The effect of cyclic AMP on Na+ and K+ transport systems in mouse macrophages

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

Activation of the Na(+)-K+ pump in frog erythrocytes by catecholamines and phosphodiesterase blockers

K+ and Na+ influx into frog erythrocytes incubated in standard saline was studied using 86Rb and 22Na as tracers. 10 microM isoproterenol (ISP) produced a significant increase in K+ influx for the first 15 min, which was sustained during the entire 60 min of cell incubation. Treatment of red cells with the phosphodiesterase (PDE) blockers theophylline (THEO, 1 and 5 mM) or 3-isobutyl-1-methylxanthine (IBMX, 0.5 mM) was also accompanied by an enhancement in K+ influx. A distinct additive effect on K+ influx into red cells was found when ISP and THEO or IBMX were added together. The increase in K+ transport induced by ISP plus IBMX was totally abolished by pretreatment of red cells with 0.1 mM ouabain. The ouabain-sensitive K+ influx in frog erythrocytes was elevated in the presence of ISP plus IBMX to 2.05 +/- 0.45, as compared with the control level of 0.39 +/- 0.11 mmol/L cells/hr (P < 0.001). Similar effects of ISP and IBMX on K+ influx were observed after chloride was replaced by nitrate. A dose-related increase in K+ influx into frog erythrocytes was observed at ISP concentrations of 10(-8)-10(-6) M, with a half-maximal stimulatory concentration of approximately 0.02 microM. The effects of ISP (10(-8)-10(-5) M) on K+ transport were completely abolished with 10 microM of the beta-adrenergic blocker propranolol, but alpha-adrenergic antagonists (phentolamine, prazosin, and yohimbine) did not alter the ISP-induced increase in K+ influx. The drugs tested had no effect on 22Na influx in frog red cells, but ISP produced a small decline (13%) in intracellular Na+ concentration. Thus, our study indicates that catecholamines and PDE blockers enhance K+ (86Rb) transport in frog erythrocytes mediated by Na(+)-K+ pump activity. The frog erythrocyte membrane may serve as a convenient model to investigate the hormonal modulation of the Na(+)-K+.

Evidence for the regulation of Na+, K(+)-ATPase alpha 1 gene expression through the interaction of aldosterone and cAMP-inducible transcriptional factors

Mineralocorticoid hormones such as aldosterone modulate cellular ion homeostasis at least in part through the regulation of Na+, K(+)-ATPase (NAKA) gene expression. While aldosterone acts at the transcriptional level through its ligand-inducible mineralocorticoid receptor (MR), tissue specific and other transcriptional factors may interact with the MR to modulate this regulatory response. cAMP also regulates NAKA alpha 1 gene expression which at the transcriptional level is mediated, in part, through a cAMP response element (CRE) present on a highly conserved, 48 base pair enhancer region, the PUC-1 core, of the rat NAKA alpha 1 subunit gene promoter. We have tested the hypothesis that the MR interacts with cAMP induced transcriptional factors to modulate the NAKA alpha 1 gene expression. In transient transfection studies a PUC-1 core attached to an enhancerless SV40 promoter driven reporter gene (pB1CAT) was induced by 8-bromo-cAMP in HeLa cells. Co-transfected MR expression vector inhibited the 8-bromo-cAMP inducible activity of pB1CAT. DNA binding studies suggested that the PUC-1 core binds both CREB/ATF proteins as well as the glucocorticoid hormone class of steroid receptors. These results suggest that the MR suppresses cAMP-mediated activation of PUC-1 core driven CAT activity possibly through a direct interaction with CREB/ATF transcriptional factors. This in turn suggests that the interaction of two distinct signal transduction systems, aldosterone and cAMP, may define the mineralocorticoid responsiveness of the Na+, K(+)-ATPase alpha 1 gene.

