Inhibition of [3H]norepinephrine uptake in peripheral organs of some mammalian species by ouabain

Infarction-induced cytokines cause local depletion of tyrosine hydroxylase in cardiac sympathetic nerves

Myocardial infarction causes a heterogeneity of noradrenergic transmission that contributes to the development of ventricular arrhythmias and sudden cardiac death. Ischaemia-induced alterations in sympathetic transmission include regional variations in cardiac noradrenaline (NA) and in tyrosine hydroxylase, the rate-limiting enzyme in NA synthesis. Inflammatory cytokines that act through gp130 are elevated in the heart after myocardial infarction. These cytokines decrease expression of tyrosine hydroxylase in sympathetic neurons, and indirect evidence suggests that they contribute to the local depletion of tyrosine hydroxylase in the damaged left ventricle. However, gp130 cytokines are also important for the survival of cardiac myocytes following damage to the heart. To examine the effect of cytokines on tyrosine hydroxylase and NA content in cardiac nerves we used gp130(DBH-Cre/lox) mice, which have a deletion of the gp130 receptor in neurons expressing dopamine beta-hydroxylase. The absence of neuronal gp130 prevented the loss of tyrosine hydroxylase in cardiac sympathetic nerves innervating the left ventricle 1 week after ischaemia-reperfusion compared with wild-type C57BL/6J mice. Surprisingly, restoration of tyrosine hydroxylase in the damaged ventricle did not return neuronal NA content to normal levels. Noradrenaline uptake into cardiac nerves was significantly lower in gp130 knockout mice, contributing to the lack of neuronal NA stores. There were no significant differences in left ventricular peak systolic pressure, dP/dt(max) or dP/dt(min) between the two genotypes after myocardial infarction, but ganglionic blockade revealed differences in autonomic tone between the genotypes. Stimulation of the heart with dobutamine or release of endogenous NA with tyramine generated similar responses in both genotypes. Thus, the removal of gp130 from sympathetic neurons prevents the post-infarct depletion of tyrosine hydroxylase in the left ventricle, but does not alter NA content or cardiac function.

Differential pre- and postsynaptic effects of desipramine on cardiac sympathetic nerve terminals in RHF

Right heart failure (RHF) is characterized by chamber-specific reductions of myocardial norepinephrine (NE) reuptake, beta-receptor density, and profiles of cardiac sympathetic nerve ending neurotransmitters. To study the functional linkage between NE uptake and the pre- and postsynaptic changes, we administered desipramine (225 mg/day), a NE uptake inhibitor, to dogs with RHF produced by tricuspid avulsion and progressive pulmonary constriction or sham-operated dogs for 6 wk. Animals receiving no desipramine were studied as controls. We measured myocardial NE uptake activity using [(3)H]NE, beta-receptor density by [(125)I]iodocyanopindolol, inotropic responses to dobutamine, and noradrenergic terminal neurotransmitter profiles by glyoxylic acid-induced histofluorescence for catecholamines, and immunocytochemical staining for tyrosine hydroxylase and neuropeptide Y. Desipramine decreased myocardial NE uptake activity and had no effect on the resting hemodynamics in both RHF and sham animals but decreased myocardial beta-adrenoceptor density and beta-adrenergic inotropic responses in both ventricles of the RHF animals. However, desipramine treatment prevented the reduction of sympathetic neurotransmitter profiles in the failing heart. Our results indicate that NE uptake inhibition facilitates the reduction of myocardial beta-adrenoceptor density and beta-adrenergic subsensitivity in RHF, probably by increasing interstitial NE concentrations, but protects the cardiac noradrenergic nerve endings from damage, probably via blockade of NE-derived neurotoxic metabolites into the nerve endings.

