Myocardial uptake of digoxin in chronically digitalized dogs.

1 The time course of myocardial uptake of digoxin, increase in contractility and changes in myocardial potassium concentration was studied for 90 min following an intravenous digoxin dose to long-term digitalized dogs. 2 Nineteen dogs were investigated by the use of a biopsy technique which allowed sampling before and after administration of digoxin. 3 Ten minutes after administration of digoxin the myocardial concentration increased from 60 to 306 nmol/kg tissue, the myocardial concentration of digoxin was significantly lower (250 nmol/kg tissue) after 30 min and then increased again. 4 The transmural myocardial distribution of digoxin was uniform before and 90 min after administration of digoxin in long-term digitalized dogs but at 10 min after administration, both the subepicardial and the subendocardial concentration of digoxin were significantly lower than that of the mesocardial layer. 5 During the first 10 min the dp/dtmax increased to 135% of the control level. The increase remained unchanged during the rest of the study. 6 Myocardial potassium decreased throughout the study. 7 The M-configuration of the myocardial uptake curve and the non-uniformity of myocardial distribution of digoxin observed at 10 min after administrating digoxin to long-term digitalized dogs indicate that the distribution of myocardial blood flow may be changed during chronic digitalization.     

Ouabain inhibition of the sodium-potassium pump: estimation of ED50 in different types of human leucocytes in vitro.

1. We examined the effect of ouabain on the Na(+)-K+ pump of intact mononuclear leucocytes and polymorphonuclear leucocytes by using the radioactive potassium analogue, 86rubidium, as a tracer and measuring the cellular uptake of K+ (86Rb+). Na(+)-K+ pump activity was determined as the cellular uptake of K+ (86Rb+) that is sensitive to ouabain, 10(-5) mol l-1. 2. Dose-response curves for inhibition of the Na(+)-K+ pump were obtained after exposure of the cells to various concentrations of ouabain for 2.5 h when the level of pump inhibition was considered to be at steady state. 3. The ED50 of ouabain for the effect on the Na(+)-K+ pump was estimated to be 3 X 10(-9) mol l-1 in the absence of potassium. In the presence of potassium, 3 and 6.5 X 10(-3) mol l-1, it was increased by factors of 10 and 45 respectively. In the presence of potassium, 4 X 10(-3) mol l-1, the ED50 was similar but somewhat higher when determined in an artificial medium (Ringer solution) than when determined in autologous plasma. 4. The ED50 values observed in the present study were very similar to KD values reported in the literature on ligand binding of tritiated ouabain to the same types of human leucocytes.     

Cardiac NaK ATPase activity during positive inotropic and toxic actions of ouabain.

In order to define pharmacological actions of ouabain in the dog heart, ouabain uptake and subcellular distribution and its effect on NaK ATPase (MG2+ dependent, Na+-K+-activated adenosinetriphosphate phosphohydrolase, E.C. 3.6.1.3), have been investigated in 21 open-chest dogs. A continuous infusion of ouabain (0.036 mug/kg/min) after a loading dose (20 mug/kg) produced a relatively constant plasma concentration of approximately 10(-8) M (6 ng/ml) ouabain, which induced a sustained positive inotropic response for the 300 min experimental period. In these hearts much greater binding of ouabain was noted in the NaK ATPase and microsomal fractions than in other myocardial fractions. No statistically significant inhibition of NaK ATPase activity was noted. Doubling the loading and infusion doses of ouabain raised the plasma level of ouabain to approximately 3 X 10(-8) M and produced various types of arrhythmia within an hour, which persisted for the rest of the 5 h experimental period. Under this experimental protocol there was a significant inhibition of NaK ATPase activity and increased binding of ouabain to this enzyme. This study does not support the hypothesis that there is a causal relationship between inotropic response to ouabain and NaK ATPase inhibition. It was concluded that NaK ATPase inhibition might be causally related to the development of ouabain toxicity.     

