Boutjdir M, Méry P F, Hanf R, Shrier A, Fischmeister R
Laboratoire de Physiologie Cellulaire Cardiaque, INSERM U-241, Université de Paris-Sud, Orsay, France.
Mol Pharmacol. 1990 Dec;38(6):758-65.
The diterpene forskolin is widely known for its ability to directly activate adenylyl cyclase and consequently increase intracellular cAMP. In cardiac cells, one result is a cAMP-mediated increase in the L-type Ca2(+)-channel current (ICa). However, forskolin was also shown recently to affect a number of ionic channels in noncardiac cells by mechanisms that do not involve activation of adenylyl cyclase. The present study reveals such an effect of forskolin on cardiac Ca2+ channels. Indeed, under appropriate conditions, forskolin was found to cause an inhibition of ICa. Although the stimulation of adenylyl cyclase and ICa requires micromolar concentrations of forskolin, the inhibitory effect of forskolin was observed in the nanomolar range of concentrations, i.e., 2-3 orders of magnitude lower. This high affinity forskolin inhibition of ICa was observed when ICa was previously enhanced via a cAMP-dependent pathway, but not when ICa was at its basal level or when the current was elevated by the dihydropyridine Bay K 8644. The inhibitory effect occurred at a site of action remote from adenylyl cyclase, because forskolin similarly inhibited ICa that had been previously elevated by isoprenaline (a beta-adrenergic agonist) or directly by intracellular perfusion with cAMP. Under these conditions, forskolin was inhibitory when applied to either side of the cell membrane, but only in its lipid-soluble form. The inhibitory effect of forskolin appeared to be independent of membrane potential and was not accompanied by a change in the time constants of ICa activation and inactivation. This may indicate that forskolin mainly reduces the number of functional Ca2+ channels without changing the gating of individual channels. However, the reduction in ICa amplitude was not equally distributed among the different exponential components that constitute ICa, which suggests that forskolin also modifies the resting state of the channels. This novel high affinity forskolin inhibition of ICa may take place at some step in the pathway between cAMP and Ca2+ channel phosphorylation and/or at Ca2+ channels only after they have been phosphorylated.
二萜类化合物福斯高林以其直接激活腺苷酸环化酶并因此增加细胞内cAMP的能力而广为人知。在心肌细胞中，一个结果是cAMP介导的L型Ca2(+)通道电流（ICa）增加。然而，最近也发现福斯高林通过不涉及腺苷酸环化酶激活的机制影响非心肌细胞中的多种离子通道。本研究揭示了福斯高林对心脏Ca2+通道的这种作用。实际上，在适当条件下，发现福斯高林会导致ICa受到抑制。虽然刺激腺苷酸环化酶和ICa需要微摩尔浓度的福斯高林，但在纳摩尔浓度范围内（即低2 - 3个数量级）就观察到了福斯高林的抑制作用。当ICa先前通过cAMP依赖性途径增强时，观察到了这种高亲和力的福斯高林对ICa的抑制作用，但当ICa处于基础水平或电流由二氢吡啶Bay K 8644升高时则未观察到。抑制作用发生在远离腺苷酸环化酶的作用位点，因为福斯高林同样抑制先前由异丙肾上腺素（一种β - 肾上腺素能激动剂）或直接通过细胞内灌注cAMP升高的ICa。在这些条件下，当以脂溶性形式应用于细胞膜的任一侧时，福斯高林具有抑制作用。福斯高林的抑制作用似乎与膜电位无关，并且不伴随着ICa激活和失活时间常数的变化。这可能表明福斯高林主要减少功能性Ca2+通道的数量，而不改变单个通道的门控。然而，ICa幅度的降低在构成ICa的不同指数成分中分布并不均匀，这表明福斯高林也改变了通道的静息状态。这种新型的高亲和力福斯高林对ICa的抑制作用可能发生在cAMP与Ca2+通道磷酸化之间的途径中的某个步骤，和/或仅在Ca2+通道被磷酸化之后发生在Ca2+通道上。
