Comp. Biochem. Physiol.. 1977. Vol. 58C, pp. 153 to 155. Peryamon Press. Printed in Great Britain THE SODIUM PUMP ACTIVITY AND ACETYLCHOLINE SENSITIVITY OF NEURONAL MEMBRANE* S. N. AYRAPETYANAND V. L. ARVANOV Institute of Experimental Biology. Academy of Sciences of Armenian SSR, B. Sevag 9, 375044 Erevan, U.S.S.R (Received 24 February 1977) Abstract--1. The relation of Ach-sensitivity of the nerve cell membrane to the activity of the Na pump was studied. 2. The depolarizing effect of Ach is greater when the Na pump is in an inactive state. 3. Activation of Na pump work leads to the decrease and inactivation to the increase of the Ach-effect on the membrane conductivity. INTRODUCTION We have previously demonstrated (Ayrapetyan & Gevorgyan, 1971) that acetylcholine (Ach) has a double effect on the sodium pump activity: on the one hand the application of Ach increases the mem- brane conductivity for Na ions, thus promoting their intracellular concentration and intensifying the Na pump work, and on the other hand acting as an in- hibitor of the membrane Na-K-ATPase activity. Consequently, it can be suggested that the effect of Ach on the membrane potential (MP) depends not only on the passive, but also on the active ion-trans- porting systems of the membrane. In our preceding * The results of these investigations were reported at the Symposium Neurone Concept in 1976, Tihany, Hungary, 26-28 August, 1976. experiments (Ayrapetyan, 1969a, b) we have already demonstrated, that the temperature-dependent part of the neurone MP is conditioned by the work of electro- genie Na pump, which activity is gradually inhibited as a result of: (1) a long-term maintenance in in vitro state; (2) in K+-free solution; (3) in cold; (4) by the action of ouabain, etc. At the same time Bregestovsky and others (Bregestovsky et al., 1972) demonstrated that cold increased the depolarising effect of Ach on the cell membrane. They explained the temperature- dependent effect of Ach by the prolongation of the life-time of Ach-receptor complex. MATERIALS AND METHODS The whole circumoesophagel ganglion of Helix armenica was isolated. Experiments were carried out on the neurones. These neurones were exposed by using a very fine forceps [o) ~Ach ~Ringer t Ringer cold ~Ach cold • . , ,.I:,,P~II~ _f'l ~l ' * ~ACh t Ringer ~Ringer cold tAch cold ( c ) I¢~-= .~ ..... _~ iii ~ ~Aeh ~Ringer cold ~Ach cold ' I 2 min Fig. 1. The relation of Ach action on the MP to the temperature and the duration of preparation maintenance in in vitro state. The records of the electrical neuron activity were made, (al 5 min. (b) 50 min, (c) 90 min following the puncture of the membrane. 153