Effect of dietary cardiac glycosides on blood pressure regulation in rats

To investigate the possible physiological significance of dietary cardiac glycosides in blood pressure regulation, the blood pressure of normal Sprague Dawley rats raised on a regular diet, which naturally contains large amounts of Na+-pump inhibitors, was compared with that of rats on a purified synthetic diet, which contains no Na+-pump specific inhibitors, and with that of rats on a synthetic diet supplemented with 10 µg·mL-1ouabain or 10 µg·mL-1convallatoxin in the drinking water. After 6 weeks on the synthetic diet, the systolic blood pressure in the synthetic diet group was significantly elevated (145 ± 5 vs. 128 ± 4 mmHg, P < 0.05). At 10 weeks it reached a plateau (154 ± 3 vs. 122 ± 3 mmHg, P < 0.05). Plasma renin activity and Na+level were significantly higher in animals fed synthetic diets than in the regular diet group (P < 0.01). Administration of either losartan or lisinopril or a switch to a low salt synthetic diet (0.03% sodium) normalized the synthetic diet-induced high blood pressure. Supplementation of the synthetic diet with the cardiac glycosides delayed the onset of the increase in blood pressure for 4 weeks. Plasma aldosterone levels were approximately doubled in the cardiac glycoside-treated groups. Higher plasma Na+levels and hematocrit values present in the synthetic diet group were normalized by the glycoside supplements. These results suggest that supplemental dietary cardiac glycosides exert bidirectional effects on blood pressure regulation through actions that modulate extracellular fluid and electrolyte balance.Key words: cardiac glycosides, convallatoxin, ouabain, ouabain-like substance, purified synthetic diet, high blood pressure, renin-angiotensin system.

Transmural heterogeneity of norepinephrine uptake in failing human hearts

OBJECTIVES

The aim of this study was to determine whether regional heterogeneity in myocardial sympathetic neural function measured by the uptake of norepinephrine could account for the spatial heterogeneity of beta-adrenergic receptor down-regulation that occurs in the failing human heart.

BACKGROUND

Myocardial beta-adrenergic receptor density and function are diminished in patients with chronic heart failure. Down-regulation occurs predominantly in the subendocardium, suggesting that local rather than systemic alterations in sympathetic neural function may be responsible. Although some studies have implicated hypofunction of cardiac sympathetic nerves with defective norepinephrine uptake, others suggest increased cardiac sympathetic nerve activity with unimpaired uptake.

METHODS

We measured norepinephrine uptake by incubating transmural slices of the left ventricle from 19 patients who had chronic heart failure and three nonfailing control hearts with [3H]norepinephrine with or without desipramine, a neuronal uptake blocker. The density of uptake sites was measured in subepicardial and subendocardial myocyte regions with light microscopic autoradiography.

RESULTS

Although the amount of [3H]norepinephrine uptake varied considerably in failing ventricles, uptake was directly proportional (r = 0.46, p < 0.05) to beta 1-adrenergic receptor density measured in additional slices with radioligand binding assays. In addition, marked transmural heterogeneity in [3H] norepinephrine uptake was consistently observed in failing ventricles. Uptake in subendocardial myocyte regions was significantly less than in subepicardial regions (mean [ +/- SD] subepicardial/subendocardial uptake ratio 4.7 +/- 4.8, p < 0.01). The extent of transmural heterogeneity in norepinephrine uptake was similar in patients with idiopathic and ischemic cardiomyopathy. In contrast, nonfailing hearts exhibited more uniform transmural [3H]norepinephrine uptake (subepicardial/subendocardial uptake ratio 1.8 +/- 1.2, p = NS).

CONCLUSIONS

Specific [3H]norepinephrine accumulation is approximately fivefold lower in subendocardial regions of failing left ventricles than in subepicardial regions. These findings support the hypothesis that a subendocardial defect in norepinephrine uptake may chronically elevate local interstitial catecholamine levels and thereby down-regulate beta-adrenergic receptors in a spatially heterogeneous distribution.

Cardiac sympathetic nervous activity in congestive heart failure. Evidence for increased neuronal norepinephrine release and preserved neuronal uptake

BACKGROUND

Increased concentrations of norepinephrine in coronary sinus plasma reported in congestive heart failure (CHF) could result from increased cardiac sympathetic nerve firing and norepinephrine release or from failure of neuronal uptake mechanisms to recapture released norepinephrine. We have applied neurochemical indexes of cardiac sympathetic nerve function in heart failure patients to delineate the underlying neural pathophysiology.

