Distinct localization and modulation of Ca_v1.2 and Ca_v1.3 L‐type Ca²⁺ channels in mouse sinoatrial node

Abstract

Key points• In the sinoatrial node (SAN), Ca v 1 voltage-gated Ca 2+ channels mediate L-type currents that are essential for normal cardiac pacemaking.• Both Ca v 1.2 and Ca v 1.3 Ca 2+ channels are expressed in the SAN but how their distinct properties affect cardiac pacemaking is unknown.• Here, we show that unlike Ca v 1.2, Ca v 1.3 undergoes voltage-dependent facilitation and colocalizes with ryanodine receptors in sarcomeric structures.• By mathematical modelling, these properties of Ca v 1.3 can improve recovery of pacemaking after pauses and stabilize SAN pacemaking during excessively slow heart rates.• We conclude that voltage-dependent facilitation and colocalization with ryanodine receptors distinguish Ca v 1.3 from Ca v 1.2 channels in the SAN and contribute to the major impact of Ca v 1.3 on pacemaking.Abstract Dysregulation of L-type Ca 2+ currents in sinoatrial nodal (SAN) cells causes cardiac arrhythmia. Both Ca v 1.2 and Ca v 1.3 channels mediate sinoatrial L-type currents. Whether these channels exhibit differences in modulation and localization, which could affect their contribution to pacemaking, is unknown. In this study, we characterized voltage-dependent facilitation (VDF) and subcellular localization of Ca v 1.2 and Ca v 1.3 channels in mouse SAN cells and determined how these properties of Ca v 1.3 affect sinoatrial pacemaking in a mathematical model. Whole cell Ba 2+ currents were recorded from SAN cells from mice carrying a point mutation that renders Ca v 1.2 channels relatively insensitive to dihydropyridine antagonists. The Ca v 1.2-mediated current was isolated in the presence of nimodipine (1 μM), which was subtracted from the total current to yield the Ca v 1.3 component. With strong depolarizations (+80 mV), Ca v 1.2 underwent significantly stronger inactivation than Ca v 1.3. VDF of Ca v 1.3 was evident during recovery from inactivation at a time when Ca v 1.2 remained inactivated. By immunofluorescence, Ca v 1.3 colocalized with ryanodine receptors in sarcomeric structures while Ca v 1.2 was largely restricted to the delimiting plasma membrane. Ca v 1.3 VDF enhanced recovery of pacemaker activity after pauses and positively regulated pacemaking during slow heart rate in a numerical model of mouse SAN automaticity, including preferential coupling of Ca v 1.3 to ryanodine receptor-mediated Ca 2+ release. We conclude that strong VDF and colocalization with

G protein-gated I KACh channels as therapeutic targets for treatment of sick sinus syndrome and heart block

“…To this end, we used mice in which Ca v 1.2 channels have been rendered insensitive to dihydropyridines (DHPs) via point mutation, eliminating channel sensitivity to these drugs (Ca v 1.2 DHP−/− mice) (26). Indeed, because the mouse heart expresses both Ca v 1.2 and Ca v 1.3 channels, Ca v 1.2 DHP−/− mice permit isolation of the effects of selective inhibition of Ca v 1.3-mediated I Ca,L (26,27). In vivo telemetric recordings of Ca v 1.2 DHP−/− mice injected with the DHP blocker amlodipine showed heart rate slowing that was quantitatively similar to the heart rate slowing observed in Ca v 1.3 −/− mice (SI Appendix, Fig.…”

Mechanisms underlying the cardiac pacemaker: the role of SK4 calcium-activated potassium channels

“…In fact, their sustained activity in phase 2 of an AP (plateau) is necessary for a proper interaction between the sarcomeric molecules and a contraction of the myocytes. Ca v 1.2 and Ca v 1.3 are expressed in the SAN pacemaker cells [61] , but unlike their role in the working myocytes, these currents are essential for generating the action potential upstroke of the "low-velocity" pacemaker cells. Ca v 1.2 is a ubiquitous current that is expressed equally in all human heart regions, whereas Ca v 1.3 was expressed more specifically in the SAN and the human AVN [65] .…”

“…A typical trace of an ECG shows the "P-wave", the "P-Q interval" (isoelectric line), the "QRS complex", the "T-wave" and the "U-wave". [60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75] in the SAN). This "SAN overdrive suppression" on the other elements of the conductive network explains its leading role in driving the pacemaker.…”

“…Two different subfamilies of voltage-gated calcium channels are expressed in the heart pacemaker tissue [59][60][61] , the highthreshold "L-type" (Ca v 1.2, Ca v 1.3) and the low-threshold "T-type" channels (Ca v 3.1, Ca v 3.2), which are distinguished by their different electrophysiological and pharmacological properties. The high-threshold L-type channels begin to be activated at -30 mV, whereas the T-type channels are activated at more hyperpolarizing voltages (approximately -60 mV).…”

High Heart Rate in Pregnancy Is Modulated by Augmented Expression of an Ion Channel, HCN-2, in Pacemaker Tissue

“…[11][12][13] The contribution of both T-and L-type calcium channels to the pacemaker potential or pacemaker depolarization is an important area that our model in its present form cannot accurately recapitulate. 14 It is also known that in the mouse sinoatrial node, primary pacemaker activity (heart rate) is modulated by the expression of Na + channels. The resulting activation of inward current takes place very late in the diastolic depolarization and enhanced excitability.…”