Membrane Na+ K+ ATPase inhibition related dyslipidemia and insulin resistance in neuropsychiatric disorders

There are several reports in literature implicating cholesterol metabolism in the pathogenesis of neuronal degenerations, oncogenesis, functional neuropsychiatric disorders and multiple sclerosis. Biosynthesis of cholesterol takes place by the isoprenoid pathway, which also produces digoxin, an inhibitor of membrane Na(+)-K+ ATPase. Inhibition of this enzyme results in intracellular Mg++ deficiency which can influence cholesterol metabolism. Digoxin also influences transport of tryptophan and tyrosine which are precursors of various neurotransmitters. Alterations in digoxin, membrane Na(+)-K+ ATPase and also in neurotransmitters have been reported in the disorders mentioned above. In view of this, serum lipid profile, activity of plasma HMG CoA reductase (the major rate limiting step in the isoprenoid pathway), RBC membrane Na(+)-K+ ATPase activity, serum Mg++ concentration, concentration of digoxin and concentration of serum neurotransmitters were studied in some neuropsychiatric disorders. The serum serotonin level was increased while that of serum dopamine and noradrenaline was reduced. Serum digoxin levels were high and RBC membrane sodium-potasium ATPase activity and serum magnesium were reduced. There was a reduction in HDL cholesterol and increase in plasma triglycerides (pattern similar to insulin resistance and syndrome X) in most of the disorders studied. The HMG CoA reductase activity was high, the serum total cholesterol was increased while RBC membrane cholesterol was reduced in most of the cases. The significance of increased digoxin with consequent inhibition of membrane Na(+)-K+ ATPase in relation to changes in cholesterol metabolism and insulin resistance type of dyslipidemia is discussed in this paper.     

Erythrosin B inhibits high affinity ouabain binding in guinea-pig heart Na+-K+-ATPase without influence on cardiac glycoside induced contractility.

Binding of [3H]-ouabain to guinea-pig heart membranes enriched in Na+-K+-ATPase revealed two different cardiac glycoside binding sites. High affinity binding was obtained at a KD = 2.2 X 10(-7) mol 1(-1) (Bmax = 16.8 pmol ouabain mg-1 protein) whereas low affinity ouabain binding occurred at a KD much greater than 10(-6) mol 1(-1). To discover whether the two ouabain binding sites are functional in guinea-pig heart muscle, erythrosin B, an inhibitor of the high affinity ouabain binding in rat brain tissue, was tested in guinea-pig isolated heart muscle preparations. Erythrosin B proved to be a potent inhibitor of the Mg2+ (Na+)-dependent-, as well as Na+-K+-activated ATPase (ID50 = 9 X 10(-6) mol 1(-1). Contractility of guinea-pig isolated papillary muscles, however, was not influenced by erythrosin B in concentrations up to 1 X 10(-5) mol 1(-1). Only very high concentrations (4 X 10(-4) mol 1(-1) resulted in a slightly negative inotropic effect (about 20%). Erythrosin B dose-dependently inhibited [3H]-ouabain binding to the Na+-K+-ATPase (KD = - 3.6 X 10(-6) mol 1(-1). In a concentration of 1 X 10(-5) mol 1(-1) the dye abolish high affinity [3H]-ouabain binding without affecting the low affinity binding sites. In contrast, in guinea-pig isolated atria, no functional antagonism between erythrosin B (5 X 10(-5) mol 1(-1) and ouabain was observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vitamin D-induced myocardial lesions and the protection by carbocromen

Effects of high doses of vitamin D on rat hearts were investigated 72 h after the administration of 500 000 U/kg. Histological examination showed disseminated focal necrosis with reactive round cell and granulocytic infiltration in the heart. The calcium content was increased by about 70%. These findings indicate vitamin D-induced myocardial lesions. Heart mitochondrial calcium binding and uptake activities were lower than in control animals. Na+-K+-ATPase activity of heart washed particles was reduced by the treatment with vitamin D whereas neither calcium binding and uptake activities of cardiac sarcoplasmic reticulum nor myofibrillar ATPase activity were affected. Carbocromen (20 and 100 mg/kg/d) treatment reduced vitamin D-induced decreases in mitochondrial calcium accumulating ability and Na+-K+-ATPase activity of heart washed particle, suggesting a beneficial effect of carbocromen against vitamin D-induced cardiac injury.     

Sodium fluoride produces a K+ efflux by increasing intracellular Ca2+ through Na+-Ca2+ exchange

Acute fluoride intoxication increases intracellular calcium (Cai), manifested by increased twitch tension in cardiac muscle, and by potassium efflux (mediated by Ca2+-dependent K+ channels) in fluoridated erythrocytes. Fluoride, like isoproterenol, stimulates adenylate cyclase, and could increase Cai via the effects of cAMP on Ca2+ channels. However, while the inotropic effects of fluoride mimicked isoproterenol in rat atria, their effects on the time course of isometric contraction were quite different. In addition, acetylcholine negated isoproterenol's effect on twitch tension but did not modulate the effects of fluoride. Further, the Ca2+ channel antagonist verapamil had no effect on fluoride-stimulated K+ efflux from erythrocytes. Fluoride also inhibits Na+-K+ ATPase, and increases intracellular Na+, so could increase Cai via Na+-Ca2+ exchange. Lanthanum, which blocks Na+-Ca2+ exchange, blocks fluoride-induced K+ efflux in erythrocytes. We conclude that the effects of fluoride on adenylate cyclase are not important in intact tissue, and that inhibition of Na+-K+ ATPase and subsequent Na2+-Ca2+ exchange may be the mechanism of increased Cai in acute fluoride toxicity.     

