Does the electrogenic Na+/K+ pump play a role in the neuronal effects of catecholamines?

Vulnerability of medium spiny striatal neurons to glutamate: role of Na+/K+ ATPase

In Huntington's disease neuronal degeneration mainly involves medium-sized spiny neurons. It has been postulated that both excitotoxic mechanisms and energy metabolism failure are implicated in the neuronal degeneration observed in Huntington's disease. In central neurons, > 40% of the energy released by respiration is used by Na+/K+ ATPase to maintain ionic gradients. Considering that impairment of Na+/K+ ATPase activity might alter postsynaptic responsivity to excitatory amino acids (EAAs), we investigated the effects of the Na+/K+ ATPase inhibitors, ouabain and strophanthidin, on the responses to different agonists of EAA receptors in identified medium-sized spiny neurons electrophysiologically recorded in the current- and voltage-clamp modes. In most of the cells both ouabain and strophanthidin (1-3 microM) did not cause significant change in the membrane properties of the recorded neurons. Higher doses of either ouabain (30 microM) or strophanthidin (30 microM) induced, per se, an irreversible inward current coupled to an increase in conductance, leading to cell deterioration. Moreover, both ouabain (1-10 microM) and strophanthidin (1-10 microM) dramatically increased the membrane depolarization and the inward current produced by subcritical concentrations of glutamate, AMPA and NMDA. These concentrations of Na+/K+ ATPase inhibitors also increased the membrane responses induced by repetitive cortical activation. In addition, since it had previously been proposed that dopamine mimics the effects of Na+/K+ ATPase inhibitors and that dopamine agonists differentially regulate the postsynaptic responses to EAAs, we tested the possible modulation of EAA-induced membrane depolarization and inward current by dopamine agonists. Neither dopamine nor selective dopamine agonists or antagonists affected the postsynaptic responses to EAAs. Our experiments show that impairment of the activity of Na+/K+ ATPase may render striatal neurons more sensitive to the action of glutamate, lowering the threshold for the excitotoxic events. Our data support neither the role of dopamine as an ouabain-like agent nor the differential modulatory action of dopamine receptors on the EAA-induced responses in the striatum.

Alteration of brain cell membrane function following cocaine exposure in the fetal guinea pig

The effect of cocaine on brain cell membrane structure and function was studied in the fetal guinea pig. We tested the hypothesis that cocaine, a potent vasoconstrictor, would result in brain cell membrane dysfunction as determined by altered activity of Na+,K(+)-ATPase and the appearance of products of membrane lipid peroxidation (conjugated dienes (CD) and fluorescent compounds (FC)). A total of 14 pregnant guinea pigs were studied at term (60 days). One hour prior to delivery, the pregnant guinea pigs were divided into 3 groups as follows: cocaine, 30 mg/kg i.p., saline placebo i.p., or 7% FiO2 for 1 h. Following cocaine, brain Na+,K(+)-ATPase activity decreased (mean +/- S.D., 25.6 +/- 9.2 vs. 54.6 +/- 3.4 mumol Pi/mg protein/h, cocaine vs. control, respectively, P < 0.01) and was similar to the hypoxia group (21.9 +/- 2.8 mumol Pi/mg protein/h). The products of lipid peroxidation did not change significantly following cocaine whereas hypoxia resulted in a rise in CD from 0 to 0.175 +/- 0.015 mumol/g brain, control vs. hypoxia, (P < 0.01), and FC from 1.13 +/- 0.15 to 1.88 +/- 0.13 micrograms quinine sulfate/g brain, control vs. hypoxia, (P < 0.01). These data show that acute fetal cocaine exposure, unlike hypoxia alone, results in a significant decrease in Na+,(K+)-ATPase activity without a significant increase in the products of lipid peroxidation, suggesting the mechanism by which cocaine affects brain cell membrane integrity is distinct from hypoxia. Inhibition of the enzyme activity may be due to a direct action of cocaine on the enzyme or due to enzyme regulation by cocaine-induced alterations in neurotransmitters.