METHODS AND RESULTS

Cardiac norepinephrine synthesis, assessed from the cardiac overflow of the norepinephrine precursor dihydroxyphenylalanine (DOPA), intraneuronal metabolism estimated from the overflow of the intraneuronal metabolite dihydroxyphenylglycol (DHPG), neuronal norepinephrine reuptake assessed from the fractional extraction of plasma-tritiated norepinephrine and production of tritiated DHPG across the heart, and norepinephrine spillover to plasma were examined in eight patients with CHF caused by coronary artery disease (left ventricular ejection fraction of 26 +/- 5%, mean +/- SEM) and 14 age-matched healthy subjects. Cardiac norepinephrine spillover was increased eightfold in CHF subjects (127 ng/min versus 14 ng/min in healthy subjects; standard error of the difference [SED], 8 ng/min; P < .002), and cardiac DOPA was increased twofold (P < .02). The fractional extraction of tritiated norepinephrine across the heart was marginally less in CHF subjects (0.63 versus 0.73 in normal subjects; SED, 0.02), but the extent to which pharmacological neuronal uptake blockade with desipramine reduced the cardiac extraction of tritiated norepinephrine (by 71% versus 73% in normal subjects) and reduced the production of tritiated DHPG derived from uptake and intraneuronal metabolism of tritiated norepinephrine was similar in CHF patients and healthy subjects.

CONCLUSIONS

The marked increase in norepinephrine spillover from the heart in CHF attributable to coronary artery disease results primarily from an increase in sympathetic nerve firing and neuronal release of norepinephrine, not from faulty neuronal reuptake of norepinephrine.

Effects of bufalin on norepinephrine turnover in canine saphenous vein

Abundant experimental data suggest that an endogenous digitalislike factor is responsible for some essential hypertension. Some forms of hypertension have also been associated with increased levels of catecholamines. We therefore designed experiments to investigate the role of digitalislike factors in the regulation of norepinephrine turnover in the neurovascular junction. We chose bufalin, an amphibian-derived compound that shares many of the physiological properties postulated as characteristic of digitalislike compounds, as a model of the mammalian compound. In vitro experiments in canine saphenous veins showed that, in addition to inhibiting norepinephrine uptake, bufalin increased norepinephrine overflow by an amount larger than could be explained solely by uptake inhibition. The effect of bufalin on norepinephrine overflow is inhibited by tetrodotoxin, which suggests a dependence of this response on Na+ influx through the neuronal membranes. We propose that Na+,K(+)-ATPase inhibition resulting in neuronal depolarization is responsible for the augmented norepinephrine turnover caused by bufalin and that these indirect effects of norepinephrine on the cardiovascular system may play a role in the etiology of hypertension.

Effect of digitalis glycosides on norepinephrine release in the heart. Dual mechanism of action

The effect of ouabain on exocytotic and nonexocytotic norepinephrine release was investigated in perfused rat and guinea pig hearts. The overflow of endogenous norepinephrine and its neuronal metabolite 3,4-dihydroxyphenylethyleneglycol (DOPEG) was determined by high-pressure liquid chromatography. DOPEG served as the indicator of free axoplasmic norepinephrine concentrations. The overflow of the norepinephrine cotransmitter neuropeptide Y (NPY) was determined by radioimmunoassay and NPY was used as marker for exocytotic release. Electrical stimulation of the left stellate ganglion resulted in exocytotic norepinephrine release in rat and guinea pig hearts. Ouabain caused an increase in stimulation-induced norepinephrine overflow from rat and guinea pig hearts by 40%. However, overflow of NPY was decreased by 40%, indicating a reduced exocytosis rate. Ouabain increased both norepinephrine and NPY overflow, suggesting enhancement of exocytosis, when neuronal catecholamine uptake (uptake1) was blocked by desipramine or when presynaptic alpha 2-adrenoceptors were inhibited by yohimbine. The results demonstrate an interaction of ouabain with both calcium-dependent exocytosis and uptake1 of norepinephrine. Under calcium-free conditions, ouabain or potassium-free perfusate resulted in norepinephrine release from hearts when the axoplasmic norepinephrine concentration was elevated by the reserpinelike agent Ro 4-1284. This release was independent from neural activity, not accompanied by NPY overflow, and suppressed by the uptake1 blocker desipramine. These findings are in keeping with carrier-mediated nonexocytotic norepinephrine release that is caused by reversal of the transport direction of the uptake1 carrier. During myocardial ischemia nonexocytotic norepinephrine release was accelerated and enhanced by inhibition of Na+,K(+)-ATPase before ischemia. This study demonstrates the potential of digitalis glycosides to interact both with transmitter exocytosis and with the neuronal catecholamine transport system by Na+,K(+)-ATPase inhibition. Interaction with the catecholamine transport system involves both inhibition of norepinephrine inward transport and induction of norepinephrine outward transport, resulting in nonexocytotic norepinephrine release.