Effect of interaction of heavy metals on (Na+ -K+) ATPase and the uptake of 3H-DA and 3H-NA in rat brain synaptosomes

The effect of interaction of Mn2+, Pb2+ and Cd2+ on (Na+ -K+) ATPase and uptake of labelled dopamine (3H-DA) and labelled noradrenaline (3H-NA) were studied in vitro in rat brain synaptosomes. The inhibition of (Na+ -K+) ATPase by Pb2+ and Cd2+ alone was concentration dependent, however, Mn2+ had almost no effect on the activity of this enzyme. Interaction of Cd2+ with either Pb2+ or Mn2+ was most powerful in inhibiting the activity of synaptosomal transport ATPase. Lower concentrations of Pb2+ increased while higher concentrations inhibited synaptosomal uptake of 3H-DA and 3H-NA. Lower concentrations of Cd2+ increased the uptake of 3H-DA while at concentrations of 100 microM, the uptake was inhibited, this metal had strong inhibitory effect on the uptake of 3H-NA. Mn2+ had inhibited the uptake of labelled amines. Interaction of Mn2+ with Pb2+ or Cd2+ produced inhibition on the uptake of 3H-DA and 3H-NA. The results of the uptake of biogenic amines in the presence of metal ions apparently had no correlation with the activity of (Na+ -K+) ATPase which is involved in the active transport of cations across cell membranes.     

内洋地黄素拮抗剂对大鼠心肌缺血再灌注损伤的保护作用

目的　结扎大鼠左冠状动脉前降支造成心肌缺血再灌注损伤 (MIR) ,观察MIR时心肌组织内洋地黄素水平的变化和内洋地黄素特异性拮抗剂地高辛抗血清对MIR时心肌的保护作用。方法　采用左冠状动脉前降支结扎 30min ,复灌4 5min建立在体大鼠MIR模型。SpraugeDawley大鼠随机分成 7组 ,每组 10只。假手术组 ,缺血再灌注模型组 ,生理盐水组 ,维拉帕米组 ,小剂量、中剂量、大剂量地高辛抗血清组。各组于再灌注 4 5min后立即取左室心尖部缺血区心肌 ,检测心肌匀浆中内洋地黄素含量、心肌细胞膜Na+ ,K+ ATP酶活性和线粒体内Ca2 + 含量。光镜及电镜下观察心肌组织形态学变化。结果　MIR时心肌组织内洋地黄素水平明显升高 ,细胞膜Na+ ,K+ ATP酶活性明显下降 ,线粒体内Ca2 + 水平升高 ,心肌组织结构发生明显损伤。中、大剂量地高辛抗血清能降低心肌组织内洋地黄素水平 ,恢复心肌细胞膜Na+ ,K+ ATP酶活性 ,降低线粒体内Ca2 + 水平 ,减轻MIR导致的心肌组织结构的损伤。结论　内洋地黄素拮抗剂地高辛抗血清对MIR大鼠心肌有明显的保护作用 ,其作用机制可能通过拮抗内洋地黄素 ,恢复心肌细胞膜Na+ ,K+ ATP酶活性 ,减轻细胞内Ca2 + 超载。     

Stimulation of platelet serotonin transport by substituted 1,4-naphthoquinone-induced oxidant stress.

