K-POTENTIAL TRAINS IN GIANT-AXONS OF THE SQUID SEPIOTEUTHIS( ACCUMULATION AND K+ CONDUCTANCE INACTIVATION DURING ACTION)

1. During action potential trains in giant axons from the squid Sepiot euthis, decline of the peak level of the undershoot potential was obse rved. The time course of the decline of the undershoot could be fitted with a three-exponential function with time constants of similar to 2 5, similar to 400 and similar to 7000 ms, respectively. 2. When the os molarity of the external solution was doubled by adding glucose (1.2 M ), the fast component of undershoot decline, but not the medium and sl ow components, was significantly reduced. 3. Under voltage clamp in hi gh osmolarity solutions where K+ accumulation was completely removed, repeated depolarizing pulses at 40 Hz (designed to mimic a train of ac tion potentials) elicited K+ currents whose peak value declined. The d ecline is consistent with inactivation of the K+ conductance (g(K)). T he decline of g(K) was fitted by a two-exponential function with time constants of similar to 400 and similar to 7000 ms, respectively. 4. I nterventions designed to modify Schwann cell physiology, such as high frequency stimulation (100 Hz, 2 min), externally applied ouabain (100 -500 mu M), L-glutamate (100 mu M), ACh (100 mu M), Co2+ (5 mM), Ba2(2 mM), or removal of external Ca2+ by EGTA, had no significant effect s on the fast, medium or slow components of undershoot decline. 5. The results suggest that the fast component of undershoot decline represe nts K+ accumulation in the space between Schwann cell and axolemma. Th e medium and slow components are the result of axonal g(K) inactivatio n. Schwann cells appear to be involved in K+ clearance only to the ext ent that they provide an efficient physical pathway for the clearance of K+ by extracellular diffusion.