Phosphodiesterases 3 and 4 regulate local Ca2+ releases and beating of pacemaker

兔SANC的自发跳动是由camp介导的、pka依赖的节律性的、局部肌浆网在舒张后期去极化期间的肌浆网Ca2+释放（lcr）控制的。虽然通过l型Ca2+通道的Ca2+内流确保LCR发生，但高基础PDE活性限制了LCR的发生。然而，PDE调节L型Ca2+电流，ICa，L的程度仍然是一个谜。我们确定了PDE亚型依赖的基础ICa、L、自发SANC放电率的控制程度；并将其与肾上腺素能受体（ar）激动剂异丙肾上腺素的效果进行比较。特异性PDE4抑制剂罗利普兰对ICa、L或自发搏动均无影响；相比之下，特异性PDE3抑制剂西洛胺增加了ICa、L和自发的SANC放电。（西洛胺+罗利普兰）同时抑制PDE3和PDE4使ICa，L升高；放大LCR尺寸（从5.90.58到8.60.50 m）；LCR周期由309.720.6缩短至214.33.9 msec；与广谱PDE抑制剂IBMX相当的自发SANC发射速率。这些效果甚至比-AR刺激产生的效果更大。因此，一致的PDE3和PDE4活性控制基础camp - pka依赖性磷酸化，抑制ICa，L，限制基础lcr和自发SANC放电速率。

Spontaneous beating of rabbit SANC is controlled by cAMP-mediated, PKA-dependent rhythmic, local subsarcolemmal Ca2+releases (LCRs) from sarcoplasmic reticulum during late diastolic depolarization. While Ca2+influx via L-type Ca2+ channels ensures LCR occurrence, high basal PDE activity limits LCRs. The extent to which PDE regulates L-type Ca2+ current, ICa,L, however, remains enigma. We determined the extent of PDE subtype-dependent control of basal ICa,L, spontaneous SANC firing rate; and compared those to the effect of -adrenergic receptor (-AR) agonist, isoproterenol. A specific PDE4 inhibitor, rolipram, had no effect, on either ICa,L or spontaneous beating; cilostamide, a specific PDE3 inhibitor, in contrast, increased both ICa,L and spontaneous SANC firing. Simultaneous inhibition of PDE3 and PDE4 by (cilostamide+ rolipram) increased ICa,L; amplified LCR size (from 5.90.58 to 8.60.50 m); decreased the LCR period (from 309.720.6 to 214.33.9 msec); and accelerated spontaneous SANC firing rate equivalent to broad-spectrum PDE inhibitor, IBMX. These effects were even greater than those produced by -AR stimulation. Thus, concerted PDE3 and PDE4 activities control basal cAMP-PKA-dependent phosphorylation and suppress ICa,L, limiting basal LCRs and spontaneous SANC firing rate.