The effect of oxidant stress produced by redox cycling of substituted 1,4-naphthoquinones on the activity of platelet (Na(+)-K+)ATPase and the active transport of serotonin (5-HT) was studied. 2-Methyl-1,4-naphthoquinone (menadione) produced a concentration-dependent (0-100 microM) and time-dependent (2-20 min) stimulation of platelet 5-HT transport. Exogenous superoxide dismutase (250 units) and/or catalase (500 units) failed to block the stimulation. Fluoxetine, an inhibitor of the platelet 5-HT transporter, blocked menadione-induced stimulation of 5-HT uptake as did ouabain, an inhibitor of platelet (Na(+)-K+)ATPase. The structure-activity relationship of select 1,4-naphthoquinones suggested that stimulation was due to redox cycling and not arylation. The kinetics of 5-HT transport revealed that menadione markedly increased the maximal rate of 5-HT transport (Vmax control = 20.6 +/- 2.0 pmol/10(8) platelets/4 min vs Vmax menadione = 46.4 +/- 3.9 pmol/10(8) platelets/4 min) but did not significantly alter the Km values. The activity of (Na(+)-K+)ATPase was determined by measuring the uptake of 86Rb+ into intact platelets. Menadione produced a concentration-dependent and time-dependent stimulation of platelet 86Rb+ uptake. These changes in platelet (Na(+)-K+)ATPase activity paralleled the changes observed in 5-HT transport and were inhibited in a concentration-dependent manner by ouabain. The data have shown that the redox cycling of 1,4-naphthoquinones caused an increase in (Na(+)-K+)ATPase activity that resulted in the stimulation of the rate of platelet 5-HT transport.     

甲基葡糖苷对豚鼠离体心房的正性肌力作用

目的　观察甲基葡糖苷对豚鼠离体心房的正性肌力作用 ,并探讨其作用机制。方法　采用豚鼠离体左右心房 ,测定药物对心房肌收缩力、右心房心率 ,以及对静息后收缩和正阶梯现象的影响 ,并测定大鼠心肌细胞膜Na+ K+ ATP酶活性。结果　甲基葡糖苷显著增强心房肌收缩力 ,减慢右心房心率 ,且呈剂量依赖性 ;能明显增强左心房静息后收缩和正阶梯现象 ,并能显著抑制大鼠心肌细胞膜Na+ K+ ATP酶活性。结论　甲基葡糖苷具有加强心房肌收缩力 ,降低右心房心率的作用 ,其正性变力作用可能与抑制心肌细胞膜Na+ K+ ATP酶活性 ,促进心肌细胞外钙内流和内钙释放有关     

Effects of ouabain on 86Rb-uptake, 3H-5-HT-uptake and aggregation by 5-HT and ADP in human platelets

In the search of sensitive models for actions of digitalis-like substances on intact cells or tissues, the effects of ouabain on human platelets were investigated. In a concentration-dependent manner ouabain 10(-8)-10(-4) M inhibited Na+-K+-ATPase activity measured as uptake of 86Rubidium (86Rb), with about 90% inhibition of the total uptake at ouabain greater than or equal to 10(-6) M. An almost identical concentration-effect curve was found for platelet uptake of 3H-serotonin (3H-5-HT). The platelet shape change reaction to exogenous 5-HT (1 X 10(-6) M) was suppressed by ouabain (10(-8)-10(-4) M) in a concentration-dependent manner, but with no clear maximum effect within the range tested. Aggregation induced by adenosine-di-phosphate (ADP 2 X 10(-6) M) was enhanced by ouabain 10(-8)-10(-6) M. At the highest concentration tested the rate of aggregation was increased by 31% and the change in light transmission by 54%. At low concentrations (less than 10(-9) M) of ouabain, there was a tendency towards increased aggregation as well as increased uptake of 86Rb, which may be a parallel to observations of positive inotropic effects of low concentration of glycosides, which do not inhibit Na+-K+-ATPase. The results show that human platelets can be used as a model tissue for studying effects of cardiac glycosides. This suggests that it may be useful for further investigations of the biological effects of agents with a similar effect profile, e.g. endogenous digitalis-like substances.     

Detection of endogenous lithium in neuropsychiatric disorders--a model for biological transmutation

The human hypothalamus produces an endogenous membrane Na(+)-K(+) ATPase inhibitor, digoxin. A digoxin induced model of cellular/neuronal quantal state and perception has been described by the authors. Biological transmutation has been described in microbial systems in the quantal state. The study focuses on the plasma levels of digoxin, RBC membrane Na(+)-K(+) ATPase activity, plasma levels of magnesium and lithium in neuropsychiatric and systemic disorders. Inhibition of RBC membrane Na(+)-K(+) ATPase activity was observed in most cases along with an increase in the levels of serum digoxin and lithium and a decrease in the level of serum Mg(++). The generation of endogenous lithium would obviously occur due to biological transmutation from magnesium. Digoxin and lithium together can produce added membrane Na(+)-K(+) ATPase inhibition. The role of membrane Na(+)-K(+) ATPase inhibition in the pathogenesis of neuropsychiatric and systemic disorders is discussed. The inhibition of membrane Na(+)-K(+) ATPase can contribute to an increase in intracellular calcium and a decrease in magnesium, which can result in a defective neurotransmitter transport mechanism, mitochondrial dysfunction and apoptosis, defective golgi body function and protein processing dysfunction, immune dysfunction and oncogenesis.     