Decreased adrenergic neuronal uptake activity in experimental right heart failure. A chamber-specific contributor to beta-adrenoceptor downregulation

The reduction of myocardial beta-adrenoceptor density in congestive heart failure has been thought to be caused by agonist-induced homologous desensitization. However, recent evidence suggests that excessive adrenergic stimulation may not produce myocardial beta-receptor downregulation unless there is an additional defect in the local norepinephrine (NE) uptake mechanism. To investigate the association between beta-adrenoceptor regulation and NE uptake activity, we carried out studies in 30 dogs with right heart failure (RHF) produced by tricuspid avulsion and progressive pulmonary artery constriction and 23 sham-operated control dogs. We determined NE uptake activity by measuring accumulation of [3H]NE in tissue slices, NE uptake-1 carrier density by [3H]mazindol binding and beta-adrenoceptor density by [3H]dihydroalprenolol binding. Compared with sham-operated dogs, RHF dogs showed a 26% decrease in beta-adrenoceptor density, a 51% reduction in NE uptake activity, and a 57% decrease in NE uptake-1 carrier density in their right ventricles. In addition, right ventricle beta-receptor density correlated significantly with NE uptake activity and NE uptake-1 carrier density. In contrast, neither NE uptake activity nor beta-receptor density in the left ventricle and renal cortex was affected by RHF. Thus, the failing myocardium is associated with an organ- and chamber-specific subnormal neuronal NE uptake. This chamber-specific loss of NE uptake-1 carrier could effectively reduce local NE clearance, and represent a local factor that predisposes the failing ventricle to beta-adrenoceptor downregulation.

Hemodynamic effects of bufalin in the anesthetized dog

Studies in Lichstein's laboratory suggest that the endogenous digitalislike substance implicated in low renin hypertension might be a steroidal dienolide derivative. If this is true, the bufadienolides should block potassium vasodilation and enhance norepinephrine vasoconstriction, constrict blood vessels, raise blood pressure, and produce natriuresis and diuresis. We have therefore examined these parameters while infusing bufalin (aglycone), a bufadienolide, intrabrachially and intravenously in the anesthetized dog. Intrabrachial infusion of 5-25 micrograms/min with brachial arterial blood flow held constant at 100 ml/min produced a dose-dependent increase in perfusion pressure with rapid onset and offset, a progressive decrease in the vasodilator response to intrabrachial injection of 1 ml iso-osmotic potassium chloride solution (but not to acetylcholine), and an increase in the vasoconstrictor response to intrabrachial injection of 0.1 microgram norepinephrine. Intravenous infusion at 5-50 micrograms/min produced a dose-dependent increase in systemic arterial blood pressure, rate of change of ventricular pressure (dP/dt), and after the highest dose, cardiac irregularities. Natriuresis and diuresis were not observed. Thus, bufalin does in fact have some of the physiological properties required to be considered a candidate for the digitalislike substance found in low renin hypertension.

