Doxorubicin Stimulates the Na+/Ca2+ Exchanger in Ventricular Cardiomyocytes

Abstract

Doxorubicin (DOX) is a widely used antineoplastic agent that exerts cardiotoxic effects. This study analyzed the effects of a clinically relevant DOX concentration (30 μM) on Ca2+ transport in rat ventricular myocytes. DOX decreased the sarcoplasmic reticulum (SR) Ca2+ content ([Ca2+]SR) (15%, p<0.001), however without significant change in Ca2+ transient amplitude and kinetics, or in the fraction of [Ca2+]SR released at a twitch. DOX abbreviated [Ca2+]i decline of caffeine-evoked transients (p<0.001). Estimates of the integrated Ca2+ fluxes attributable to individual transporters during twitch [Ca2+]i decline indicated a small depression of SR Ca2+ uptake (12%, p<0.05), but great enhancement of Ca2+ efflux via the Na+/Ca2+ exchanger (NCX) (90%, p<0.05), so that the contribution of the latter to cytosolic Ca2+ removal more than doubled. To promote NCX-mediated Ca2+ influx in the absence of electrical stimulation, extracellular [Na+] was lowered from 140 to 105 mM. In these conditions, DOX exposure increased both the rate constant of [Ca2+]i rise (12.8±1.4 vs. 4.8±0.8 nM.s-1) and the rate of spontaneous Ca2+ transients (0.18±0.04 vs. 0.05±0.01 Hz, p<0.05). After SR function inhibition with thapsigargin, spontaneous Ca2+ transients were absent, but NCX-mediated increase in [Ca2+]i remained greater in the presence of DOX (190±35 vs. 74±18 nM.min-1, p<0.01), which was abolished by inhibition of reverse NCX with 10 μM KB-R7943 (−29±6 nM.min-1, p<0.01). The results indicate that DOX altered mainly diastolic Ca2+ handling, causing mild inhibition of SR Ca2+ uptake, but marked increase of Ca2+ transport via NCX. The latter effect does not seem to be due to thermodynamic factors, but probably to NCX stimulation, as both Ca2+ influx and efflux were enhanced.