Interactions of cardiac glycosides with cardiac cells. III. Alterations in the sensitivity of (Na+ +K+)-ATPase to inhibition by ouabain in rat hearts

Sequestration of cardenolides inOncopeltus fasciatus: Morphological and physiological adaptations

The morphological and physiological adaptations associated with sequestration of cardenolides by the lygaeidOncopeltus fasciatus are summarized and discussed. Cardenolides are efficiently accumulated inO. fasciatus; however, the insect does not appear to suffer any physiological cost as a result of handling large amounts of these plant toxins. Morphological adaptations of the insect include a modified integument composed of a double layered epidermis with an inner layer (the dorsolateral space) specialized for cardenolide storage. Special weak areas of the cuticle are found on both the thorax and abdomen, which rupture when the insect is squeezed, resulting in the cardenolide-rich contents of the inner epidermal layer being released onto the body surface in the form of discrete spherical droplets. Physiological adaptations include selective sequestration of food plant cardenolides, concentration of cardenolides in the dorsolateral space, passive uptake of cardenolides at the gut and dorsolateral space requiring little energy output, reabsorption of secreted cardenolides by the Malpighian tubules, high in vivo tolerance to cardenolides, and the presence of cardenolide-resistant Na,K-ATPases.

Immunologic identification of Na+,K(+)-ATPase isoforms in myocardium. Isoform change in deoxycorticosterone acetate-salt hypertension

There are three isoforms of the catalytic (alpha) subunit of the Na+,K(+)-ATPase, each derived from a different gene, that differ in their sensitivity to inhibition by cardiac glycosides. Antibodies specific for the three isoforms were used to study Na+,K(+)-ATPase isoform expression in ventricular myocardium, where an understanding of digitalis receptor diversity is most important. In the rat heart, there is simultaneous expression of two isoforms in adult ventricle, and immunofluorescence studies demonstrated that both isoforms are expressed uniformly in cardiomyocytes. Hypertension and hypertrophy have been reported to selectively depress alpha 2 isoform mRNA levels, and we show in the present study that alpha 2 protein levels were correspondingly depressed in rats made hypertensive by uninephrectomy and treatment with deoxycorticosterone acetate and a high-salt diet. In the human heart, where mRNA for all three alpha isoforms has been reported, we detected all three isoform proteins (alpha 1, alpha 2, and alpha 3). Two isoforms (alpha 1 and alpha 3) predominated in the macaque heart; dissection of the heart showed uniformity of isoform expression in different ventricular regions but markedly less alpha 3 in the atrium. Finally, isoform-specific antibodies were used to detect which alpha isoforms were expressed in the ventricles of several commonly used experimental animals to test the correlation of isoform expression with cardiac glycoside-response heterogeneity. Two isoforms (alpha 1 and alpha 3) were found in canine myocardium, whereas only one (alpha 1) was found in sheep and guinea pig. Expression of Na+,K(+)-ATPase isoforms can thus be readily followed and related to the physiology of the digitalis receptor.

Different ouabain sensitivities of Na+/K(+)-ATPase from Poekilocerus bufonius tissues and a possible physiological cost

1. The properties of Na+/K(+)-transporting ATPase in microsomal preparation from mid-gut of the grasshopper, Poekilocerus bufonius, were investigated and compared with the same enzyme from brain and excretory system. 2. Two components of ATPases activity are present in the three tissues studied. 3. The physiochemical properties of Na+/K(+)-transporting ATPase from mid-gut, brain and excretory system (hind-gut plus Malpighian tubules) are essentially the same. 4. The calculated values of PI50 were 2 (I50 = 1 x 10(-2) M), 3.7 (I50 = 2 x 10(-4) M) and 6.4 (I50 = 3.98 x 10(-7)) for Na+/K(+)-ATPase from mid-gut, excretory system and brain, respectively. The mid-gut contains the most ouabain-resistant Na+/K(+)-ATPase. 5. The results suggest that P. bufonius have developed some tolerance to toxic cardiac glycosides (CGS), but there is a possibility of autotoxicity as indicated by the presence of ouabain-sensitive ATPase from brain tissue. 6. It was concluded that the dissimilarities of Na+/K(+)-ATPases from different tissues of P. bufonius are probably due to tissue-dependent differences in ouabain sensitivity (or isoenzymes pattern) available in the same insect. 7. The atrophy of female flight muscle of P. bufonius suggests the possibility of physiological cost inflicted on insects consuming poisonous plants.

