Species difference in temperature dependence of cardiac (Na+ + K+)-ATPase activity

Suppression of Na+/K+-ATPase activity during estivation in the land snail Otala lactea

Entry into the hypometabolic state of estivation requires a coordinated suppression of the rate of cellular ATP turnover, including both ATP-generating and ATP-consuming reactions. As one of the largest consumers of cellular ATP, the plasma membrane Na+/K+-ATPase is a potentially key target for regulation during estivation. Na+/K+-ATPase was investigated in foot muscle and hepatopancreas of the land snail Otala lactea, comparing active and estivating states. In both tissues enzyme properties changed significantly during estivation: maximal activity was reduced by about one-third, affinity for Mg.ATP was reduced (Km was 40% higher), and activation energy (derived from Arrhenius plots) was increased by approximately 45%. Foot muscle Na+/K+-ATPase from estivated snails also showed an 80% increase in Km Na+ and a 60% increase in Ka Mg2+ as compared with active snails, whereas hepatopancreas Na+/K+-ATPase showed a 70% increase in I50 K+ during estivation. Western blotting with antibodies recognizing the alpha subunit of Na+/K+-ATPase showed no change in the amount of enzyme protein during estivation. Instead, the estivation-responsive change in Na+/K+-ATPase activity was linked to posttranslational modification. In vitro incubations manipulating endogenous kinase and phosphatase activities indicated that Na+/K+-ATPase from estivating snails was a high phosphate, low activity form, whereas dephosphorylation returned the enzyme to a high activity state characteristic of active snails. Treatment with protein kinases A, C or G could all mediate changes in enzyme properties in vitro that mimicked the effect of estivation, whereas treatments with protein phosphatase 1 or 2A had the opposite effect. Reversible phosphorylation control of Na+/K+-ATPase can provide the means of coordinating ATP use by this ion pump with the rates of ATP generation by catabolic pathways in estivating snails.

Digitalis: new actions for an old drug

The mechanisms by which digitalis causes its therapeutic and toxic actions have been studied for nearly a half century, revealing a great deal about cardiac cell regulation of intracellular ions via the Na-K-ATPase (NKA) and how it is altered by cardiac glycosides. However, recent observations suggest that digitalis may have additional effects on cardiac cell function in both the short and long term that include intracellular effects, interactions with specific NKA isoforms in different cellular locations, effects on intracellular (including nuclear) signaling, and long-term regulation of intracellular ionic balances through circulating ouabain-like compounds. The purpose of this review is to examine the current status of a number of the newest and most interesting developments in the study of digitalis with a particular focus on cardiac function, although we will also discuss some of the new advances in other relevant cardiovascular effects. This new information has important implications for both our understanding of ionic regulation in normal and diseased hearts as well as for potential avenues for the development of future therapeutic interventions for the treatment of heart failure.

Temperature effects on ion transport across the erythrocyte membrane of the frog Rana temporaria

Unidirectional K+ and Na+ influxes in the frog erythrocytes incubated in Cl- or NO(3)- media with 2.7 mM K+ were measured using 86Rb and 22Na as tracers. K+ influx was inhibited by 35-55% in the presence of 0.2-1.0 mM furosemide but it was unaffected by 0.1-0.2 mM bumetanide. Furosemide at a concentration of 0.5 mM had no effect on K+ loss from the frog red cells incubated in a nominally K(+)-free medium. Together with our previous studies the data support the existence of K-Cl cotransport and the absence of Na-K-2Cl cotransport in the frog erythrocyte membrane. Cell cooling from 20 to 5 degrees C caused a decrease in K+ influx and K+ efflux via the K-Cl cotransporter (3.2- and 3.7-fold, respectively) giving an apparent energy of activation (EA) of about 60 kJ/mol and Q10 value of 2.5. Only small decline (approximately 30%) in the ouabain-sensitive K+ influx was found as temperature was changed from 20 to 5-10 degrees C. Low values of Q10 (approximately 1.5) and EA (27.3 kJ/mol) were obtained for passive K+ influx in the frog erythrocytes (ouabain-insensitive in NO(3)- medium) at temperature within 5-20 degrees C. However, the temperature coefficients were greater for passive Na+ influx and passive K+ efflux (Q10 approximately 2.4-2.5 and EA approximately 56-58 kJ/mol). The temperature dependence of all ion transport components displayed discontinuities showing no changes at temperature between 5 and 10 degrees C. Thus, cooling of the frog red cells is associated with a greater decrease of Na+ influx and K+ efflux than passive and active K+ influx. These data indicate that the preservation of a relative high activity of the Na,K-pump during cell cooling and also the temperature-induced changes in the K-Cl cotransport activity and ion passive diffusion contribute to maintenance of ion concentration gradients in the frog erythrocytes at decreased temperature.