Catecholamines and their metabolites

The research on biosynthesis, physiology, pharmacology, regulation and degradation of catecholamines has continuously increased for more than 50 years. This is not unexpected because of the fact that catecholamines are involved in so many life processes such as nerve conduction, blood circulation and hormone regulations in health and disease. This demands that methods for their determination should be improved, and in fact during the years a number of analytical methods have been published. About 20 years ago radioenzyme techniques with thin-layer chromatographic (TLC) separation of radiolabelled catecholamine derivatives were developed which greatly contributed to our knowledge of physiological concentrations of catecholamines in biological media, particularly in plasma and brain. Radioimmune methods were successful for analysis of a number of analytes, but for catecholamines radioimmunoassays developed slowly. We believe that the greatest potential for radioimmunochemical methods lies in their ability to localize catecholamines and metabolites at the cellular and subcellular levels. With the advent of gas chromatographic-mass spectrometric (GC-MS) and high-performance liquid chromatographic (HPLC) procedures analysis of catecholamines improved greatly., The equipment for GC-MS is expensive and requires technical skillfulness, but in experienced hands a lot of new biological data have emerged. An outstanding quality with GC-MS is that the method offers the ability to identify unknown compounds and is relatively free from interferences from extraneous compounds. In comparison with GC-MS, HPLC is versatile and has gained a widespread use. Applications for research in the catecholamine field are numerous. In general, the sensitivity and specificity are satisfactory with HPLC, but it should be borne in mind that a number of pitfalls can obscure the results. This involves both sample handling, clean-up and chromatographic procedures. At present, HPLC is the most expanding field in chromatographic determination of catecholamines and their metabolites. This is particularly the case for HPLC with electrochemical detection which has revolutionized our analytical potential in this field. These chromatographic procedures continue to develop. The prerequisites for further improved methods such as capillary zone electrophoresis and combined HPLC-MS are at hand and hopefully will soon come into more general use for analysis of catecholamines in biological samples.

In vivo measurement of neuronal uptake of norepinephrine in the human heart

Neuronal uptake (Uptake-1) of the sympathetic neurotransmitter norepinephrine from the circulation in the human heart was assessed in vivo with three techniques. 1) Cardiac removal of intravenously infused tracer-labeled norepinephrine was measured before and after Uptake-1 blockade with desipramine; 2) the difference between the fractional extraction of radioactive norepinephrine and of radioactive isoproterenol, which is not a substrate for neuronal uptake, was used to estimate the removal of norepinephrine by Uptake-1 in the heart compared with other vascular beds (arm, leg, brain, and lungs); and 3) regional arteriovenous differences in radioactive and endogenous dihydroxyphenylglycol (DHPG), an exclusively intraneuronal metabolite of norepinephrine, were compared in these beds. In untreated patients, cardiac removal of radioactive norepinephrine averaged 79%, whereas in desipramine-treated patients, cardiac removal of radioactive norepinephrine averaged 19%, a value similar to that of isoproterenol in untreated patients (14%), confirming that in the heart the non-neuronal removals of isoproterenol and norepinephrine were similar. In the heart, 69% of delivered norepinephrine was estimated to be removed by Uptake-1, a much higher percentage than that in the arm (14%), leg (7%), brain (10%), and lungs (4%). The cardiac arteriovenous increment in endogenous DHPG (137%) far exceeded that of the other beds (49%, 26%, 39%, and -19%, respectively), and radioactive DHPG in the great cardiac vein exceeded arterial levels by 113%, whereas in the other beds, arterial radioactive DHPG exceeded venous levels. The results indicate that the human heart is exceptionally dependent on neuronal uptake for in vivo removal of circulating norepinephrine.

Effects of high potassium or low sodium diet on vascular Na+,K+-ATPase activity and blood pressure in young spontaneously hypertensive rats

These studies were designed to investigate whether the antihypertensive effects of high potassium or low sodium diets are related to changes in vascular Na+,K+-adenosine triphosphatase (ATPase) activity. Vascular Na+,K+-ATPase was measured as ouabain-sensitive rubidium uptake in aorta incubated in buffer or plasma from spontaneously hypertensive rats (SHR) fed either a high potassium, a low sodium, or a normal diet for 2 weeks. The high potassium diet significantly increased Na+,K+-ATPase activity, whereas the low sodium diet significantly decreased activity. There was no evidence of a ouabainlike factor in plasma. The increased pump activity on the high potassium diet appeared to be due to an increase in maximum activity (Vmax) of the enzyme, rather than to an increased affinity for potassium. Potentially, an increase in Na+,K+-ATPase activity could contribute to the antihypertensive effect of potassium by hyperpolarizing the cell membrane. The decrease in vascular Na+,K+-ATPase activity on a low sodium diet probably is unrelated to its depressor effect, but it may be a homeostatic mechanism for maintaining sodium balance in the animal.

