Asai T, Pelzer S, McDonald T F
Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia, Canada.
Mol Pharmacol. 1996 Nov;50(5):1262-72.
Low-to-moderate concentrations (< or = 3 microM) of forskolin (FSK) stimulated L-type Ca2+ current (ICa,L) and activated Cl- current (IC1) in guinea pig ventricular myocytes investigated under standard whole-cell conditions at 35 degrees. These stimulatory effects reached a steady state after several minutes and smoothly decayed after a short lag period on removal of the drug. Short (2-3 min) exposures to higher concentrations (10-100 microM) of FSK frequently had a multiphasic effect on ICa,L; marked stimulation during the first minute quickly faded during the next 1-2 min, and removal of the drug caused secondary stimulation that lasted for several minutes. Because the amplitude of cAMP-dependent ICl remained stable during the fade and secondary stimulation of ICa,L, the latter modulation of ICa,L seemed to be the result of a cAMP-independent inhibitory action of FSK on Ca2+ channels. Under conditions in which the stimulation of cAMP by FSK was slowed (22 degrees), rapid application of 10-30 microM FSK revealed that inhibition occurred within < 1 sec. In myocytes dialyzed with channel-up-modulating cAMP solution. 0.01-1 microM FSK had no effect on up-modulated currents, whereas high FSK rapidly and reversibly inhibited ICa,L by < or = 42% without affecting ICl. High FSK also inhibited ICa,L in myocytes dialyzed with protein kinase A inhibitor. External but not internal application of the inactive analog 1,9-dideoxy-FSK (30-100 microM) inhibited basal ICa,L. The inhibition was dependent on holding potential and involved a speeding up of ICa,L inactivation and a slowing of recovery from inactivation. We conclude that FSK inhibits cardiac ICa,L by reducing the availability of Ca2+ channels.
在35摄氏度的标准全细胞条件下研究豚鼠心室肌细胞时，低至中等浓度（≤3微摩尔）的福斯可林（FSK）刺激了L型钙电流（ICa,L）并激活了氯电流（IC1）。这些刺激作用在几分钟后达到稳定状态，在去除药物后的短暂延迟期后平稳衰减。短时间（2 - 3分钟）暴露于较高浓度（10 - 100微摩尔）的FSK通常对ICa,L有多相作用；在第一分钟内显著刺激，在接下来的1 - 2分钟内迅速消退，去除药物会引起持续数分钟的二次刺激。由于在ICa,L的消退和二次刺激期间，依赖cAMP的ICl幅度保持稳定，ICa,L的后一种调节似乎是FSK对钙通道的非cAMP依赖性抑制作用的结果。在FSK刺激cAMP的速度减慢（22摄氏度）的条件下，快速施加10 - 30微摩尔的FSK表明抑制在<1秒内发生。在用通道上调调节cAMP溶液透析的心肌细胞中，0.01 - 1微摩尔的FSK对上调调节的电流没有影响，而高浓度的FSK迅速且可逆地抑制ICa,L达≤42%，而不影响ICl。高浓度的FSK也抑制用蛋白激酶A抑制剂透析的心肌细胞中的ICa,L。无活性类似物1,9 - 二脱氧 - FSK（30 - 100微摩尔）的外部而非内部应用抑制基础ICa,L。这种抑制依赖于钳制电位，涉及ICa,L失活的加速和失活后恢复的减慢。我们得出结论认为，FSK通过降低钙通道的可用性来抑制心脏ICa,L。
