Ca2+/calmodulin potentiates I Ks in sinoatrial node cells by activating Ca2+/calmodulin-dependent protein kinase II

Abstract

The slow component of the delayed rectifier K(+) current (I Ks) plays an important role in the repolarization of action potentials in cardiac pacemaker cells and ventricular myocytes, and is regulated by various signaling pathways. Recent evidence has shown that calmodulin (CaM) is involved in modulation of diverse ion channels in cardiac myocytes under physiological and pathophysiological conditions. In the present study, we examined regulation of I Ks by Ca(2+)/CaM in guinea pig sinoatrial (SA) node cells using the whole-cell patch-clamp method. The density of I Ks was larger during intracellular dialysis with a higher Ca(2+) concentration (pCa 7, Ca (+)) compared to that with a low Ca(2+) concentration (pCa 10, Ca (-)). Intracellular application of CaM (400 nM) markedly potentiated I Ks with a Ca (+) pipette solution but not with a Ca (-) solution, thus showing that CaM potentiates I Ks in an intracellular Ca(2+)-dependent manner. Intracellular application of a specific Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor, autocamtide-2 inhibitory peptide (AIP, 500 nM), markedly reduced I Ks activity in the presence of higher intracellular Ca(2+). Similarly, bath application of another inhibitor, KN-93 (1 μM) also significantly suppressed I Ks. Finally, the stimulatory action on I Ks of Ca(2+)/CaM was abolished by pretreatment with KN-93. Taken together, these observations suggest that Ca(2+)/CaM stimulates I Ks in guinea pig SA node cells through activation of CaMKII. This enhancement of I Ks by CaMKII may be involved in modulation of SA node automaticity under physiological or pathophysiological condition.

The importance of Ca2+-dependent mechanisms for the initiation of the heartbeat

“…Phospholamban regulating the uptake of Ca 2+ into the SR is another important target. Observations concerning the effects of KN93, KN92, and autocamtide inhibitor peptide (AIP) on spontaneous action potentials in guinea pig SAN myocytes (Xie et al, 2015 ) were broadly similar to those in the rabbit.…”

“…Xie et al ( 2015 ) have recently presented evidence that the slow component of the delayed rectifier current (I Ks ) is regulated by a Ca 2+ -dependent process thought to be CaMKII in guinea-pig SA node. This current was found to be decreased when the cytosolic Ca 2+ concentration was reduced from 10 −7 to 10 −10 M (buffered by EGTA in a ruptured patch conditions).…”

Cytosolic calcium ions exert a major influence on the firing rate and maintenance of pacemaker activity in guinea-pig sinus node

“…In this regard, our data are in agreement with the observations of Noma's group (Himeno et al, 2011 ) in exhibiting significant action potential prolongation. Cytosolic calcium also enhances delayed rectifier potassium currents, particularly I Ks (Xie et al, 2015 ). Cytosolic calcium chelation would therefore be expected to reduce the repolarizing potassium current and both prolong action potential duration and lead to depolarization of the most negative diastolic potential, which is indeed seen in our cells.…”

Potassium channels in the sinoatrial node and their role in heart rate control

“…Unlike I Ks regulation, there is no agreement on whether I Kr currents can be increased by β adrenoreceptor modulation [74,75]. Uniquely in the SAN, calcium-calmodulin kinase II may activate I Ks and intriguingly this could affect calcium handling and channel modulation too [76]. …”