Energy metabolism of reticulocytes: two different sources of energy for Na+K(+)-ATPase activity

Total energy production in rabbit reticulocytes amounted to 136.52 +/- 6.50 mumol ATP h-1 ml-1 of reticulocytes: 88.3 per cent was provided by oxidative phosphorylation, whereas only 11.7 per cent by aerobic glycolysis. Na+K(+)-ATPase accounted for 23 per cent, i.e. 27.65 +/- 2.55 mumol ATP h-1ml-1 of reticulocytes, in the overall energy consumption in reticulocytes of rabbits. Under basal conditions ATP for Na+K(+)-ATPase activity was derived exclusively from oxidative phosphorylation. However, when the activity of Na+K(+)-ATPase was increased due to the stimulation of adenylate cyclase by (-)-isoprenaline, the additional energy required was provided by aerobic glycolysis. These results indicate that two different compartments, one cytosolic and the other mitochondrial, provide energy for Na+K(+)-ATPase activity in reticulocytes.

Regulation of Na+ and K+ contents in rat thymocytes

A modified nystatin technique allowed the investigation of the initial rate of Na+ efflux as a function of internal Na+ content under steady-state conditions in rat thymocytes. This kinetic study showed that 1) ouabain-sensitive Na+ efflux as a function of internal Na+ can be adjusted by a three-sites kinetic model, with a maximal pump rate of 581 +/- 79 mmol.l cells-1.h-1 and an apparent dissociation constant for internal Na+ of 10.0 +/- 1.0 mmol/l cells (mean +/- SE of 3 experiments), 2) bumetanide-sensitive Na+ efflux was extremely low compared with the pump efflux (approximately 1%), and 3) ouabain- and bumetanide-resistant Na+ efflux was almost a linear function of internal Na+ content (as expected for a Na+ leak). This "all-pump" mechanism of thymocyte Na+ regulation was confirmed by non-steady-state experiments showing that 1) ouabain induced a rapid net Na+ gain and K+ depletion in fresh thymocytes and completely blocked the recovery of normal cation contents in Na+-loaded-K+-depleted thymocytes, and 2) bumetanide was unable to modify thymocyte Na+ and K+ contents. Na+ extrusion by Na+-loaded thymocytes was unaffected by prostaglandin E2, isoproterenol, or platelet-aggregating factor (PAF) and was slightly impaired in the adult spontaneously hypertensive rat of the Okamoto strain (10% lower rate constant for net Na+ extrusion, P less than 0.05). Concerning cell Na+ regulation, our results do not support the concept that rat thymocytes are more representative of vascular cells than enucleated erythrocytes.

Inhibitory action of norepinephrine on sodium transport in vascular smooth muscle cells in culture

Cultured vascular smooth muscle cells from porcine aortas incubated in Na+ -free medium rapidly release their intracellular Na+ contents (Nai) (23 +/- 4% of baseline after 60 min incubation, mean +/- SEM of 18 experiments). Total Nai release was inhibited by 35-40% after addition of ouabain and by 60-70% after addition of ouabain + bumetanide. Norepinephrine inhibited ouabain and bumetanide-sensitives Na+ efflux with an IC50 of about 10(-9)-10(-8) M. Addition of the alpha-adrenergic agonist phenylephrine (10 microM) to the cells mimicked the inhibitory action of norepinephrine on Nai release. Conversely, the beta-adrenergic agonist isoproterenol was without effect on Nai release. Simultaneous addition of 10 microM norepinephrine and the alpha-adrenergic antagonist phentolamine prevented any effect of norepinephrine on the rate of Nai decline. In A-10 cultured vascular smooth muscle cells, the alpha-adrenergic agonist phenylephrine (10 microM) inhibited 40.0 +/- 8.1% of ouabain-sensitive Rb+ influx and 70.7 +/- 6.9% of bumetanide-sensitive Rb+ influx (mean +/- SEM of three experiments). 50% inhibition of bumetanide-sensitive Rb+ influx was obtained with about 5 x 10(-7) M of phenylephrine. Our results show that in vascular smooth muscle cells a [Na+, K+, Cl-]-cotransport system is able to catalyze outward Na+ movements (in Na+ -free media) of a similar order of magnitude to those of the Na+, K+ pump and that alpha-adrenergic stimulation markedly inhibits Na+ efflux (and Rb+ influx) through these two transport systems.

