Vinogradova Tatiana M, Sirenko Syevda, Lyashkov Alexey E, Younes Antoine, Li Yue, Zhu Weizhong, Yang Dongmei, Ruknudin Abdul M, Spurgeon Harold, Lakatta Edward G
Laboratory of Cardiovascular Science, Gerontology Research Center, NIA, NIH, 5600 Nathan Shock Dr, Baltimore, MD 21224-6825, USA.
Circ Res. 2008 Apr 11;102(7):761-9. doi: 10.1161/CIRCRESAHA.107.161679. Epub 2008 Feb 14.
Spontaneous beating of rabbit sinoatrial node cells (SANCs) is controlled by cAMP-mediated, protein kinase A-dependent local subsarcolemmal ryanodine receptor Ca(2+) releases (LCRs). LCRs activated an inward Na(+)/Ca(2+) exchange current that increases the terminal diastolic depolarization rate and, therefore, the spontaneous SANC beating rate. Basal cAMP in SANCs is elevated, suggesting that cAMP degradation by phosphodiesterases (PDEs) may be low. Surprisingly, total suppression of PDE activity with a broad-spectrum PDE inhibitor, 3'-isobutylmethylxanthine (IBMX), produced a 9-fold increase in the cAMP level, doubled cAMP-mediated, protein kinase A-dependent phospholamban phosphorylation, and increased SANC firing rate by approximately 55%, indicating a high basal activity of PDEs in SANCs. A comparison of specific PDE1 to -5 inhibitors revealed that the specific PDE3 inhibitor, milrinone, accelerated spontaneous firing by approximately 47% (effects of others were minor) and increased amplitude of L-type Ca(2+) current (I(Ca,L)) by approximately 46%, indicating that PDE3 was the major constitutively active PDE in the basal state. PDE-dependent control of the spontaneous SANC firing was critically dependent on subsarcolemmal LCRs, ie, PDE inhibition increased LCR amplitude and size and decreased LCR period, leading to earlier and augmented LCR Ca(2+) release, Na(+)/Ca(2+) exchange current, and an increase in the firing rate. When ryanodine receptors were disabled by ryanodine, neither IBMX nor milrinone was able to amplify LCRs, accelerate diastolic depolarization rate, or increase the SANC firing rate, despite preserved PDE inhibition-induced augmentation of I(Ca,L) amplitude. Thus, basal constitutive PDE activation provides a novel and powerful mechanism to decrease cAMP, limit cAMP-mediated, protein kinase A-dependent increase of diastolic ryanodine receptor Ca(2+) release, and restrict the spontaneous SANC beating rate.
兔窦房结细胞（SANCs）的自发搏动受cAMP介导、蛋白激酶A依赖的局部肌膜下兰尼碱受体Ca²⁺释放（LCRs）控制。LCRs激活内向Na⁺/Ca²⁺交换电流，增加舒张末期去极化速率，从而提高窦房结细胞的自发搏动率。窦房结细胞中的基础cAMP水平升高，提示磷酸二酯酶（PDEs）对cAMP的降解可能较低。令人惊讶的是，用广谱PDE抑制剂3'-异丁基甲基黄嘌呤（IBMX）完全抑制PDE活性后，cAMP水平升高了9倍，cAMP介导的、蛋白激酶A依赖的受磷蛋白磷酸化增加了一倍，窦房结细胞发放频率提高了约55%，表明窦房结细胞中PDEs具有较高的基础活性。对特异性PDE1至-5抑制剂的比较显示，特异性PDE3抑制剂米力农使自发发放频率加快约47%（其他抑制剂的作用较小），并使L型Ca²⁺电流（I(Ca,L)）幅度增加约46%，表明PDE3是基础状态下主要的组成型活性PDE。PDE依赖的对窦房结细胞自发发放的控制关键依赖于肌膜下LCRs，即PDE抑制增加了LCR的幅度和大小，缩短了LCR周期，导致更早且增强的LCR Ca²⁺释放、Na⁺/Ca²⁺交换电流以及发放频率增加。当用ryanodine使兰尼碱受体失活时，尽管PDE抑制仍能增加I(Ca,L)幅度，但IBMX和米力农均无法放大LCRs、加速舒张期去极化速率或增加窦房结细胞发放频率。因此，基础组成型PDE激活提供了一种新的强大机制，可降低cAMP、限制cAMP介导的、蛋白激酶A依赖的舒张期兰尼碱受体Ca²⁺释放增加，并限制窦房结细胞的自发搏动率。
