Further study of the correlation between Na-pump activity and membrane chemosensitivity

Na+/K+-pump and neurotransmitter membrane receptors

Na⁺/K⁺-pump is an electrogenic transmembrane ATPase located in the outer plasma membrane of cells. The Na⁺/K⁺-ATPase pumps 3 sodium ions out of cells while pumping 2 potassium ions into cells. Both cations move against their concentration gradients. This enzyme's electrogenic nature means that it has a chronic role in stabilizing the resting membrane potential of the cell, in regulating the cell volume and in the signal transduction of the cell. This review will mainly consider the role of the Na⁺/K⁺-pump in neurons, with an emphasis on its role in modulating neurotransmitter receptor. Most of the literature on the modulation of neurotransmitter receptors refers to the situation in the mammalian nervous system, but the position is likely to be similar in most, if not all, invertebrate nervous systems.

The Na,K pump regulates decreases in the cholinosensitivity of neurons in the common snail to a cellular analog of habituation: the role of cellular calcium

The effects of the Na,K pump inhibitor ouabain on the depth of depression of cholinosensitivity of defensive behavior command neurons LPa2, LPa3, RPa3, and RPa2 were studied in the common snail using a cellular analog of habituation; the role of intracellular Ca2+ in these effects was analyzed. Integral acetylcholine-evoked transmembrane currents (ACh currents) were recorded by two-electrode membrane voltage clamping. In one group of neurons, extracellular application of ouabain (0.1 mM) by addition to the bathing solution evoked increases in the depression of the ACh current evoked by rhythmic application of mediator (with interstimulus intervals of 1-3 min) while neurons of the other groups responded with decreases in depression. After spontaneous diffusion of the Ca2+ ion chelator BAPTA (1 mM) from the intracellular microelectrode for 60-150 min, ouabain only increased the level of depression of the ACh current. After intracellular injection of CaCl2 (100 mM), ouabain only decreased the level of depression of the ACh current. It was concluded that inhibition of the Na,K pump modifies depression of the cholinosensitivity of neurons in the cellular model of habituation. The direction of the effect depends on the basal concentration of intracellular Ca2+.

Ouabain-sensitive and insensitive acetylcholine receptors on the membrane of the same neuron in Helix pomatia

Using internally dialyzed neurons of Helix pomatia as a model, the effect of structural analogs of acetylcholine (ACh) were investigated for their cholinomimetic properties on A- and B-types of ACh-responses. Specifically we analyzed choline esters of N-para- and ortho-alkoxybenzoyl-beta-alanines, (CH(3)O-, C(2)H(5)O-, C(3)H(7)O-, iso-C(3)H(7)O-, C(4)H(9)O-, iso-C(4)H(9)O-, C(5)H(11)O-, iso-C(5)H(11)O-), (in all, 16 combinations). The compounds evoked differing sensitivities of response to factors de-activating the Na-K-pump (ouabain and K-free solution). Most compounds resembled ACh: ionic currents caused by these compounds were inhibited by Na-K pump blockers in the case of A-type responses - B-type responses were insensitive to these factors. Ionic currents induced by choline esters of p-, o-propoxy- and iso-propoxybenzoyl-beta-alanines were insensitive to ouabain and K-free solution in the case of A- and B-type responses. Ionic currents induced by the choline ester of p-butoxybenzoyl-beta-alanine were inhibited by ouabain and K-free solution on both types of neuron. The results allow us to classify choline esters of p-, o-propoxy- and iso-propoxybenzoyl-beta-alanines as N-cholinomimetics, while the choline ester of p-butoxybenzoyl-beta-alanine can be considered as M-cholinomimetic. We conclude that both M- and N-type ACh receptors exist on the membrane of the same neurons.

Cellular and molecular mechanisms of nitric oxide-induced heart muscle relaxation

1. The nitric oxide (NO) donor S-nitro-N-acetyl-penicillamine (SNAP) inhibits Helix aspersa heart activity and relaxes muscles. 2. K-free saline and ouabain both depress SNAP-induced relaxation in most experiments, but in a few preparations they either had no effect or potentiated SNAP-induced relaxation. 3. Na-K pump reactivation following preincubation in K-free saline leads to the pronounced transient relaxation of heart muscle, the magnitude of which depends on the duration of preincubation. 4. 0.1 mM SNAP inhibited the ouabain sensitive part of 86Rb uptake, which reflects Na-K pump activity. This inhibition is potentiated by phospholipase C. 5. SNAP increased cGMP levels in the heart. 6. These results indicate that SNAP-induced relaxation depends on Na and Ca gradients across the membrane, which suggests that Na:Ca exchange is involved in the mechanisms of SNAP-induced relaxation. It is postulated that SNAP elicits its inhibitory effect on the heart through a cGMP-dependent Na:Ca exchange.