High sensitivity of the Na+, K+-pump of human red blood cells to genins of cardiac glycosides

1. Four different cardiac glycosides (ouabain, digitoxin, digoxin and gitoxin) and their corresponding genins were tested on Na+, K+-pump fluxes measured under steady-state and initial rate conditions (non equilibrium conditions) in human and rat erythrocytes and in mouse macrophages. 2. In human red cells, Na+, K+-pump fluxes exhibited up to 8 fold higher sensitivity to genins than to glycosides. In addition genins, but not the corresponding glycosides, exhibited double reactivity with regard to the erythrocyte Na+, K+-pump (with the exception of gitoxigenin). A weak reactivity component was similar to the one of the corresponding glycosides (IC50 of about 10(-6) M) and a high reactivity component exhibited IC50 values varying from 0.1 to 0.5 X 10(-6) M for digitoxigenin and ouabagenin respectively. 3. In contrast with human red cells, the initial rate of Na+, K+-pump fluxes in rat erythrocytes and mouse macrophages was less sensitive to genins than to the corresponding cardiac glycosides. 4. Dihydroouabain was 3, 10 and 75 times less active than ouabain in inhibiting the initial rate of Na+, K+-pump fluxes in human and rat erythrocytes and in mouse macrophages respectively. 5. In conclusion, Na+, K+-pump fluxes measured under initial rate conditions in human erythrocytes exhibit an unusually high sensitivity to genins of cardiac glycosides. This property probably results from the fast binding rate constants of genins and the slow association rates of glycosides to human red cells.

Sodium transport kinetics in erythrocytes from spontaneously hypertensive rats

Rat erythrocytes with five different amounts of Na+ content have been prepared by using a new, nondetrimental Na+-loading method (net NaHPO4-influx through the anion carrier). This method allowed the determination of 1) maximal translocation rates and apparent dissociation constants for internal Na+ of the Na+-K+ pump, outward Na+-K+ cotransport, and Na+-Li+ countertransport and 2) rate constants of Na+ leak in erythrocytes from spontaneously hypertensive rats of the Okamoto strain and Wistar-Kyoto normotensive controls aged 2 to 26 weeks. Two major abnormalities were found in erythrocytes from spontaneously hypertensive rats: 1) a decreased cotransport affinity for internal Na+, which was constantly observed from 2 to 26 weeks of age (mean intracellular Na+ content for half-maximal stimulation of outward Na+-K+ cotransport = 33.1 +/- 7.0 [SD] mmol/L cells in spontaneously hypertensive rats vs 16.7 +/- 4.7 mmol/L cells in Wistar-Kyoto rats; p less than 0.001), and 2) a decreased maximal pump rate in adult (15- to 26-week-old) spontaneously hypertensive as compared with that for age-matched Wistar-Kyoto rats (9-37 vs 34-70 mmol/L cells/hr). Therefore, the low cotransport affinity for internal Na+ appears to be a stable, possibly genetic defect of spontaneously hypertensive rats. Conversely, the decreased maximal pump rate may be a secondary event, possibly reflecting the appearance of endogenous pump inhibitors in the plasma of adult spontaneously hypertensive rats.