Activation of alpha-adrenoceptors indirectly facilitates sodium pumping in frog motoneurons

The effects of clonidine on Na+ pumping in motoneurons of the isolated frog spinal cord was investigated using sucrose gap recordings from ventral roots. Na+ pump activity, induced in motoneurons either by tetanizing the dorsal root or by rapidly exposing the cord to normal medium following 30 min in K(+)-free Ringer's solution (K(+)-activated hyperpolarizations), was increased by application of clonidine (100 microM). These actions of clonidine were blocked by the preferential alpha 2-adrenergic antagonist yohimbine, but not by alpha 1-adrenergic antagonist prazosin or the beta-blocker propranolol. Clonidine's effects on Na+ pumping appeared to be indirect (presumably via interneurons) because its effects on K(+)-activated hyperpolarizations were reduced by tetrodotoxin (TTX) or high concentrations of Mg2+. This indirect mechanism involved activation of non-NMDA excitatory amino acid receptors. Thus, in the presence of clonidine, CNQX, but not APH, limited the ability of clonidine to enhance K(+)-activated hyperpolarizations. The AMPA receptor may play a role in the process, K(+)-activated hyperpolarizations were augmented by the presence of AMPA; NMDA had no effect. The present results are consistent with the idea that activation of alpha 2-adrenoceptors produces the following: the release of excitatory amino acids from interneurons; the activation of non-NMDA receptors on motoneurons; increased Na+ influx and loading and increased Na+ pump activity.

Inhibition by dopamine of (Na(+)+K+)ATPase activity in neostriatal neurons through D1 and D2 dopamine receptor synergism

The (Na(+)+K+)ATPase, an integral membrane protein located in virtually all animal cells, couples the hydrolysis of ATP to the countertransport of Na+ and K+ ions across the plasma membrane. In neurons, a large portion of cellular energy is expended by this enzyme to maintain the ionic gradients that underlie resting and action potentials. Although neurotransmitter regulation of the enzyme in brain has been reported, such regulation has been characterized either as a nonspecific phenomenon or as an indirect effect of neurotransmitter-induced changes in ionic gradients. We report here that the neurotransmitter dopamine, through a synergistic effect on D1 and D2 receptors, inhibits the (Na(+)+K+)ATPase activity of isolated striatal neurons. Our data provide unequivocal evidence for regulation by a neurotransmitter of a neuronal ion pump. They also demonstrate that synergism between D1 and D2 receptors, which underlies many of the electrophysical and behavioural effects of dopamine in the mammalian brain, can occur on the same neuron. In addition, the results support the possibility that dopamine and other neurotransmitters can regulate neuronal excitability through the novel mechanism of pump inhibition.

Analysis of the hyperpolarizing effect of catecholamines on canine cardiac Purkinje fibres

1. The hyperpolarization induced by catecholamines on barium-depolarized (0.2-0.8 mM BaCl) canine cardiac Purkinje fibres, in vitro, was studied by use of conventional microelectrode recordings of transmembrane electrical potentials. 2. Noradrenaline, adrenaline and isoprenaline hyperpolarized Purkinje fibres in a concentration-dependent manner from a threshold concentration around 5 nM. The three catecholamines were shown to be approximately equipotent. Tachyphylaxis was observed when the interval between catecholamine applications was less than 15 min. 3. Atenolol (10 microM) blocked the hyperpolarization reversibly and theophylline (0.5 mM) potentiated it. 4. Tetrodotoxin (5 microM) did not affect the hyperpolarization induced by isoprenaline. Acetylcholine and histamine, up to 10 microM, were not effective in hyperpolarizing Purkinje fibres. 5. Low extracellular potassium concentrations (zero and 1 mM) did not affect the hyperpolarization, but high extracellular potassium concentrations (10-20 mM), markedly reduced the effect of isoprenaline (100 nM). 6. Reduction of the extracellular sodium concentration produced a roughly proportional reduction in the isoprenaline-induced hyperpolarization. The hyperpolarization was reversibly blocked in 34 mM sodium Tris-Tyrode solution. 7. The hyperpolarization was not reduced in Tyrode solution containing 0.6 mM calcium, but was drastically reduced in zero-calcium Tyrode solution. This effect was reversible. 8. Addition of verapamil (5-10 microM) diminished the hyperpolarization, in a concentration-dependent manner. This effect was partially reversed after washing. 9. Ouabain (0.7-1 microM) significantly reduced the isoprenaline-induced hyperpolarization, but 2,4-dinitrophenol (0.2 mM) did not affect it. 10. Caesium chloride (20 mM) abolished the hyperpolarization. The blockade was only partially reversed upon washing. 11. It is suggested that the hyperpolarization induced by a short exposure to catecholamines is mainly due to an increase in potassium permeability (PK). A mechanism involving calciumdependent potassium channels might underlie the increase in PK.

