Binding of dihydrodigitoxin to beef and human cardiac (Na+ + K+)-ATPase: evidence for two binding sites in cell membranes

Multiple digitalis receptors A molecular perspective

The Na,K-ATPase is the only established receptor for cardiac glycosides like digoxin or ouabain. There are now known to be three different isoforms of its principal subunit. These isoforms can differ from one another in their intrinsic affinity for cardiac glycosides. Recent work examines the molecular structure of the binding site. The relative level of expression of the isoforms in cardiac tissue is modified in several developmental, hormonal, and pathological states, contributing to alterations in the digitalis sensitivity of the tissue.

Binding properties and biological effects of oxidized-ouabain on cultured neonatal-rat cardiac myocytes. Implications on the mechanism of action of the digitalis-glycosides

Mild oxidation of ouabain with NaIO4, causes the cleavage of the bond between C2' and C3' of the rhamnose ring, leaving the steroid moiety intact. The oxidized ouabain (ox-ouabain) was examined on spontaneously contracting cultured rat-cardiac myocytes. Two classes of binding sites, with high and low affinities, were detected for both ox-ouabain and unmodified ouabain. The dissociation constants (KD) were found to be similar for both compounds, but the rate constants of association (ka) and dissociation (kd) of the low affinity sites were higher for ox-ouabain as compared with ouabain. Displacement experiments showed that ox-ouabain and ouabain bind to the same sites. The effects of ox-ouabain and ouabain on the activity of Na+, K(+)-ATPase were determined in microsomal preparations. Similar dose-response curves for the inhibition of the enzyme activity were determined for both drugs. Inhibition was observed only at concentrations above 10(-6) M. The biological effects of the drugs were examined by their capacity to induce positive inotropic or toxic effects. Concentrations of ox-ouabain which induced positive inotropic effects (increase in amplitude of systolic cell motion), ranged from 5 x 10(-8) M to 5 x 10(-6) M, as compared with 10(-7) M to 5 x 10(-7) M with ouabain. "Toxic" effects (decrease in the amplitude of systolic motion, increased beating frequencies and elevation in the position of maximal relaxation) was observed only with 10(-5) M ox-ouabain as compared with 10(-6) M ouabain. The mechanism of the inotropic action of ox-ouabain at the lower concentration range was investigated by measuring the effect of the drugs on 86Rb+ (analogue of K+) influx. Dose-response curves of effects of ouabain and ox-ouabain on 86Rb+ influx were bi-phasic. At low concentrations stimulation was observed, whereas at high concentrations 86Rb+ influx was inhibited. Ox-ouabain stimulated 86Rb+ influx by lower concentrations and to a greater extent than ouabain. A part of 86Rb+ influx into cardiac myocytes is mediated by the K+/Na+/Cl- cotransporter, which can be inhibited by loop diuretic drugs such as bumetanide. We have previously shown that ouabain, at low concentrations, stimulates the activity of the cotransporter. It is shown in the present work that ox-ouabain stimulates the activity of the cotransporter by lower concentrations and to a greater extent than ouabain.(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular sodium affects ouabain interaction with the Na/K pump in cultured chick cardiac myocytes

Whether a given dose of ouabain will produce inotropic or toxic effects depends on factors that affect the apparent affinity (K0.5) of the Na/K pump for ouabain. To accurately resolve these factors, especially the effect of intracellular Na concentration (Nai), we have applied three complementary techniques for measuring the K0.5 for ouabain in cultured embryonic chick cardiac myocytes. Under control conditions with 5.4 mM Ko, the value of the K0.5 for ouabain was 20.6 +/- 1.2, 12.3 +/- 1.7, and 6.6 +/- 0.4 microM, measured by voltage-clamp, Na-selective microelectrode, and equilibrium [3H]ouabain-binding techniques, respectively. A significant difference in the three techniques was the time of exposure to ouabain (30 s-30 min). Since increased duration of exposure to ouabain would increase Nai, monensin was used to raise Nai to investigate what effect Nai might have on the apparent affinity of block by ouabain. Monensin enhanced the rise in Na content induced by 1 microM ouabain. In the presence of 1 microM [3H]ouabain, total binding was found to be a saturating function of Na content. Using the voltage-clamp method, we found that the value of the K0.5 for ouabain was lowered by nearly an order of magnitude in the presence of 3 microM monensin to 2.4 +/- 0.2 microM and the magnitude of the Na/K pump current was increased about threefold. Modeling the Na/K pump as a cyclic sequence of states with a single state having high affinity for ouabain shows that changes in Nai alone are sufficient to cause a 10-fold change in K0.5. These results suggest that Nai reduces the value of the apparent affinity of the Na/K pump for ouabain in 5.4 mM Ko by increasing its turnover rate, thus increasing the availability of the conformation of the Na/K pump that binds ouabain with high affinity.

Inhibition of Na-K pump current in guinea pig ventricular myocytes by dihydroouabain occurs at high- and low-affinity sites

Binding of cardiac glycosides to the Na+,K+-dependent ATPase has been shown to occur at both high- and low-affinity sites. However, recent reports suggest that glycoside-induced inhibition of electrogenic Na-K pump current occurs with simple first-order binding kinetics at relatively low-affinity sites. This implies that high-affinity binding sites have little to do with Na-K pump inhibition during exposure to cardiac glycosides. To better understand the role of the high-affinity site, we investigated the concentration dependence of Ipump inhibition by dihydroouabain (DHO) in guinea pig ventricular myocytes through use of wide-pore patch pipettes to "fix" internal Na+ activity at approximately 30 mM and to voltage clamp at -40 mV (T = 34 degrees C). DHO was found to have no effect on membrane conductance at a holding potential of -40 mV. Holding current was monitored and the difference between steady-state holding current before and during external exposure to nine concentrations (range, 0.01-1,000 microM) of DHO was measured and normalized to cellular membrane capacitance. The concentration dependence of the inhibition of Na-K pump current was biphasic and well fitted to a two-binding site model with inhibitory KD values of 0.05 microM and 64.5 microM. This is consistent with previously reported 3H-ouabain binding studies in guinea pig myocardium. These findings indicate that the electrogenic properties of the Na-K pump can be inhibited by glycoside binding to both high- and low-affinity sites.

