The inotropic effect of cardioactive glycosides in ventricular myocytes requires Na⁺-Ca²⁺ exchanger function

Glycoside-induced cardiac inotropy has traditionally been attributed to direct Na⁺-K⁺-ATPase inhibition, causing increased intracellular [Na⁺]and consequent Ca²⁺ gain via the Na⁺-Ca²⁺ exchanger (NCX). However, recent studies suggested alternative mechanisms of glycoside-induced inotropy: (1) direct activation of sarcoplasmic reticulum Ca²⁺ release channels (ryanodine receptors; RyRs); (2) increased Ca²⁺ selectivity of Na⁺ channels (slip-mode conductance); and (3) other signal transduction pathways. None of these proposed mechanisms requires NCX or an altered [Na⁺] gradient. Here we tested the ability of ouabain (OUA, 3 μM), digoxin (DIG, 20 μM) or acetylstrophanthidin (ACS, 4 μM) to alter Ca²⁺ transients in completely Na⁺-free conditions in intact ferret and cat ventricular myocytes. We also tested whether OUA directly activates RyRs in permeabilized cat myocytes (measuring Ca²⁺ sparks by confocal microscopy). In intact ferret myocytes (stimulated at 0.2 Hz), DIG and ACS enhanced Ca²⁺ transients and cell shortening during twitches, as expected. However, prior depletion of [Na⁺ ] i (in Na⁺-free, Ca²⁺-free solution) and in Na⁺-free solution (replaced by Li⁺) the inotropic effects of DIG and ACS were completely prevented. In voltage-clamped cat myocytes, OUA increased Ca²⁺ transients by 48 ± 4% but OUA had no effect in Na⁺-depleted cells (replaced by N-methyl-D-glucamine). In permeabilized cat myocytes, OUA did not change Ca²⁺spark frequency, amplitude or spatial spread (although spark duration was slightly prolonged). We conclude that the acute inotropic effects of DIG, ACS and OUA (and the effects on RyRs) depend on the presence of Na⁺ and a functional NCX in ferret and cat myocytes (rather than alternate Na⁺-independent mechanisms).