Inhibition of spontaneous activity of rabbit atrioventricular node cells by KB-R7943 and inhibitors of sarcoplasmic reticulum Ca2+ ATPase

The atrioventricular node (AVN) can act as a subsidiary cardiac pacemaker if the sinoatrial node fails. In this study, we investigated the effects of the Na-Ca exchange (NCX) inhibitor KB-R7943, and inhibition of the sarcoplasmic reticulum calcium ATPase (SERCA), using thapsigargin or cyclopiazonic acid (CPA), on spontaneous action potentials (APs) and [Ca2+](i) transients from cells isolated from the rabbit AVN. Spontaneous [Ca2+](i) transients were monitored from undialysed AVN cells at 37 degrees C using Fluo-4. In separate experiments, spontaneous APs and ionic currents were recorded using the whole-cell patch clamp technique. Rapid application of 5 KB-R7943 slowed or stopped spontaneous APs and [Ca2+](i) transients. However, in voltage clamp experiments in addition to blocking NCX current (I-NCX) KB-R7943 partially inhibited L-type calcium current (I-Ca,I-L). Rapid reduction of external [Na+] also abolished spontaneous activity. inhibition of SERCA (using 2.5 p.M thapsigargin or 30 mu M CPA) also slowed or stopped spontaneous APs and [Ca2+], transients. Our findings are consistent with the hypothesis that sarcoplasmic reticulum (SR) Ca2+ release influences spontaneous activity in AVN cells, and that this occurs via [Ca2+](i)-Jr-activated I-NCX: however, the inhibitory action of KB-R7943 on I-Ca,I-L means that care is required in the interpretation of data obtained using this compound. (C) 2010 Elsevier Ltd. All rights reserved.