Metabotropic GABA receptors regulate acetylcholine responses on snail neurons

1. The A type of acetylcholine response of Helix neurons is downmodulated by low concentrations of GABA that do not elicit any measurable change in membrane potential or conductance. 2. We find that these physiological actions are associated with an increase in both intracellular cyclic AMP levels and 45Ca2+ influx. 3. The modulation of the acetylcholine response by GABA is blocked when the neurons are injected with EGTA to prevent a rise in intracellular Ca2+ concentration or when tolbutamide, an inhibitor of protein kinase A, is applied. 4. These results are consistent with the effects of GABA being mediated by a metabotropic GABA receptor that is activated at very low GABA concentrations and mediates modulation of the acetylcholine response via regulation of intracellular Ca2+ and cyclic AMP levels.

Interactions of anticholinesterases with Achatina fulica acetylcholine responses and electrogenic sodium pump

The dose-dependent effects of the anticholinesterases, neostigmine and mycotoxin territrem-B, were determined on: (i) Cl(-)-responses of voltage clamped Achatina fulica neurons to microperfused acetylcholine; (ii) the 4 K(+)-induced outward currents evoked by an electrogenic sodium pump in the same neuron; and (iii) acetylcholinesterase activity of Achatina fulica ganglionic homogenates. Both compounds at low doses potentiated the peak acetylcholine responses. However, they had different effects at higher (> 1 microM) doses in that neostigmine now antagonized acetylcholine responses, while territrem-B still produced a maximal potentiation. At all doses neostigmine produced a dose-dependent inhibition of acetylcholinesterase activity. The cholinolytic effect of high doses of neostigmine was associated with the inhibition of 4 K(+)-induced current in the same neuron, while territrem-B neither altered the K(+)-induced current nor antagonized acetylcholine responses. The cholinolytic effect of neostigmine was completely antagonized by the inhibition of electrogenic sodium pump by ouabain or by perfusion with K(+)-free solution. These results suggest that neostigmine at high concentrations inhibits the electrogenic sodium pump and that the cholinolytic effect of high doses of neostigmine is secondary to this action. Territrem-B, on the other hand, had no effect on the electrogenic sodium pump and had no effect on the neuronal membrane properties other than to inhibit acetylcholinesterase. Thus, territrem-B may be a useful tool for studying the interaction between acetylcholinesterase and acetylcholine receptors.

Interaction of concanavalin A and wheat germ agglutinin with Helix acetylcholine receptors

The effects of lectins concanavalin A (Con A) and wheat germ agglutinin (WGA), were studied on acetylcholine (ACh) responses of physically isolated internally dialyzed Helix aspersa neurons using the concentration clamp method and on the binding of [3H]alpha-bungarotoxin to the cluster of neurons. Con A and WGA have different simple sugar specificity and produced different actions on ACh-evoked Cl conductance responses, which were antagonized by Con A (5 micrograms/ml) but were not altered by WGA. Con A depressed ACh responses when applied extracellularly while it had no effect on ACh responses of the same neuron when added to the intracellular solution, thus indicating that Con A specific glycoproteins are exposed on the surface of the neuron. The studies of the effect of Con A on the properties of the ACh binding site (receptor) have demonstrated, that (1) the onset of desensitization of ACh responses of the dialyzed neurons, determined from the decay of ACh-current from peak to plateau in the continued presence of agonist and best fitted by a double exponential function, was accelerated by Con A; (2) Con A depressed the maximal ACh induced current in a dose response relationship and altered the Hill coefficients; (3) Con A depressed the binding of [3H]alpha-bungarotoxin to the cluster of neurons. These results indicate that Con A receptors on the surface of the neuronal membrane play a regulatory role in the ACh-receptor system and suggest that binding of lectin molecules to their receptors leads to inhibition of binding of ACh to ACh-receptors and to acceleration of the kinetics of desensitization of ACh receptors. All the effects of Con A, that is, on the peak amplitude, desensitization, dose-response relationship of ACh induced current and binding of [3H]alpha-bungarotoxin, could be recovered by D-mannose, a competitive inhibitor of Con A binding to its receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of cAMP, Ca2+, and phorbol esters on ouabain-induced depression of acetylcholine responses in Helix neurons

