Effect of Cardiac Glycosides on Action Potential Characteristics and Contractility in Cat Ventricular Myocytes: Role of Calcium Overload

Abstract

There is increasing evidence that cardiac glycosides act through mechanisms distinct from inhibition of the sodium pump but which may contribute to their cardiac actions. To more fully define differences between agents indicative of multiple sites of action, we studied changes in contractility and action potential (AP) configuration in cat ventricular myocytes produced by six cardiac glycosides (ouabain, ouabagenin, dihydroouabain, actodigin, digoxin, and resibufogenin). AP shortening was observed only with ouabain and actodigin. There was extensive inotropic variability between agents, with some giving full inotropic effects before automaticity occurred whereas others produced minimal inotropy before toxicity. AP shortening was not a result of alterations in calcium current or the inward rectifier potassium current, but correlated with an increase in steady-state outward current (I ss ), which was sensitive to KB-R7943, a Na ϩ -Ca 2ϩ exchange (NCX) inhibitor.Interestingly, I ss was observed following exposure to ouabain and dihydroouabain, suggesting that an additional mechanism is operative with dihydroouabain that prevents AP shortening.Further investigation into differences in inotropy between ouabagenin, dihydroouabain and ouabain revealed almost identical responses under AP voltage clamp. Thus all agents appear to act on the sodium pump and thereby secondarily increase the outward reverse mode NCX current, but the extent of AP duration shortening and positive inotropy elicited by each agent is limited by development of their toxic actions. The quantitative differences between cardiac glycosides suggest that mechanisms independent of sodium pump inhibition may result from an altered threshold for calcium overload possibly involving direct or indirect effects on calcium release from the sarcoplasmic reticulum.

Digitalis: new actions for an old drug

“…Table 1 summarizes data for ouabain, dihydroouabain, and ouabagenin at which 50% of I p is inhibited (IC 50 , Table 1, column A). The order of potency is in general agreement with that of inotropy (106). However, ratios of IC 50 for I p to effective concentration at 50% of maximal inotropic effect (EC 50 , Table 1, column B) are different (ouabagenim Ͻ ouabain Ͻ dihydroouabain, A/B), demonstrating variation in the relationship between pump inhibition and inotropy.…”

“…When intracellular Na ϩ is increased during Na ϩ pump inhibition, Ca 2ϩ efflux is diminished even at resting potential where forward-mode exchange would ordinarily remove the Ca 2ϩ that entered during systole (7). In addition, Ca 2ϩ influx via the exchanger in the reverse mode at potentials positive to E NCX is actually promoted by the same increase in Na ϩ resulting from pump inhibition thus causing positive inotropy and the development of SR Ca 2ϩ overload toxicity (68,106). Consequently, cardiotonic steroids that cause both an increase in Na i ϩ and membrane depolarization secondary to Na ϩ pump inhibition, such as ouabain and digoxin, respond with an increase in force as a result of changes in E NCX .…”

“…However, a singular action via NKA inhibition and activation of NCX precludes the possibility that different glycosides might possess different cellular actions and toxicto-therapeutic ratios, because it requires a fixed relationship among intracellular Na ϩ concentration ([Na ϩ ] i ), [Ca 2ϩ ] i , inotropy, and toxicity. Profound differences are known to exist between agents, however, in toxic-to-therapeutic ratios in vivo (2,84) and in action potential configuration in vitro (43,56,106,123,130,131). For example, Karagueuzian and Katzung (60) and Ruch et al (106) reported a wide variety of effects on resting potential, action potential characteristics, inotropy, and the development of toxicity for a series of different glycosides.…”

Late Sodium Current Inhibition Alone with Ranolazine Is Sufficient to Reduce Ischemia- and Cardiac Glycoside-Induced Calcium Overload and Contractile Dysfunction Mediated by Reverse-Mode Sodium/Calcium Exchange

“…Intracellular calcium (Ca 2ϩ i ) overload is a key contributor to the mechanical and electrical dysfunction observed in cardiac ischemia-reperfusion (IR) injury and cardiac glycoside toxicity (Barry, 1987;Tani, 1990;Ferrari et al, 1993;Silverman and Stern, 1994;Piper et al, 2003;Ruch et al, 2003). Excessive Ca 2ϩ influx mediated by reverse mode (RM) operation of the cardiac Na ϩ -Ca 2ϩ exchanger 1.1 (NCX1.1) is thought be the major mediator of the observed Ca 2ϩ i overload.…”

Protective effect of distillated Nerium oleander on heart of type 2 diabetic rats

“…There are many studies suggesting that these ionic currents have been affected in the animal models of diabetes (10,11,12,13). Applications of cardiac glycosides to healthy normal cardiomyocytes on the other hand results not only in shortening of the measured APD values but also resulted in a decrement in the measured I CaL in a time dependent manner (14,15). Among the many other explanations (changes in the steady state inactivation or decrease in maximum conductance of the channel), reduction in the calcium equilibrium potential can be the best answer for why the L type calcium currents were reduced with the cardiac glycosides application.…”