EFFECTS OF OSMOTIC STRESS ON THE ELECTRICAL ACTIVITIES OF THE GIANT AXON OF A MARINE OSMOCONFORMER, SABELLA PENICILLUS

Reprint requests should be addressed to Dr Treherne. The giant axons of the polychaete, Sabella penicillus , can withstand, in vitro , abrupt changes in osmotic and ionic concentration of the bathing medium in the range measured in the blood of this osmoconformer (543–1236 m-osmol) at different external salinities. Isosmotic dilution of the external ions (i.e. when osmotic concentration was maintained by sucrose) induced a modest hyperpolarization of the axonal membrane and a rapid decline in the overshoot of the action potential. In contrast, abrupt hyposmotic dilution resulted in a relatively slow and complex decline in overshoot in the absence of axonal hyperpolarization. A slow potassium depolarization and rate of decrease in overshoot in sodium-free conditions suggests that there is a reduced intercellular access to the axon surfaces following exposure to hyposmotic media. It is suggested that this restricted access could provide short-term protection from fluctuations in blood osmotic concentration.