Decreased Na+-K+-ATPase activity and [3H]ouabain binding sites in various tissues of spontaneously hypertensive rats

Na+-K+-ATPase activity and [3H]ouabain binding were studied in cerebral cortex, kidney and heart isolated from spontaneously hypertensive rats (SHR) in both the prehypertensive (6 week old) and the hypertensive stages (14 week old). Na+-K+-ATPase activity of heart and kidney was found to be decreased by about 38 and 16% in the prehypertensive and hypertensive stages of SHR respectively; that of cerebral cortex decreased by 23.5% only in the hypertensive stages. Similar results were obtained by pretreatment of membranes with either 0.001% Triton X-100 or by increasing the K concentration from 4.7 to 12.7 mM in the Krebs solution. No significant differences in microsomal protein yield were noted between prehypertensive or hypertensive SHR and the age-matched WKY rats. The study of binding of [3H]ouabain to cerebral cortex, kidney and heart showed that the decreased Na+-K+-ATPase in hypertensive SHR was due to a 31.6, 21.8 and 41.3% reduction in the number of high affinity binding sites respectively, while the affinity constants (Kd) of ouabain binding sites on this enzyme in cerebral cortex, kidney and heart of the normotensive WKY rats were 26.5, 455.9 and 74.7 nM respectively and those from the hypertensive SHR were not altered. The plasma K concentration of the SHR in the prehypertensive and hypertensive stages was 4.07 and 4.13 mM, respectively, significantly less than that of the age-matched WKY rats. It appears that the decrease of plasma K and Na+-K+-ATPase activity in heart and kidney in SHR is derived from a genetic defect and may be related to the abnormal Na handling in this genetically hypertensive strain.

Abnormalities of membrane transport in hypertension

In this review, postulated passive and active fluxes of sodium, potassium, and calcium across the sarcolemma of the normal vascular smooth muscle cell are first summarized. Some practical problems encountered in their measurement are also mentioned. The review then considers how these fluxes appear to be altered in various forms of hypertension in animals and humans. Emphasis is given to abnormal fluxes of sodium and potassium due to altered sodium pump activity and permeability. Increasing evidence indicates that sodium retention due to increased sodium intake or decreased sodium excretion causes hypertension by releasing a humoral pressor substance from brain. This substance, which may be the putative natriuretic hormone, inhibits Na+, K+-ATPase and sodium pump activities in blood vessels and heart, thereby increasing contractile activity. In the genetic models of hypertension, the primary defect appears to be increased permeability of the vascular smooth muscle cell wall to sodium; pump activity increases to compensate for the increased inward leak of sodium. This may also be the case in patients with heritable essential hypertension. The possible consequences of super-imposing the sodium pump inhibitor on the primary defect are also considered. This may occur when animals with genetic hypertension or patients with heritable essential hypertension retain sodium subsequent to increased sodium intake and/or decreased ability to excrete sodium. Such superimposition should raise intracellular sodium concentration to high levels since now the pump would not fully compensate for the increased inward leak of sodium.

The effects of acetylstrophanthidin and ouabain on the sympathetic adrenergic neuroeffector junction in canine vascular smooth muscle

We performed experiments to determine the effects of acetylstrophanthidin (ACS) and ouabain on the adrenergic neuroeffector junction in dog saphenous veins. In quiescent strips incubated with 3H-norepinephrine (3H-NE), the drugs caused contraction and a progressive increase in overflow of 3H-NE and O-methylated metabolites; 3,4-dihydroxyphenylglycol (DOPEG) decreased. Tissue uptake of 3H-NE was partially inhibited. After surgical sympathectomy, both contraction and 3H-NE overflow were markedly attenuated. Following chemical sympathectomy with 6-hydroxydopamine, ouabain contractions were 11% of control, whereas the contractions due to exogenous norepinephrine were exaggerated. The initial overflow of 3H-NE was unaffected by tetrodotoxin, but the later and larger overflow with prolonged exposure was depressed. The former occurred in the absence of Ca2+, but the latter was Ca2+ dependent. Inhibition of the neuronal amine carrier by cocaine or desipramine and blockade of the neuronal alpha-adrenoceptors with phentolamine or phenoxybenzamine attenuated the release of 3H-NE evoked by ACS and ouabain. During electrical stimulation, ACS augmented the overflow of 3H-NE. This was attenuated by cocaine, desipramine, and the alpha-adrenolytic drugs. ACS, like pargyline, augmented the overflow of 3H-NE evoked by tyramine and depressed that of DOPEG. These experiments suggest that acetylstrophanthidin and ouabain (1) cause contraction of vascular smooth muscle by displacement of norepinephrine from neuronal stores, (2) reduce neuronal monoamine oxidase activity, (3) facilitate and may trigger Ca2+-dependent exocytotic release of norepinephrine, (4) partially inhibit the neuronal amine carrier mechanism but do not interfere with extraneuronal disposition of norepinephrine, and, finally (5) may have unexplained interactions with prejunctional alpha-adrenoceptors.

