demonstrated that hydra tissue is isosmotic to a solution with an osmotic strength of 40 to 60 milliosmol, and that the gut of the animal is approximately isosmotic to the hydra tissue. These experimental results have led to the conclusion that ionic and osmotic regulation of the intact hydra is maintained by an active transport of sodium with passive movement of water and an anion and with isosmotic flow from the tissue into the gut.