Molecular genetics of Na,K-ATPase

Researchers in the past few years have successfully used molecular-genetic approaches to determine the primary structures of several P-type ATPases. The amino-acid sequences of distinct members of this class of ion-transport ATPases (Na,K-, H,K-, and Ca-ATPases) have been deduced by cDNA cloning and sequencing. The Na,K-ATPase belongs to a multiple gene family, the principal diversity apparently resulting from distinct catalytic alpha isoforms. Computer analyses of the hydrophobicity and potential secondary structure of the alpha subunits and primary sequence comparisons with homologs from various species as well as other P-type ATPases have identified common structural features. This has provided the molecular foundation for the design of models and hypotheses aimed at understanding the relationship between structure and function. Development of a hypothetical transmembrane organization for the alpha subunit and application of site-specific mutagenesis techniques have allowed significant progress to be made toward identifying amino acids involved in cardiac glycoside resistance and possibly binding. However, the complex structural and functional features of this protein indicate that extensive research is necessary before a clear understanding of the molecular basis of active cation transport is achieved. This is complicated further by the paucity of information regarding the structural and functional contributions of the beta subunit. Until such information is obtained, the proposed model and functional hypotheses should be considered judiciously. Considerable progress also has been made in characterizing the regulatory complexity involved in expression of multiple alpha-isoform and beta-subunit genes in various tissues and cells during development and in response to hormones and cations. The regulatory mechanisms appear to function at several molecular levels, involving transcriptional, posttranscriptional, translational, and posttranslational processes in a tissue- or cell-specific manner. However, much research is needed to precisely define the contributions of each of these mechanisms. Recent isolation of the genes for these subunits provides the framework for future advances in this area. Continued application of biochemical, biophysical, and molecular genetic techniques is required to provide a detailed understanding of the mechanisms involved in cation transport of this biologically and pharmacologically important enzyme.

Purification of cardiac (Na+,K+)-activated adenosine triphosphatase from rat

A procedure is described for preparation of highly active (Na+,K+)-ATPase from rat heart which has a specific activity of 200-600 mumol Pi/mg/h. The procedure is simple and can be applied to small amounts of heart muscle (approximately 1 g). The ATPase activity was more than 90% sensitive to ouabain (at concentrations up to 1 mM). The ouabain sensitivity is biphasic with about 20% of the ATPase activity being inhibited at approximately 3 X 10(-7) M ouabain.

Rat cardiac ventricle has two Na+,K+-ATPases with different affinities for ouabain: developmental changes in immunologically different catalytic subunits

The mechanism of the inotropic effect of cardiac glycosides on the heart has long been controversial. Inotropic effects at low concentrations of cardiac glycosides indicate more than one class of receptor or more than one cellular mechanism. In the brain of the rat, high- and low-affinity cardiac glycoside receptors have been shown to be associated with two structurally different isoforms of the catalytic subunit of the Na+,K+-ATPase, termed alpha and alpha(+). Evidence is presented here that the high- and low-affinity sites in rat cardiac ventricle are associated with Na+,K+-ATPase catalytic subunit forms similar to the alpha(+) and alpha forms in the brain. Membranes from the rat ventricle contained polypeptides with the electrophoretic mobilities of alpha and alpha(+), which could be stained by isoform-specific anti-Na+,K+-ATPase antibodies on electrophoretic blots. Both polypeptides also displayed Na+-stimulated phosphorylation with [gamma-32P]ATP. Inhibition of Na+,K+-ATPase activity by ouabain demonstrated the presence of both high- and low-affinity ATPases proportional to the presence of the alpha(+) and alpha polypeptides. The ratios of the two isoforms changed with postnatal maturation, paralleling known changes in cardiac physiology and cardiac glycoside sensitivity. Cardiac glycoside sensitivity can evidently be regulated at the level of gene expression by developmental signals.