Effect of diphenolic laxatives on Na+-K+-activated ATPase and cyclic nucleotide content of rat colon mucosa in vivo

1. The effect of bisacodyl and oxyphenisation on the Na+-K+- and Mg2+-activated ATPase and on the mucosa levels of cAMP and cGMP was investigated in transporting ligated loops of the rat colon in acute studies and in chronic feeding experiments 2. The specific activity of the Na+-K+-ATPase was lowered in both types of experiments, concomitantly with a reduction in net sodium absorption. The specific activity of the Mg2+-activated ATPase was unaffected. 3. The cAMP content per mg protein was elevated and the cGMP content decreased in the acute experiments in which the effect on transport was most marked. The content of cyclic nucleotides returned to normal within 2 h whereas absorption, Na+-K+-ATPase specific activity and the mucosal potential difference were still significantly depressed at that time. In chronic experiments with bisacodyl, cAMP was not affected and cGMP was increased in colon loops exhibiting reduced absorption. 4. The results indicate that the inhibition of the Na+-K+-activated ATPase by diphenolic laxatives may play a role in the inhibition of intestinal fluid absorption caused by these compounds. The increase of cAMP in acute experiments could point to a cAMP-mediated stimulation of secretory processes under this condition.     

Differential effect of potassium on the action of digoxin and digoxigenin in guinea-pig heart.

The effect of potassium on the binding of digoxin or digoxigenin to isolated Na+, K+-ATPase was compared with that of potassium on the positive inotropic action of the agents in guinea-pig hearts. The binding of digoxigenin to the enzyme in vitro was reduced to a greater extent by potassium than was the binding of digotoxin. The digoxigenin-induced increase in the force of contraction of left atrial preparations estimated at steady state was reduced at higher potassium concentrations. Potassium had a lesser effect when digoxin was used as the inotropic agent. In contrast, potassium concentrations. Potassium had a lesser effect when digoxin was used as the ininotropic agent. In contrast, potassium reduced the rate of development and also the rate of loss of the positive inotropic action of digoxin observed with left atrial and Langendorff preparations, respectively, to a greater extent than those of digoxigenin. The loss of the positive inotropic effect was more rapid with digoxigenin than with digoxin at each KCl concentration. These data support the contention that the extent of the interaction of digitalis with Na+,K+-ATPase determines the degree of the positive inotropic effect.     

Inhibition of electrogenic sodium transport across toad urinary bladder by the mycotoxin patulin.

The effects of the mycotoxin patulin (4-hydroxy-4H-furo[3,2c]pyran-2(6H)-one) on short circuited intact toad bladder and on Na+-K+, activated ATPase were examined in an attempt to elucidate the relationship between toxin, the Na+-K+ ATPase enzyme system and associated active sodium transport. Patulin inhibited transbladder short circuit current and Na+-K+ ATPase from isolated bladder preprations. The effect was exponentially dependent on time. A significantly slower rate of inhibition was achieved within 15-30 min. The results are compatible with the assumption that Na+-K+ ATPase is associated with the pump mechanism since patulin inhibited enzyme activity and concomitantly reduced the rate of electrogenic Na+ transport. A significant correlation suggested a cause-effect relationship.     

Effects of canrenone on aorta and right ventricle of the rat

Canrenone is a major active metabolite of spironolactone and, in addition to the antimineralocorticoid effect, shares with the parent compound the action as a partial agonist with respect to ouabain on the Na+-K+ ATPase. We have investigated whether canrenone, through its action on Na+-K+ ATPase, reverses rat aorta contractions induced by ouabain and has vasorelaxant properties unrelated to its interaction with ouabain. Contractile responses of endothelium-deprived aorta to 1 mM ouabain, 0.1 microM phenylephrine, 10 microM serotonin, and 60 mM K+ were relaxed by canrenone (50-250 microM), with maximum inhibitions of 85.3%, 55.3%, 56.7%, and 64.2%, respectively. Canrenone shifted to the right the concentration-response curve for Ca2+ in depolarized aorta and did not affect the response to 10 mM caffeine. In rat right ventricular strips driven at 0.1 Hz, canrenone exerted negative inotropic effect. The relaxation of ouabain-induced contraction may be due, at least in part, to an interaction between canrenone and ouabain on the Na+-K+ ATPase. Inhibition of calcium entry through calcium channels either in aorta or ventricles is the most parsimonious hypothesis of mechanism underlying the effect of canrenone on contractile responses of rat aorta to agonists and high K+ and the negative inotropic effect on ventricular strips.     