Mechanism, prevention and therapy of sodium-dependent hypertension

This review considers the mechanism, prevention and therapy of sodium-dependent, low-renin, presumably volume-expanded, hypertension. Certain evidence suggests that in susceptible persons the basic problem is a genetic or acquired deficiency in the ability of the kidney to excrete sodium and hence water. This places them at a disadvantage in a society such as ours in which the salt intake is uniformly high, to a large extent because of the salt content in commercially processed foods. Other evidence suggests that the blood pressure level rises in part because the volume expansion evokes the release of an unknown, slowly-acting, pressor agent which operates by stimulating the contractility of cardiovascular muscle through suppression of the cellular sodium-potassium pump, much in the manner of the cardiac glycosides. Several investigators and the Select Committee on GRAS Substances suggest that the incidence of salt-dependent hypertension could be significantly decreased in a society such as ours if salt intake were reduced from the present level of approximately 10 g/day to 12 g/day. An obvious starting point is a reduction of the salt content in processed foods. The Food and Nutrition Board of the National Research Council suggest that a judicious combination of dietary sodium restriction and the use of an appropriate diuretic is the most rational approach to the treatment and management of diseases characterized by retention of sodium.

Ouabain stimulation of noradrenaline transport in guinea pig heart

Cardiac glycosides such as ouabain inhibit Na+-dependent high-affinity noradrenaline uptake in sympathetic nerve terminals by interacting with (Na+ + K+)ATPase located in the neuronal membrane. It has been suggested that the augmentation of noradrenaline concentration at the neuromuscular junctions following the inhibition of its uptake may contribute to the ouabain-induced cardiac arrhythmia seen with toxic doses of the drug. Although it is generally accepted that the biochemical basis for the pharmacological response of cardiac glycosides must involve specific interaction of this class of drugs with the cardiac cell membrane Na+, K+-exchange pump, the precise molecular mechanism for the improvement of muscle performance by ouabain is controversial. Many investigators claim that cardiac glycosides increase myocardial force of contraction by inhibiting (Na+ + K+)ATPase. According to this hypothesis, such an inhibition might increase the intracellular concentration of Na+, which may augment the activity of the transmembrane Na+, Ca2+-exchange pump described by several workers. We have found that ouabain stimulates Na+-dependent transport of noradrenaline by guinea pig heart sympathetic nerve terminals.

Effect of chronic ethanol treatment on specific [3H]ouabain binding to (Na+ + K+)-ATPase in different areas of cat brain

Chronic ethanol administration to cats increased specific [3H]ouabain binding by 63% in cerebral cortex, 47% in cerebellum, 84% in amygdala, and 100% in hippocampus when the binding assays were performed in the presence of 160 nM [3H]ouabain. There was no significant change in specific [3H]ouabain binding in hypothalamus, thalamus, corpus striatum, and brain stem following chronic ethanol ingestion. Scatchard analysis revealed that enhancement of specific [3H]ouabain binding following chronic ethanol treatment in some areas of cat brain is primarily due to changes in densities of ouabain binding sites. Since ouabain is a specific inhibitor of (Na+ + K+)-ATPase the present observations suggest that the molecular mechanism for the enhancement of (Na+ + K+)-ATPase activity after chronic ethanol ingestion may be due to increased net rate of synthesis of (Na+ + K+)-ATPase molecules or exposure of non-functional enzyme system following conformational change of plasma membrane.