Screening of inotropic drugs on isolated rat and guinea pig hearts

A technique is described for screening the effects of inotropic drugs on isolated rat or guinea pig hearts perfused at constant coronary pressure and at a frequency of 6 Hz. Their performances, including function curves, were recorded using an intraventricular balloon. Both preparations became either sensitive from initially having been insensitive, or more sensitive from having been slightly sensitive at the outset, to inotropic interventions, provided the external calcium concentration was reduced to 0.25 mM for the rat and 0.50 mM for the guinea pig. The inotropic effect of drugs such as isoproterenol, forskolin, and theophylline was only slightly altered by lowering [Ca]o. Amrinone, sulmazole, and beta agonists such as xamoterol, cicloprolol, pindolol, or RU 42173 almost never caused an inotropic effect at the serum calcium concentration of 2.50 mM, whereas they did provoke a positive response at low [Ca]o. Other compounds such as ouabain, salbutamol, and pimobendan were toxic at high [Ca]o, although at reduced [Ca]o their positive effect on contractility was quite evident.

Deformability and electrolyte changes of erythrocytes in connection with open heart surgery

Red cell damage was studied in 16 patients undergoing open heart surgery. The damage was assessed by red cell deformability measuring red cell filtration rate (RFR) using a microfiltration technique. Simultaneous estimations of red cell sodium, potassium and water content were carried out. During and after cardiopulmonary bypass (CPB) RFR decreased with simultaneous loss of potassium and gain in sodium. These ionic changes are known to induce an increase in cellular calcium content via raised calcium influx. Since elevated intracellular calcium content provokes reduced deformability of the red cells the observed reduction in RFR in connection with CPB might at least partly be secondary to the reduction in the K+-Na+ ratio. A comparison between the regression lines between K+/Na+ and RFR for cold blood not subjected to the trauma caused by the heart-lung machine and for patient blood suggests that approximately 20% of the reduction in RFR observed in connection with CPB is due to the reduction in K+/Na+. The severity of the erythrocyte electrolyte changes was positively correlated to the amount of oxygen used in the heart-lung machine. The results suggest that the flow of oxygen and blood through the heart-lung machine should be kept as low as possible.

Magnitude of ouabain-sensitive respiration of lamb hepatocytes (Ovis aries)

In lamb hepatocyte preparations with viabilities greater than 90%, ouabain-sensitive respiration accounted for approximately 50% of the total cellular O2 consumption. Lamb hepatocyte preparations with viability of less than 50% exhibited lower (P less than 0.05) total and ouabain-sensitive respiration. The decrease in ouabain-sensitive respiration in these preparations entirely accounted for the drop in total respiration.

Species variation in the ouabain sensitivity of cardiac Na+/K+-ATPase. A possible role for membrane lipids

The role of membrane lipid composition on the modulation of ouabain sensitivity of cardiac Na+/K+-ATPase has been studied in vitro using several animal species. The animals can be grouped as ouabain-sensitive and ouabain-insensitive species. Ouabain-sensitive species (I50; 0.5-2.2 microM) include sheep, marmoset, pig and the guinea pig, whilst rat and mouse form the ouabain-insensitive group (I50; 100-105 microM). Although no species variation in the distribution of major phospholipid classes was observed, significant differences were apparent in the proportions of certain saturated and unsaturated phospholipid fatty acids. Thus, there was a marked increase in the relative proportion of docosahexaenoic (22:6, omega-3) acid in the Na+/K+-ATPase preparations from the rat and mouse compared to ouabain-sensitive species. Despite these differences, all animals had similar proportions of total saturated (sigma SAT) and total unsaturated (sigma Unsat) fatty acids. On the other hand, a good correlation between the unsaturation index of membrane lipids and I50 value for ouabain was observed. It is proposed that acyl chain characteristics (unsaturation and/or chain length) rather than the head group of the phospholipid molecule play a major role in the modulation of Na+/K+-ATPase to inhibition by ouabain.

Cardiac glycoside receptors in cultured heart cells--II. Characterization of a high affinity and a low affinity binding site in heart muscle cells from neonatal rats