pH and temperature effects on kinetics of creatine kinase in aqueous solution and in isovolumic perfused heart. A 31P nuclear magnetization transfer study

Phosphorylated metabolites concentrations and creatine kinase kinetics are measured by 31P NMR in solution and in isovolumic perfused rat hearts submitted to hypo- and hyperthermia as well as to acidosis (37 degrees C). In the organ, temperature variation from 40 to 25 degrees C induces an increase of phosphocreatine (PCr) stores, a decrease of Pi and ADP concentrations, but does not affect the ATP pool. Creatine kinase forward flux (Vfor) is gradually reduced when the temperature is lowered both in vitro and in perfused heart. In normothermic and hypothermic conditions, a relationship is found between cardiac performance (rate pressure product, RPP), Vfor and ATP synthesis estimated through the myocardial oxygen consumption rate (MVO2). At 40 degrees C however, the RPP is reduced although both Vfor and MVO2 increase. In vitro experiments show an optimum pH of 7.7 for the forward creatine kinase reaction. In perfused heart submitted to acidosis, a decrease of PCr concentration is observed, whereas ATP and ADP contents remain unchanged. Heart creatine kinase flux increased as in hyperthermia. These high fluxes are attributed to the coupling of the creatine kinase reaction with energy consuming or producing reactions: the increase of energy demand related to non-contractile processes could explain the high MVO2 and Vfor observed in those conditions.

Temperature dependence of embryonic cardiac gap junction conductance and channel kinetics

We have investigated the effects of temperature on the conductance and voltage-dependent kinetics of cardiac gap junction channels between pairs of seven-day embryonic chick ventricle myocytes over the range of 14-26 degrees C. Records of junctional conductance (Gj) and steady-state unit junctional channel activity were made using the whole-cell double patch-clamp technique while the bath temperature was steadily changed at a rate of about 4 degrees C/min. The decrease in Gj upon cooling was biphasic with a distinct break at 21 degrees C. In 12 cell pairs, Q10 was 2.2 from 26 to 21 degrees C, while between 21 and 14 degrees C it was 6.5. The mean Gj at 22 degrees C (Gj22) was 3.0 +/- 2.1 nS, ranging in different preparations from 0.24 to 6.4 nS. At room temperature, embryonic cardiac gap junctions contain channels with conductance states near 240, 200, 160, 120, 80 and 40 pS. In the present study, we demonstrate that cooling decreases the frequency of channel openings at all conductance levels, and at temperatures below 20 degrees C shifts the prevalence of openings from higher to lower conductance states: all 240 pS openings disappear below 20 degrees C; 200 pS openings are suppressed at 17 degrees C; below 16 degrees C 160 and 120 pS events disappear and only 80 and 40 pS states are seen. Temperature also affected the voltage-dependent kinetics of the channels. Application of a 6 sec, 80 mV voltage step across the junction (Vj80) caused a biexponential decay in junctional conductance. Decay was faster at lower temperatures, whereas the rate of recovery of Gj after returning to Vj0 was slowed. Cooling reduced the fast decay time constant, increased both recovery time constants, and decreased the magnitude of Gj decay, thus leaving a 10-16% larger residual conductance (Gss/Ginit, +/- 80 mV Vj) at 18 than at 22 degrees C. From these results we propose that embryonic chick cardiac gap junctions contain at least two classes of channels with different conductances and temperature sensitivities.