Stimulation of beta-adrenoceptors inhibits calcium-dependent potassium-channels in mouse macrophages

K+ efflux in mouse macrophages exhibited a rate constant (kK) of 0.67 +/- 0.04 (h)-1 (mean +/- SEM of 16 experiments). This was strongly stimulated by increasing concentrations of the Ca2+ ionophore A23187 up to a maximal value of 4.01 +/- 0.25 (h)-1 with an IC50 of 7.6 +/- 1.9 microM (mean +/- SEM of 6 experiments). Similar results were obtained with the Ca2+ ionophore ionomycin. Binding experiments with 3H-dihydroalprenolol revealed a high density of beta-adrenergic receptors (97.5 +/- 5.2 fmol/mg protein) with apparent dissociation constant of 2.03 +/- 0.06 nM. Isoproterenol at a concentration of 10(-6)-10(-5) M induced a two- to threefold stimulation of endogenous levels of cyclic AMP (cAMP). A23187-stimulated K+ efflux was partially inhibited by stimulation of adenylate cyclase with isoproterenol, forskolin or, PGE1; exogenous cAMP; and inhibition of phosphodiesterase with MIX (1-methyl-3-isobutylxanthine). Maximal inhibition of K+ efflux was obtained by simultaneous addition of isoproterenol and MIX. In dose-response curves, the isoproterenol-sensitive K+ efflux was half-maximally inhibited (IC50) with 2-5 X 10(-10) M of isoproterenol concentration. Propranolol was able to completely block the effect of isoproterenol, with an IC50 of about 1-2 X 10(-7) M. Isoproterenol and MIX were also able to partially inhibit ionomycin-stimulated K+ efflux. Isoproterenol and MIX did not inhibit A23187-stimulated K+ efflux in an incubation medium where NaCl was replaced by sucrose (or choline), suggesting the involvement of an Na+:Ca2+ exchange mechanism. Our results show that stimulation of beta-adrenoceptors in mouse macrophages counterbalances the opening of K+ channels induced by the calcium ionophore A23187. This likely reflects a decrease in cytosolic free calcium content via a cAMP-mediated stimulation of Na+:Ca2+ exchange.

Involvement of K+ movements in the membrane signal induced by PAF-acether

We investigated the effects of PAF-acether and its specific antagonist BN 52021 on Na+ and K+ transport systems in human red cells and mouse macrophages. PAF-acether and BN 52021 exhibited specific and opposite effects on a Cs+-stimulated, K+ efflux in human red cells. PAF-acether increased and BN 52021 decreased the apparent dissociation constant for external Cs+ without significant effects on the maximal rate of K+ translocation. In mouse macrophages, PAF-acether stimulated a quinidine-sensitive K+ efflux. In the presence of the Ca2+-ionophore A 23187, PAF-acether and BN 52021 showed opposite effects (stimulation and inhibition respectively). For most cells, membrane potential is dependent on K+-permeability. In addition, opening of potential-dependent Ca2+ channels appears to be associated with cell activation in several models. We thus propose that the specific interaction of PAF-acether with a K+:K+ exchange increases Ca2+ uptake through transitory changes in membrane potential. This in turn may lead to a more permanent membrane hyperpolarization through to opening of Ca2+ dependent, K+ channels.

Involvement of natriuretic hormones and Na+ transport in the antihypertensive action of canrenone

Recent studies in essential hypertensive patients and rats with genetic hypertension strongly suggested that the development of primary hypertension takes place by a transient and chronic "cascade" of events (i) excess Na+ intake, (ii) secretion of natriuretic factors, (iii) abnormal cells Na+ homeostasis in the vascular wall, due to the presence of inherited abnormalities in different Na+ transport systems, and (iv) increase in cytosolic free Ca2+ content and catecholamines. Canrenone, an antihypertensive drug, behaves like a partial agonist at the digitalis-receptor site of the Na+, K+ pump. We observed here that (i) a 4 hr preincubation of human red cells with this compound increases its antagonistic properties against ouabain, (ii) in cultured smooth muscle cells, canrenone counterbalances the increase in cytosolic free Ca2+ induced by ouabain, and (iii) in a model of experimental hypertension with increased endogenous "ouabain-like" factors (rats with reduced renal mass), the administration of canrenone tends to normalize Na+, K+-pump activity and decrease blood pressure.