Examination of the role of calcium in the adrenaline-induced hyperpolarization of bullfrog sympathetic neurons

The adrenaline-induced hyperpolarization, which was recorded in neurons of bullfrog paravertebral sympathetic ganglia by means of the sucrose gap technique, was antagonized by 1 mM 4-aminopyridine. The response was unaffected by drugs which influence intracellular Ca2+ movements or Ca2+-sensitive K+ conductances, i.e. 100 or 200 microM Cd2+, 60 microM dantrolene Na+, 10 mM tetraethylammonium bromide, 0.5-2.0 microM apamin or 70 microM (+)-tubocurarine chloride. The spontaneous, rhythmic hyperpolarizations which occur in ganglionic neurons in the presence of 5 mM caffeine and reflect activation of Ca2+-sensitive K+ conductances following mobilization of intracellular Ca2+, were examined by means of intracellular recording. These responses were often biphasic, comprising a transient rapid early phase and a slow late phase. Tetraethylammonium (10 mM) and 0.5-2.0 microM apamin antagonized the rapid early phase and 70 microM (+)-tubocurarine chloride antagonized both phases of the response. Neither phase of these spontaneous, rhythmic, caffeine-induced hyperpolarizations were affected by 1 mM 4-aminopyridine. Although the adrenaline-induced hyperpolarization was antagonized by 50 microM 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate and by 50 microM quinidine, the majority of the results argue against the hypothesis that mobilization of intracellular Ca2+ is required for activation of the K+ conductance thought to underlie the adrenaline-induced hyperpolarization.

Adrenaline causes potassium influx in skeletal muscle and potassium efflux in cardiac muscle in rats: the role of Na/K ATPase

Previous in vitro evidence suggests that adrenaline causes K influx in skeletal muscle by stimulating a ouabain sensitive Na/K ATPase membrane pump. However in rabbits, adrenaline induced hypokalaemia was not significantly altered by pretreatment with digoxin (50 micrograms/kg). Rats were infused with adrenaline or saline after being given a tracer dose of 42KCl. Adrenaline caused a highly significant uptake of 42K in skeletal muscle and a decrease in 42K uptake in ventricle. Rats were also studied after receiving a high dose of digoxin (1.4 mg/kg) which by itself produced a significant increase in plasma K, a decrease in plasma Na and a decreased uptake of 42K in ventricle and lung. These results suggest that adequate widespread Na/K ATPase inhibition had been achieved by this dose of digoxin but despite this, adrenaline still caused hypokalaemia and also still caused significant 42K tissue uptake by skeletal muscle. These results suggest that adrenaline causes K influx by skeletal muscle and K efflux by cardiac tissue. Furthermore, the former mechanism was not inhibited by pretreatment with digoxin.

Adrenaline-induced K+ efflux results in sodium pump stimulation in a sympathetic ganglion

The potassium-activated hyperpolarization (KH) was used as an index of electrogenic Na+ pumping in bullfrog sympathetic ganglia. This response was evoked by storing ganglia in K-free Ringer's solution and briefly introducing normal Ringer's solution containing 2 mM K+ at regular intervals. The apparent EC50 for K+ was 2.21 mM (range 0.88-3.54 mM, for n = 5) and at least 10 mM K+ was required to produce a maximal KH response. Adrenaline, which produces membrane hyperpolarization by increasing K+ conductance (gK), increased the amplitude of KH responses. When the K+ efflux accompanying the adrenaline-induced hyperpolarization (AdH) was blocked with 2 mM Ba2+, the KH was no longer potentiated. It is suggested that the K+ moving out of the cells during the AdH accumulates extracellularly and stimulates the Na+ pump.