Two functional Na+/K(+)-ATPase isoforms in the left ventricle of guinea pig heart

Guinea pig left ventricular muscle contains two distinct molecular forms of the Na+/K(+)-ATPase catalytic alpha subunit. Sarcolemmal vesicles highly enriched in Na+/K(+)-ATPase were isolated by a new procedure that yielded specific activities of 60-100 mumol Pi.h-1.mg-1. SDS/PAGE of isolated sarcolemma after reduction and alkylation of the sulfhydryl groups and identification on immunoblots with specific anti-(alpha subunit) antibodies indicated the presence of two major polypeptides of 100 kDa and 103 kDa, respectively. The two alpha subunits were functional: the dose/response curves of Na+/K(+)-ATPase activity with ouabain, dihydroouabain and digitoxigenin were biphasic, revealing the presence of high-affinity [concentration of drug causing 50% inhibition (IC50) = 10 nM] and low-affinity (IC50 = 2 microM) forms with proportional contributions of 55% and 45%, respectively. The involvement of the high-affinity form in the positive inotropic effect of digitalis and of the low-affinity sites in both inotropy and toxicity are consistent with the literature data on rodents.

Rat vs. rabbit ventricle: Ca flux and intracellular Na assessed by ion-selective microelectrodes

Trans sarcolemmal Ca movements in rabbit and rat ventricular muscle were compared using extracellular double-barreled Ca-selective microelectrodes. In rabbit ventricle, steady-state twitches were associated with transient extracellular Ca (Cao) depletions, indicative of Ca uptake during the twitch. In contrast, steady-state twitches in rat ventricle were associated with net cellular Ca extrusion. Rest periods in rabbit ventricle lead to a net loss of cell Ca and resumption of stimulation induces a net uptake of Ca by the cells. Conversely, in rat ventricle rest periods lead to cellular Ca gain and resumption of stimulation induces a net Ca loss from the cells. Thus stimulation is associated with net Ca gain in rabbit ventricle and net Ca loss in rat ventricle. These observations provide an explanation for some of the functional differences between rat and rabbit ventricle (e.g., negative force-frequency staircase and rest potentiation in rat vs. positive staircase and rest decay in rabbit). Resting intracellular Na activity (alpha iNa) was 12.7 +/- 0.6 mM in rat and 7.2 +/- 0.5 mM in rabbit ventricle. This alpha iNa in rat ventricle is sufficiently high that Ca entry via Na-Ca exchange is thermodynamically favored at the resting membrane potential. This may explain why rest potentiation is observed in rat ventricle. In contrast, the lower alpha iNa in rabbit ventricle would favor Ca extrusion via Na-Ca exchange at rest (and consequent rest decay). In rat ventricle, the increase of intracellular [Ca] ([Ca]i) associated with contraction, coupled with the short action potential duration, strongly favor Ca extrusion via Na-Ca exchange and explain the observed Cao accumulation observed during twitches in rat. The high plateau of the rabbit ventricular action potential tends to prevent Ca extrusion via Na-Ca exchange during the contraction and explains the Cao depletions observed in rabbit. It is concluded that the higher alpha iNa and shorter action potential duration in rat vs. rabbit ventricle can explain many of the functional differences observed in these tissues.

Difference between neuronal and nonneuronal (Na+ + K+)-ATPases in their conformational equilibrium

Several experiments were carried out to study the difference between two isozymes (alpha(+) and alpha) of (Na+ + K+)-ATPase in the conformational equilibrium. Rat brain (Na+ + K+)-ATPase was much more thermolabile than the kidney enzyme. Both enzymes were protected from heat inactivation not only by Na+ and K+, but also by choline in varying degrees, though there was a difference between the two enzymes in the protection by the ligands. The brain enzyme was partially protected from N-ethylmaleimide (NEM) inactivation by both Na+ and K+, but the effects of the ligands on NEM inactivation of the kidney enzyme were more complex. Though ligands differentially affected the thermostability and NEM sensitivity of the two enzymes, the effects were not simply related to the conformational states. The sensitivity of phosphoenzyme (EP) formed in the presence of ATP, Na+, and Mg2+ to ADP or K+ and K+-p-nitrophenyl phosphatase (pNPPase) was then studied as a probe of the differences in the conformational equilibrium between the two isozymes. The EP of the brain enzyme was partially sensitive to ADP, while those of the heart and kidney enzymes were not. At physiological Na+ concentrations the percentages of E1P formed by the brain and kidney enzymes were determined to be about 40-50 and 10-20% of the total EP, respectively. The hydrolytic activity of pNPP in the presence of Li+, a selective activator at catalytic sites of the reaction, was much higher in the kidney enzyme than in the brain enzyme. The inhibition of K+-stimulated pNPPase by ATP and Na+ was greater in the latter enzyme than in the former. These results suggest that neuronal and nonneuronal (Na+ + K+)-ATPases differ in their conformational equilibrium: the E1 or E1P may be more stable in the alpha(+) than in the alpha during the turnover, and conversely the E2 or E2P may be more stable in the latter than in the former.

