Regional differences in K channels of abdominal and circumesophageal segments of the crayfish medial giant axon.

Intracellular recordings reveal that the membrane of the circumesophageal region of the medial giant axon of crayfish responds to replacement of C1 with propionate differently from that of the abdominal region of the same axon. The connective hyperpolarizes in the propionate saline, whereas the abdominal region undergoes the transient depolarization that is expected when a permeant anion (Cl) is replaced with an impermeant one (propionate). The hyperpolarization of the connectives is accompanied by an increased conductance, a decreased length constant, and an increase in threshold current for intracellular stimulation. These effects are specific for the connectives and for propionate. They do not occur on replacing Cl with other large anions, isethionate, methane sulfonate, or glucuronate. The effects of propionate are independent of Na or Ca and result from an increased K conductance. The hyperpolarization induced by propionate is increased in a K-free saline, where the resting potential (EM) is considerably positive to the emf of the K battery (EK). It is abolished in elevated Ko when EM=EK.