Stimulation of specific [3H]-ouabain binding to microsomal preparations from rat heart and skeletal muscle by thyroid hormones: effects of 6-hydroxydopamine

1. Surgical thyroidectomy decreased specific [3H]-ouabain binding to heart ventricular microsomes by 43% and gastrocnemius muscle microsomes by 34%. Administration of triiodothyronine to euthyroid rats enhanced specific [3H]-ouabain binding to heart and skeletal muscle membrane by 60% and 33% respectively. 2. Treatment of thyroidectomized rats with triiodothyronine increased specific [3H]-ouabain binding by 44% in skeletal muscle membrane preparation and 428% in cardiac microsomes. 3. Specific [3H]-ouabain binding decreased by 55% in heart and 53% in gastrocnemius muscle preparations following chemical sympathectomy with 6-hydroxydopamine. 4. Treatment with triiodothyronine of euthyroid rats which had been sympathectomized did not significantly alter specific [3H]-ouabain binding to heart or skeletal muscle membrane preparations. 5. Administration of triiodothyronine to thyroidectomized and sympathectomized rats increased specific [3H]-ouabain binding by 80% in heart and 83% in skeletal muscle membrane preparations. 6. These results suggest that triiodothyronine may influence specific [3H]-ouabain binding to thyroid hormone nonresponsive tissue such as sympathetic nerve endings. Therefore, the present observations are incompatible with the hypothesis that induction of (Na+ +K+)-adenosine triphosphatase of skeletal muscle membrane is the molecular mechanism for the calorigenic actions of thyroid hormones.

Effect of arrhythmogenic doses of deslanoside on the uptake of monoamines in brain tissue and in cardiac tissue

The purpose of our study was to determine whether a toxic arrhythmogenic dose of digitalis administered to an in vivo preparation would affect the neuronal uptake of norepinephrine, serotonin and dopamine in brain tissue and norepinephrine in cardiac tissue. This was investigated by intoxicating anesthetized cats with deslanoside, removing cardiac and brain tissue at the onset of ventricular fibrillation, and examining the ability of brain tissue to accumulate [3H]-NE, [3H]-T-HT and [3H]-DA and cardiac tissue to accumulate [3H]-NE. It was found that deslanoside inhibited uptake of [3H]-NE into the left ventricle and [3H]5-HT into the area postrema. These selective effects may reflect greater blood flow to these regions or different sensitivities of the transport mechanisms for these amines. This inhibition of uptake into both left ventricular tissue and area postrema may contribute to some of the cardiovascular and emetic effects seen with digitalis drugs.

[3H]-Ouabain binding to peripheral organs of cats: effect of ethanol

1 The specific [3H]-ouabain binding to microsomal fractions derived from cat heart, liver, spleen, and kidney increased significantly following chronic administration of ethanol. 2 Since ouabain binds exclusively to cell membrane (Na+ + K+)-adenosine triphosphatase ((Na+ + K+)-ATPase), these results provide evidence for an increase in number of (Na+ + K+)-ATPase macromolecules during chronic alcoholism. 3 The importance of the increase in number of (Na+ + K+)-ATPase molecules in the adaptive increase in ethanol metabolism and cardiac myopathy in chronic alcoholism is discussed.

The sodium-potassium pump in volume expanded hypertension

Decreased arterial Na+-K+ pump and cardiac Na+, K-+ATPase activities have now been demonstrated in several types of experimental volume expanded hypertension. The changes are not secondary to elevated pressure since they also occur in veins and right ventricle where the pressure is not elevated. Decreased arterial Na+-K+ pump activity can be reproduced by acute volume expansion of the normal rat and plasma extracts from this rat suppress pump activity when applied to arteries from another rat. Suppression of Na+-K+ pump activity in arteries, veins and heart, with ouabain for example, leads to increased contractile activity. Thus the volume expansion, reduced pump activity, and hypertension appear to be causally related through an ouabain-like humoral agent. Certain other evidence suggests that the pump defect extends to the sympathetic nerve endings, thereby reducing the efficiency of neural compensatory mechanisms.