Na-K pump site density and ouabain binding affinity in cultured chick heart cells

The possible existence of multiple [3H]ouabain binding sites and the relationship between ouabain binding and Na-K pump inhibition in cardiac muscle were studied using cultured embryonic chick heart cells. [3H]ouabain bound to a single class of sites in 0.5 mM K (0.5 Ko) with an association rate constant (k+1) of 3.4 X 10(4) M-1.s-1 and a dissociation rate constant (k-1) of 0.0095 s-1 [corrected]. Maximal specific [3H]ouabain binding RT to myocyte-enriched cultures is 11.7 pmol/mg protein and Kd is 0.43 microM in 0.5 Ko, whereas Kd,apparent is 6.6 microM in 5.4 Ko. The number of binding sites per myocyte was calculated by correcting for the contribution of fibroblasts in myocyte-enriched cultures using data from homogeneous fibroblast cultures (RT = 3.3 pmol/mg protein; Kd = 0.19 microM in 0.5 Ko). Equivalence of [3H]ouabain binding sites and Na-K pumps was implied by agreement between maximal specific binding of [3H]ouabain and 125I-labeled monoclonal antibody directed against Na+-K+-ATPase (approximately 2 X 10(6) sites/cell). However, [3H]ouabain binding occurred at lower concentrations than inhibition of ouabain-sensitive 42K uptake in 0.5 Ko. Further studies in both 0.5 K and 5.4 Ko showed that ouabain caused cell Na content Nai to increase over the same range of concentrations that binding occurred, implying that increased Nai may stimulate unbound Na-K pumps and prevent a proportional decrease in 42K uptake rate. The results show that Na-K pump inhibition occurs as a functional consequence of specific ouabain binding and indicate that the Na-K pump is the cardiac glycoside receptor in cultured heart cells.

Decreased digitalis receptor activity in acute rejecting canine transplanted heart

In the course of acute rejection, myocardial tissue undergoes massive transformation and we hypothetized that for digitalis-like substances, receptor binding characteristics might be altered. Ten canine heterotopic cardiac allografts were carried out and were harvested once rejection had developed (8-10 days post-transplant). Microsomal membrane fractions of those grafts and of native hearts were isolated. Radioligand binding studies were carried out in a medium containing 5 mM Tris PO4, 50 mM Tris HCl, 5 mM MgCl2, pH 7.4 at 37 degrees C, using 3H-ouabain as the ligand. Saturation experiments (n = 10) indicate the presence of one homogeneous population of high affinity binding sites with an affinity constant (Kd) of 8-13 nM and a maximum binding capacity (Bmax) of 47 +/- 3.5 pmol/mg protein. Both saturation and competition binding studies illustrate the fact that acute rejection resulted in a significant decrease in Bmax (43%) without significant alteration in Kd value. These studies indicate that digitalis-like substances might not exert significant inotropic activity during rejection, but this hypothesis must be confirmed by in vivo haemodynamic experiments.

Origin of differences of inhibitory potency of cardiac glycosides in Na+/K+-transporting ATPase from human cardiac muscle, human brain cortex and guinea-pig cardiac muscle

The inhibitory potency of altogether 95 steroidal compounds (including cardenolides, bufadienolides and their glycosides) on the Na/K-ATPases (Na+/K+-transporting ATPases, EC 3.6.1.37) from human cardiac muscle, human brain cortex and guinea-pig cardiac muscle was compared to probe the complementary chemotopology of the inhibitor binding site areas on the three enzyme variants. The changes of potency, resulting from systematic variations of the geometry of steroid skeleton and the character as well as the structure of side chains at C3 or/and C17 of steroid backbone, allowed the following major conclusions. With the human cardiac and cerebral enzyme forms, the paired K0.5 (K'D) values for 77 steroid derivatives, covering seven orders of ten, were highly correlated. On an average, the total of compounds showed a 1.5-fold higher affinity to the cardiac enzyme. This tiny differentiation did not appear to be connected with an important difference in the chemotopology of the complementary subsites for steroid nucleus binding on the two enzyme forms. With the human and guinea-pig cardiac enzyme variants, the K0.5 values for 69 steroid derivatives, covering six orders of ten, were determined. For 41 5 beta, 14 beta-androstane derivatives only, the paired K0.5 values showed a close correlation. Here, the human enzyme variant exhibited 27-fold higher affinity. However, the paired K0.5 values determined on both enzymes for 28 steroid derivatives of differing structural features were but poorly correlated. Essentially, the geometries of the steroid nucleus determined the differential contributions of the side chains at C3 and C17 to the integral inhibitory potency on the two enzyme variants. Thus, the species differences in the potency of cardiac glycosides were traced to species differences in the complementarity of the steroid binding subsites. Hence, estimates of the potency of new steroidal compounds obtained on the guinea-pig cardiac enzyme can be neither quantitatively nor qualitatively easily extrapolated to the human cardiac enzyme. The extrathermodynamic analysis of the data opened major new insights in the structure-activity relationships concerning the role of C14 beta-OH, the character of the lead structure in cardioactive steroid lactones, and the significance of the configuration of A/B ring junction.