Abstract
Spontaneous firing of sinoatrial node cells (SANC) is regulated by sarcoplasmic reticulum (SR) generated local subsarcolemmal calcium releases (LCRs). LCRs appear during diastolic depolarization (DD) and activate an inward sodium-calcium exchange current to accelerate the DD rate and thus spontaneous SANC firing. Vascular endothelial growth factor (VEGF) receptors VEGFR1 and VEGFR2 activate PLC, and suppression of VEGFR-PLC signaling decreases calcium transients and contractility in ventricular myocytes. We tested the idea that VEGFR-PLC signaling may contribute to normal spontaneous beating of SANC.
We observed that expression of VEGFR1 (assessed by RT-qPCR) in rabbit sinoatrial node was comparable to that of β1-adrenergic receptors, but less than that in ventricle. The pan VEGFR1/2/3 inhibitor PTK787/ZK222584 (10 μmol/L) in a time-dependent manner: (1) suppressed spontaneous SANC beating rate by ∼50% (perforated patch-clamp technique); (2) markedly decreased the LCR size and number per each spontaneous cycle (confocal microscopy, calcium indicator Fluo-3) and prolonged the LCR period (the interval between action potential-induced calcium transient and occurrence of subsequent LCR). The PTK787/ZK222584-induced increase in the LCR period (from 348.2±30.3 to 619.9±103.4 msec; P<0.05) predicted the concomitant increase in the spontaneous cycle length (from 405.6±31.1 to 702.1±105.1 msec; P<0.05), suggesting that calcium cycling could be a major target of VEGFR-dependent regulation of SANC firing. All effects of PTK787/ZK222584 were reversible upon washout.
To elucidate whether signaling of VEGFR1 or VEGFR2 regulated spontaneous SANC firing, we employed a selective VEGFR2 inhibitor ZM-323881 (5 μmol/L), which suppressed spontaneous beating rate in only 2 of 10 SANC. These results indicate that VEGFR1, but not VEGFR2, is likely a key receptor that modulates automaticity in majority of SANC. To clarify downstream targets from VEGFR1 we employed PLC inhibitor U-73122, which decreased the LCR size, number and prolonged the LCR period. The inactive analog U-73343 was without effect. Because LCRs are critically dependent upon the SR calcium load, supplied by L-type calcium current (ICa,L), we examined effects of U-73122 on ICa,L. U-73122, but not U-73343, markedly suppressed ICa,L amplitude by ∼50%, leading to a decrease in the calcium influx and, as a result, to decrease in the LCR parameters, prolongation of the LCR period and spontaneous SANC cycle length. Thus, basal VEGFR1 signaling activates PLC, which modulates intracellular SR calcium cycling and LCR characteristics in SANC. We conclude that VEGFR1-PLC is a novel mechanism involved in the regulation of normal automaticity of cardiac pacemaker cells.
Acknowledgement/Funding
Intramural Research Program, National Institute on Aging, NIH
Sarcoplasmic Reticulum Ca
2+
Pumping Kinetics Regulates Timing of Local Ca
2+
Releases and Spontaneous Beating Rate of Rabbit Sinoatrial Node Pacemaker Cells