The binding of [3H]ouabain has been studied in (Na+ + K+)-ATPase enriched cardiac cell membranes, as well as in cardiac muscle and non-muscle cells in culture--all obtained from hearts of neonatal rats. The binding has been correlated with ouabain-induced inhibition of (Na+ + K+)-ATPase (cardiac cell membranes) and the inhibition of active (86Rb+ + K+)-influx (cardiac muscle and non-muscle cells in culture). Furthermore, the effect of ouabain on the amplitude of cell-wall motion and contraction velocity has been studied in electrically driven cardiac muscle cells. In muscle and non-muscle cells, two classes of ouabain binding sites have been identified. In rat heart muscle cells, the high affinity binding site has a dissociation constant (KD) of 3.2 X 10(-8) M and a binding capacity (B) of 0.2 pmole/mg protein (80,000 sites/cell); the values for the low affinity binding site are: KD = 7.1 X 10(-6) M; B = 2.6 pmole/mg protein (10(6) sites/cell). The binding to both types of binding sites is depressed by K+ and abolished after heat denaturation of the cells. The kinetics of [3H]ouabain binding to rat heart muscle cells (association and dissociation rate constants, K+- and temperature-dependence of association and dissociation processes) have been characterized. In rat heart muscle and non-muscle cells, the binding of [3H]ouabain to the low affinity site results in inhibition of the (86Rb+ + K+)-influx (EC50 = 1.3 and 1.5 X 10(-5) M ouabain), a decrease in cell-K+ (EC50 = 1.9 and 1.4 X 10(-5) M) and an increase in cell-Na+ (10(-5)-10(-4) M). The ouabain-induced positive inotropic effect (increase in amplitude of cell-wall motion, increase in contraction velocity) in cardiac muscle cells is observed only at ouabain concentrations greater than or equal to 5 X 10(-6) M, and it is therefore probably attributed to occupation of the low affinity binding site. Coupling of occupation of the low affinity site by ouabain with drug-induced inhibition of the sodium pump and with drug-induced positive inotropic action is further substantiated by kinetic measurements. In contrast, occupation of the high affinity binding site does not produce any measurable inhibition of the sodium pump activity or positive inotropy.(ABSTRACT TRUNCATED AT 400 WORDS)

Two binding sites for ouabain in cardiac cell membranes

Cardiac glycoside receptors were defined by simultaneous measurement of 3H-ouabain binding and its effects on cardiac cell membranes, contracting cardiac muscle and cultured cardiac cells. These measurements show that: Rat and guinea pig cardiac cell membranes have two specific ouabain binding sites. In both species, ouabain binding to the high affinity site on cell membranes correlates with the positive inotropic effect in contracting cardiac muscle. Inhibition of (Na+ + K+)-ATPase activity corresponds to binding to the low affinity site. This questions the hypothesis that (Na+ + K+)-ATPase inhibition is necessary for ouabain-induced positive inotropy. K+ may induce an heterogeneity in the ouabain binding sites of the digitalis-sensitive cat and human heart.

Binding sites for ouabain in human and rat erythrocytes and in rat heart cells

The presence and properties of two classes of binding sites for digitalis in erythrocytes and in heart cells are reviewed. Methods to distinguish between these two binding sites are summarized for intact cells and for their membranes. Our biochemical data do not suggest a physiological role for each class of sites. The membrane sites were modified by varying (a) the content of cholesterol or (b) the distribution of fatty acids leading to changes in the microviscosity thus affecting the degree of binding of ouabain to the two classes of sites. Oxidation of ouabain with periodate forms a di-aldehyde which could bind via Schiff base to the digitalis sites but could also attach covalently to these sites. The role of the sugar moiety in the process of ouabain binding becomes of increasing importance. A mild, controlled periodate oxidation of ouabain, especially in presence of phosphate cleaves only the bond between C-2 and C-3 of the rhamnose without affecting the steroid moiety. The periodate oxidation provided additional information for assigning a distorted chair conformation or a transient boat conformation for rhamnose in ouabain. It was also established by 1H NMR spectroscopy that this chair conformation is a 1C4 pyranose ring.

Effects of calcium on the heterogeneity of the Na+, K+-ATPase forms in rat heart

The sensitivity of the Na+, K+-ATPase to ouabain has been studied in sarcolemma vesicles isolated from normal rat heart. Two enzyme forms exhibiting high and low sensitivities to ouabain have been observed in Ca2+-free perfused heart. The half-maximal inhibitory effects occurred with 1-2 X 10(-8) M ouabain. The high sensitivity form undetectable in hearts maintained at a physiological Ca2+ level might represent altered low affinity sites or latent enzyme forms unmasked by low calcium concentrations. The heterogeneity of the Na+, K+-ATPase forms was found to be also modulated by the K+/ouabain antagonism, addition of K+ accentuating the heterogeneity. These in vitro results associated with in vivo experiments on isolated rat heart working under isovolumic conditions suggested that lowering Ca2+ has qualitative and quantitative effects. Low Ca2+ concentrations increased the sensitivities to ouabain and the amplitudes of both the enzyme inhibition and the positive inotropic effects.