顺铂对结直肠癌细胞增殖及侵袭能力的影响

【摘要】目的 探讨顺铂对人结直肠癌细胞SW480增殖及侵袭力的影响。方法 以SW480细胞为研究对象,设定未经处理的SW480细胞为对照组,给予不同浓度顺铂进行不同时间干预,应用MTT法、Transwell侵袭小室、定磷法检测SW480细胞的增殖、侵袭力及Na + -K + -ATP酶活性。结果 生理浓度顺铂（70μmol/L）在48h即可抑制SW480 细胞的增殖,与72和96h相比抑制率差异无统计学意义;70μmol/L顺铂处理48h时,即可降低穿越Matrigel膜基质的细胞数;分别采用17.5、35、70和140µmol/L4个梯度浓度的顺铂作用于SW480细胞48h后,35、70和140µmol/L组细胞中Na + -K + -ATP酶活性均显著升高,且顺铂浓度达70µmol/L时Na + -K + -ATP酶即可达到较高活性。结论 Na + - K + -ATP酶活性的降低可能导致对结直肠癌细胞增殖和侵袭能力调节作用减弱,可能与结直肠癌细胞SW480耐顺铂有关。     

The early and late effects of digoxin treatment on the sodium transport, sodium content and Na+K+- ATPase or erythrocytes.

1 Erythrocyte sodium content, sodium transport (ouabain sensitive sodium flux Eos, and ouabain sensitive efflux rate constant ERCos) sodium, potassium activated ouabain sensitive adenosine triphosphatase (Na+K+ATPase) and plasma digoxin were measured in patients during acute digitalisation and in patients who were on long-term digoxin treatment. 2 In the six patients who were studied during digitalisation, the ERCos and Na+K+ATPase activity decreased and erythrocyte sodium content increased during days 2-4 treatment, but there was no change in Eos. 3 In 39 patients on long term digoxin therapy (2-119 months) the erythrocyte sodium content was normal, but the erythrocyte Na+K+ATPase activity was higher than the control group. When the results from these 39 patients were divided according to the duration of treatment it was found that the erythrocyte sodium content was higher in patients treated for 2-4 months than in patients treated for longer periods and the erythrocyte Na+K+ATPase activity increased with duration of treatment. In eight patients (duration of treatment greater than 29 months) in whom ERCos and Eos were measured, ERCos and Eos were higher than the control group. 4 The results suggest that the effects of digoxin on erythrocytes which occur during acute digoxin treatment do not persist in the long term. 5 The possible explanation for the higher ERCos, Eos and Na+K+ATPase activity in patients treated with digoxin for more than 2 months is discussed.     

The mode of inhibition of the Na+-K+ pump activity in mast cells by calcium.

1 The inhibition by calcium of the Na(+)-K+ pump in the plasma membrane of rat peritoneal mast cells was studied in pure populations of the cells by measuring the ouabain-sensitive uptake of the radioactive potassium analogue, 86rubidium (86Rb+). 2 Exposure of the cells to calcium induced a time- and concentration-dependent decrease in the ouabain-sensitive K+(86Rb+)-uptake of the cells without influencing the ouabain-resistant uptake. The development of the inhibition required the presence of potassium in the medium in the millimolar range (1.5-8.0 mM), and it did not occur at a concentration of potassium (0.24 mM) that is probably rate limiting for the pump activity. In the presence of 1 mM calcium full inhibition developed almost immediately and was not readily reversed. The inhibition was not significantly reduced by 15 min incubation with 1.2 mM EGTA. 3 The inhibitory action of calcium did not develop when the mast cells were incubated in a potassium-free medium, which is known to block Na(+)-K+ pump activity and allow accumulation of sodium inside the cells. Likewise, increasing the sodium permeability of the plasma membrane by monensin abolished the inhibition of the pump activity. In both cases, incubation of the cells with 4.7 mM potassium and tracer amounts of 86Rb+ resulted in a very large uptake of K+ (86Rb+) into the cells (up to 2 nmol per 10(6) cells min-1), indicating a high activity of the Na(+)-K+ pump.(ABSTRACT TRUNCATED AT 250 WORDS)