Role of L-type Cav1.3 Ca2+ channels in Ca2+ handling and SAN pacemaker activity altered by external conditions

Abstract

Membrane currents and Ca 2+ handling generate Sinoatrial node (SAN) automaticity. Several studies showed the negative effect of membrane current impairment, while it is unknown how abnormal Ca 2+ affect pacemaking. To investigate SAN automaticity when Ca 2+ handling is altered by external conditions. We video recorded contraction amplitude and spontaneous rate of SAN cells. In WT, 0.9 mM external Ca 2+ ([Ca 2+ ] 0 ) reduces cell shortening, while 3 mM increased it, compared to control solution (1.8 mM [Ca 2+ ] 0 ). Lowering [Ca 2+ ] 0 to 0.9 mM did not affect WT automaticity, while high [Ca 2+ ] 0 dysregulate it without changing the average rate. L-type channels (LTCCs) are the main Ca 2+ source in SAN cells, with Ca v 1.3 as the dominant isoform and Ca v 1.2 less expressed. To discern their role during abnormal [Ca 2+ ] 0 , we used mice lacking Ca v 1.3 channels (Ca v 1.3 −/− ). As in WT, 0.9 mM [Ca 2+ ] 0 decreased cell shortening in Ca v 1.3 −/− , while only a tendency to increase was determined by 3 mM [Ca 2+ ] 0 . Also, high [Ca 2+ ] 0 significantly reduced spontaneous rate in Ca v 1.3 −/− cells. Stimulation of WT cells with the LTCC dihydropyridine agonist BayK 8644 (BayK) increased cell shortening and spontaneous rate. Under BayK, Ca v 1.3 −/− cells showed similar increase of contraction amplitude but reduced rate acceleration than WT. We repeated this protocol with a mouse where Ca v 1.2 is dihydropyridines insensitive (Cav1.2 DHP−/− ). Cav1.2 DHP−/− SAN cells responded like WT to abnormal [Ca 2+ ] 0 . Instead, BayK caused rate increase equivalent to WT but reduced contraction shortening in Cav1.2 DHP−/− . Conclusion Abnormal increase of Ca 2+ influx dysregulates automaticity in WT cells, likely through hyper-stimulation of Ca 2+ activated currents. Moreover, selective stimulation of Ca v 1.2 and/or Ca v 1.3 with BayK suggests that Ca v 1.2 maintain the bulk of Ca 2+ allowing proper contraction, while Ca v 1.3 mainly carry the influx of Ca 2+ needed to trigger SAN automaticity.