1. Using internal perfusion and concentration-clamp procedures applied to Helix neurons, the effects of cAMP, Ca2+, and phorbol esters on ouabain-induced depression of acetylcholine Cl-dependent responses were determined. 2. Intracellular cAMP (10(-4) M) depressed those acetylcholine responses which were blocked by ouabain but had no effect on ouabain-insensitive acetylcholine responses. In the presence of elevated intracellular cAMP, ouabain had no further depressant effect on these acetylcholine responses. Both elevated cAMP and ouabain reduced the acetylcholine response without altering the current-voltage curves. 3. An increase in intracellular Ca2+ concentration depressed the amplitude of current induced by application of acetylcholine in neurons with ouabain-sensitive responses and shifted the dose-response relationship to the right. However, elevated Ca2+ did not reduce the maximal response induced by acetylcholine, nor did it prevent the reduction of that response by ouabain. 4. 12-O-Tetradecanoylphorbol-13-acetate (TPA), a potent stimulator of protein kinase C activity, caused depression of both the ouabain-sensitive and the ouabain-insensitive acetylcholine responses. The inhibitory effect of TPA was markedly enhanced after addition of ATP to the intracellular medium and was greatly reduced by cooling to 5 degrees C. The blocking effect of ouabain, however, reexamined in the presence of TPA. 5. These observations are consistent with the hypothesis that the depression of acetylcholine induced Cl--responses in Helix neurons is a result of an increase in intracellular cAMP concentration but is unrelated to activation of protein kinase C or increases in intracellular Ca2+.

Ouabain blocks some rapid concentration-induced clamp acetylcholine responses on Helix neurons

1. The effects of ouabain, a potent inhibitor of Na(+)-K+ ATPase, were determined on the transmembrane responses of internally dialyzed Helix neurons to rapid acetylcholine (ACh) application using the "concentration clamp" technique. 2. Ouabain selectively depressed "A"-type responses to ACh, which are due to a selective increase in membrane permeability to chloride. In contrast, the "B"-type responses, due primarily to an increase in monovalent cation permeability, was unaffected. 3. The blockade of the Cl- responses was not associated with a change of the reversal potential of the response. Ouabain depressed the maximal response without shifting the dose-response curve. 4. Ouabain caused an increase in the time constant of decay of the ACh current, but the value in the presence of ouabain was not different from that of a lower concentration of ACh determined so as to give a response of the same peak amplitude. Therefore, the effect of ouabain is not on the process of receptor desensitization directly.

The pharmacological characteristics of two types of cholinoreceptors in the membrane of dialyzed neurons

1. The ramped voltage clamp technique was developed as a rapid means of studying the effects of certain nicotinic and muscarinic agents on ionic involvement and conductance changes during acetylcholine (ACh) responses of Helix pomatia neurons. 2. Atropine was found to be a potent cholinolytic on A-type neurons, ACh responses of which are blocked by ouabain and mediated by Na+ and Cl- permeabilities, while d-tubocurarine blocked B-type ACh responses which are insensitive to ouabain and mediated by Na+ and K+ permeabilities. 3. Nicotinic agent butyrylcholine was found to be a potent cholinomimetric on B-type cells. 4. The results suggest that ACh receptors on A-type cells are more "muscarinic" while those on B-type cells are more "nicotinic". 5. It was also suggested that both muscarinic and nicotinic ACh receptors may coexist in the Helix neuronal membrane and the possibility of ACh interacting with one of them is determined by the level of phosphorylation of the membrane proteins.

The effects of short-chain fatty acids on the neuronal membrane functions of Helix pomatia. II. Cholinoreceptive properties

We have examined the effects of short-chain fatty acids on acetylcholine (ACh)-induced transmembrane currents using internally dialyzed neurons of Helix. Decenoic acid, which increased the fluidity of excitable membranes, caused dramatic changes in the voltage sensitivity of ACh currents consisting of an ACh-induced increase in membrane permeability for K+ and Na+ ions and a shift of the Erev of these ACh responses to more positive potentials. Valeric acid, which did not change the membrane fluidity, had no effect on this type of ACh response. Changes of the ENa and ECl had no effect on the size of the decenoic acid-induced shift of the Erev. But the influence of decenoic acid on the voltage sensitivity of ACh-induced currents almost disappeared after the change of the EK by the reduction of the internal K concentration. Decenoic acid had no effect on ACh responses in which K+ ions were not involved in the generation of ACh-induced currents. The results suggest that decenoic acid-induced changes in membrane fluidity modulate cholinoreceptive properties of the neuronal membrane by the inhibition of the K+ carrier involved in the generation of ACh responses.