Two types of sodium channels in the cockroach giant axon

S48 TWO TYPES OF SODIUM CHANNELS IN THE COCKROACH GIANT AXOX HIROHU YAWO, HIROSHI K O J I ~ and t~TOY KUNO, Department of Physiology, Kyoto University, Faculty of Medicine, Yoshida, Sakyo-ku, Kyoto 606, Japan. When the K÷ current was blocked by tetraethylammonium (TEA) and 3,4diaminopyridine in the cockroach giant axon, the action potential was followed by a prolonged plateau potential which lasted for over i00 msec. During the plateau phase, the input membrane resistance decreased by about 80%. Both the spike and plateau potentials were unaffected by changes in the external Ca 2+ concentration, whereas the amplitudes of both potentials depended upon the external Na + concentration in a manner predicted from the Nernst equation. Also, both the spike and plateau potentials were equally diminished following the application of tetrodotoxin (TTX). When the K+ current was partially blocked by low doses of TEA, slow depolarizing responses could be elicited in an all-or-none fashion in response to depolarizing current pulses. The "threshold" current necessary for initiating this slow potential was much lower than that for evoking the spike potential. This slow potential was unaffected by the external Ca 2+ concentration, depended upon the external Na+ concentration and was blocked by TTX. Thus, it is concluded that this slow potential and the plateau potential following the spike are based on the same ionic mechanism. While Na+ ions are involved in the generation of the slow potential as well as in the spike generation, these two Na + potentials could be distinguished by a difference in their thresholds for activation. Furthermore, because of different refractory periods, the spike potential could be isolated from the plateau potential following two successive depolarizing pulses. These results suggest the presence of two types of Na + channels in the cockroach giant axon, which can be distinguished by different voltage levels for activation and hy different inactivation kinetics.

CALCIUM-DEPENDENT THE RABBIT.

OUTWARD

CURRENT

IN N O D O S E

GANGLION

NEURONES

OF

KIICHIRO MORITA~ YOSHIFUMI KATAYAMA, HITOSHI TATSUMI~ Department of Autonomic Physiology, Medical Research Institute, Tokyo M e d i c a l a n d D e n t a l U n i v e r s i t y . T o k y o , 101, J a p a n . The characteristics of calcium-dependent outward current was s t u d i e d in v o l t a g e - c l a m p e d visceral primary afferent neurones excised from rabbits {n ultro. Short depolarizing voltage command (duration: 5 - 10 m s e c ) w h i c h caused calcium entry across the membrane, was followed by an outward current (Iout) with a peak amplitude o f 0.1 - I nA, a n d a t i m e c o n s t a n t of d e c a y r a n g i n g f r o m 0.5 - 4 sec. It w a s b l o c k e d by removal of c a l c i u m i o n s o r b y a d d i t i o n o f M n ++, N i ++ o r C o ++ (I - 6 m M ) ; b u t it was observed in sodium-free solution. The peak amplitude and the time constant of decay of Iout were increased with the increase of either the number or the duration of depolarizing commands. Iout was associated with an increase in membrane conductance to potassium ions. Its reversal potential was c h a n g e d in a n a p p r o x i m a t e l y Nerstein manner with the potassium concentration. I o u t w a s a u g m e n t e d a n d / o r r e d u c e d b y T E A (10 20 mM) b u t m a r k e d l y d e p r e s s e d b y B a ++ (I - 3 mM). Furthermore, I o u t w a s e n h a n c e d b y c a f f e i n e (0.1 - 10 ~ M ) a n d t r i f l u o p e r a z i n e (I 10 ~ M ) . I t is c o n c l u d e d that calcium entry during a depolarizing command leads to a long-lasting increase in potassium conductance, which may reflect the time course of the intracellular calcium sequestration processes.