Difference in phospholipid dependence between two isozymes of brain (Na+ + K+)-ATPase

The effect of phospholipase C on two isozymes (alpha (+) and alpha forms) of rat brain (Na+ + K+)-ATPase and the temperature-dependence of their activities were investigated. Phospholipase C from Clostridium welchii inhibited the activities of the enzymes treated with and without pyrithiamin or N-ethylmaleimide, a preferential inhibitor of the alpha (+) form, but the extent of the inhibition was higher in the control enzyme than in the treated enzymes. The treatment of the (Na+ + K+)-ATPase with phospholipase C altered a ratio between high- and low-affinity components for ouabain inhibition. It also caused the similar change in a ratio between the alpha (+) and alpha forms of Na+-stimulated phosphorylation from [gamma-32P]ATP. These findings indicate that the alpha (+) form of rat brain (Na+ + K+)-ATPase is more sensitive to phospholipase C than the alpha form. Analysis of Arrhenius plots of the activities of the control and pyrithiamin-treated enzymes showed that there was a difference between the two enzymes in a break point. We suggest that two isozymes of rat brain (Na+ + K+)-ATPase differ in the interaction with phospholipids or in the lipid-environment.

Cellular basis for the species differences in sensitivity to cardiac glycosides (digitalis)

The relative toxicity of numerous cardiotonic steroids (viz. ouabain, digitoxin, digoxin, convallatoxin, SC4453, bufalin, gitaloxin, digoxigenin, actodigin, oleandrin, digitoxigenin, gitoxin, strophanthidin, gitoxigenin, lanatosides A, B and C, alpha- and beta-acetyl digoxin, alpha- and beta-methyl digoxin) and related compounds towards a number of independent cell lines established from human, monkey, mouse, Syrian hamster, and Chinese hamster have been determined. All cardiac glycosides and their genins, as well as the cardiotonic alkaloid cassaine, exhibited greater than 100-fold higher toxicity towards cultured human and monkey cells in comparison to the cell lines of mouse, Syrian hamster, and Chinese hamster origins. These differences are species-related as all cell lines (both normal as well as transformed) from any one species, as well as cells from the closely related species (e.g., man and monkey or mouse, Chinese hamster, and Syrian hamster), showed similar sensitivity towards these drugs. The failure to see any significant differences in cellular toxicity for a larger number of other compounds which either bear limited structural resemblance to cardiac glycosides (viz. estradiol 17-beta-acetate, testosterone propionate, 21-acetoxy pregnenolone, beta-estradiol, digitonin, tigogenin, and tomatine) or interact with the Na+/K+ ATPase in a different manner (viz. veratridine, sanguinarine nitrate, penicillic acid, vanadium pentoxide, harmaline-HCI,5,5'-diphenyl hydantoin, quindonium bromide, and methyl quinolizinum bromide) provides strong evidence that the observed species-related differences are highly specific for cardiotonic steroids. Studies on the binding of [3H]ouabain show that, in comparison to human and monkey cell lines, no significant binding of the drug is observed in cells derived from the resistant species (i.e., mouse and Chinese hamster). The Na+/K+ ATPase from cells of the resistant species is inhibited at much higher concentrations of ouabain and digitoxin in comparison to the enzyme from human cells, and a good correlation is observed between these concentrations and those reported for inhibition of the enzyme from isolated heart muscles of the same species. These results provide strong evidence that the species-related differences in sensitivity to digitalis have a cellular basis and that the cultured cells from various mammalian species provide a useful model system for investigating the mechanism of action of cardiac glycosides.

Phospholipid composition of cardiac (Na+ + K+)-ATPases from various species

There is a difference in phospholipid composition of cardiac (Na+ + K+)-ATPase preparations between species which are sensitive to ouabain and those which are not. Sphingomyelin is higher and phosphatidylcholine is lower in the enzymes from sensitive species than in those from insensitive ones. Lysophosphatidylcholine is detectable only in the latter preparations.