Effects of AY-22,241 (Actodigin) on electrical and mechanical activity of cardiac tissues

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.

Positive inotropic effects of ouabain in isolated cat ventricular myocytes in sodium-free conditions

The inotropic and toxic effects of cardiac steroids are thought to result from Na(+)-K(+)-ATPase inhibition, with elevated intracellular Na(+)(Na)causing increased intracellular Ca(2+)(Ca) via Na-Ca exchange. We studied the effects of ouabain on cat ventricular myocytes in Na(+)-free conditions where the exchanger is inhibited. Cell shortening and Ca transients (with fluo 4-AM fluorescence) were measured under voltage clamp during exposure to Na(+)-free solutions [LiCl or N-methyl-D-glucamine (NMDG) replacement]. Ouabain enhanced contractility by 121 +/- 55% at 1 micromol/l (n = 11) and 476 +/- 159% at 3 micromol/l (n = 8) (means +/- SE). Ca transient amplitude was also increased. The inotropic effects of ouabain were retained even after pretreatment with saxitoxin (5 micromol/l) or changing the holding potential to -40 mV (to inactivate Na(+) current). Similar results were obtained with both Li(+) and NMDG replacement and in the absence of external K(+), indicating that ouabain produced positive inotropy in the absence of functional Na-Ca exchange and Na(+)-K(+)-ATPase activity. In contrast, ouabain had no inotropic response in rat ventricular myocytes (10-100 micromol/l). Finally, ouabain reversibly increased Ca(2+) overload toxicity by accelerating the rate of spontaneous aftercontractions (n = 13). These results suggest that the cellular effects of ouabain on the heart may include actions independent of Na(+)-K(+)-ATPase inhibition, Na-Ca exchange, and changes in Na.

Activation of cardiac ryanodine receptors by cardiac glycosides

This study investigated the effects of cardiac glycosides on single-channel activity of the cardiac sarcoplasmic reticulum (SR) Ca2+ release channels or ryanodine receptor (RyR2) channels and how this action might contribute to their inotropic and/or toxic actions. Heavy SR vesicles isolated from canine left ventricle were fused with artificial planar lipid bilayers to measure single RyR2 channel activity. Digoxin and actodigin increased single-channel activity at low concentrations normally associated with therapeutic plasma levels, yielding a 50% of maximal effect of approximately 0.2 nM for each agent. Channel activation by glycosides did not require MgATP and occurred only when digoxin was applied to the cytoplasmic side of the channel. Similar results were obtained in human RyR2 channels; however, neither the crude skeletal nor the purified cardiac channel was activated by glycosides. Channel activation was dependent on [Ca2+] on the luminal side of the bilayer with maximal stimulation occurring between 0.3 and 10 mM. Rat RyR2 channels were activated by digoxin only at 1 microM, consistent with the lower sensitivity to glycosides in rat heart. These results suggest a model in which RyR2 channel activation by digoxin occurs only when luminal [Ca2+] was increased above 300 microM (in the physiological range). Consequently, increasing SR load (by Na+ pump inhibition) serves to amplify SR release by promoting direct RyR2 channel activation via a luminal Ca2+-sensitive mechanism. This high-affinity effect of glycosides could contribute to increased SR Ca2+ release and might play a role in the inotropic and/or toxic actions of glycosides in vivo.

Synthesis of 17 beta-(3'-thiophenyl)-5 beta-androstane-3 beta,14 beta-diol 3-D-glucopyranoside, an anti-inotropic cardiac glucoside

In a continuing effort to synthesize cardiac glucoside analogs with modified 17 beta-functional groups for pharmacologic testing, we used 3 beta-benzyloxy-5 beta-androst-15-en-17-one as an efficient intermediate. This report describes a preparation of 17 beta-(3'-thiophenyl)-5 beta-androstane-3 beta,14 beta-diol 3-D-glucopyranoside.

Digitalis glycosides: a discussion of the similarities and differences in actions and existing controversies

The writing of this review was initiated to answer the question of whether differences in the actions of the various digitalis glycosides exist and to discuss current controversies in the research area of the digitalis glycosides. Data obtained in our laboratory indicated that the effect of digoxin on postganglionic cardiac sympathetic neural discharge in the minute prior to the occurrence of arrhythmia differed from that of ouabain. This raised the question of whether data published in other laboratories would support the contention that differences in glycosides do exist. To answer this question, a review of the literature was begun. Our survey of these studies are cited in the tables of this review. These tables summarize the actions of glycosides in vivo and in vitro in different animal models. The reader should bear in mind that the data included within the tables do not represent an inclusive summary of all studies in the literature. For detailed review articles, the reader is referred to the following references: Gillis et al; Gillis and Quest; Roberts et al; Lathers and Roberts; Farah and Alousi; Benthe; Levitt et al; Smith and Haber; Somberg; Lee and Klaus; Mason; Schwartz. Furthermore the summary of the results for each particular study cited in the table may not, in all cases, include each finding of the published data. Nevertheless, the tables do provide a summary of data obtained in various species with different glycosides in several different areas of research, and as such, represent an abridged compendium for the research working in the field of digitalis glycosides. This review has been organized firstly to consider glycoside-induced alterations in the autonomic nervous system and, secondly, to examine their direct actions on the heart.

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.

Cardiac effects of six actodigin (AY-22,241)-related semisynthetic glycosides

Six actodigin (AY-22,241)-related semisynthetic glycosides (with the C-3 natural linkage of the lactone ring to the steroid nucleus, transposed to C-2) were assayed in failing hearts of canine heart-lung preparations. Two compounds, with additional small modifications in the steroid nucleus, were incapable of reversing heart failure. Two others, an isodigoxigenin and an isogitoxigenin derivative, were only slightly active. However, the two other actodigin-derived compounds, with methyl groups at the lactone C-4, were very active. Compound 5 had the methyl group in beta position and was 2.5 times more potent than actodigin. Compound 6, with the methyl group in alpha position, had a potency similar to that of actodigin. In the anesthetized and vagotomized dog, their toxic effects (to the point of atrioventricular dissociation) were short lasting and completely reversible. Both of these agents had a wide margin of safety (relationship between the minimal therapeutic, irregularity and lethal doses).

Cardenolide analogs. An explanation for the usual properties of AY 22241

AY 22241 (AY) was compared with digitoxigenin with respect to effects on myocardial contractility and inhibition of Na+, K+-ATPase. Ay was less potent than digitoxigenin but otherwise showed the same type of rapid onset and rapid reversal of activity usually associated with genins. Since AY is a glycoside, its behaviour was regarded as anomalous since glycosides usually show both slow onset and slow reversal of activity. Attempts were made to account for the genin-like properties of AY in terms of the model for the digitalis receptor proposed by Thomas et al. (1974a,b). It is suggested that the low potency and genin-like properties of AY may be due to the fact that the steroid is attached to the lactone through the carbon atom that is alpha to the carbonyl group. This feature could result in non-alignment of the sugar residue with the sugar binding site and might also reduce the effectiveness of the interaction of the steroid moiety with its binding site. A notable feature of AY toxicity was its extremely rapid progression to irreversible fibrillation.