Takimoto K, Levitan E S
Department of Pharmacology, University of Pittsburgh, PA 15261.
Circ Res. 1994 Dec;75(6):1006-13. doi: 10.1161/01.res.75.6.1006.
Multiple voltage-gated K+ channels contribute to the repolarization phases of the cardiac action potential and are targets of several antiarrhythmic drugs. The Kv1.5 K+ channel gene is expressed in the heart, and heterologous expression of this gene generates a slowly inactivating K+ current. Previously, we found that glucocorticoids specifically upregulate pituitary Kv1.5 gene expression. To test whether these steroids might also induce Kv1.5 gene expression in the heart, cardiac channel mRNA and protein were measured by RNase protection assay and by immunoblotting with antibody specific for the extracellular domain of Kv1.5 polypeptide. Kv1.5 mRNA and immunoreactive protein appeared to be more abundant in rat ventricle than atrium. Reduction of endogenous glucocorticoids by adrenalectomy decreased ventricular Kv1.5 mRNA approximately 8-fold, which was estimated by using cyclophilin mRNA as an internal control. Kv1.5 immunoreactive protein also decreased approximately 6-fold. Injection of dexamethasone into adrenalectomized rats acted within a day to increase ventricular Kv1.5 mRNA and immunoreactive protein approximately 50-fold and approximately 20-fold, respectively. In contrast, atrial Kv1.5 mRNA expression was unaffected by either adrenalectomy or injection of the glucocorticoid agonist. Furthermore, dexamethasone-induced upregulation was specific for Kv1.5, since whole-heart Kv1.4 and Kv2.1 mRNA levels, as well as ventricular Kv2.1 mRNA expression, were unchanged. Thus, dexamethasone specifically upregulates Kv1.5 K+ channel gene expression in rat ventricle but not atrium. Glucocorticoids may affect excitability of ventricular myocytes and the efficacy of clinically useful drugs by changing the expression of the Kv1.5 K+ channel.
多种电压门控性钾离子通道参与心脏动作电位的复极化过程，并且是多种抗心律失常药物的作用靶点。Kv1.5钾离子通道基因在心脏中表达，该基因的异源表达可产生一种缓慢失活的钾电流。此前，我们发现糖皮质激素可特异性上调垂体Kv1.5基因的表达。为了检测这些类固醇是否也能诱导心脏中Kv1.5基因的表达，我们通过核糖核酸酶保护试验以及用针对Kv1.5多肽细胞外结构域的特异性抗体进行免疫印迹，来检测心脏通道的信使核糖核酸和蛋白质。Kv1.5信使核糖核酸和免疫反应性蛋白在大鼠心室中似乎比心房中更为丰富。通过肾上腺切除术降低内源性糖皮质激素后，以亲环蛋白信使核糖核酸作为内参进行估算，心室Kv1.5信使核糖核酸减少了约8倍。Kv1.5免疫反应性蛋白也减少了约6倍。给肾上腺切除的大鼠注射地塞米松，一天内即可使心室Kv1.5信使核糖核酸和免疫反应性蛋白分别增加约50倍和约20倍。相比之下，心房Kv1.5信使核糖核酸的表达不受肾上腺切除术或糖皮质激素激动剂注射的影响。此外，地塞米松诱导的上调对Kv1.5具有特异性，因为全心的Kv1.4和Kv2.1信使核糖核酸水平以及心室Kv2.1信使核糖核酸的表达均未改变。因此，地塞米松可特异性上调大鼠心室而非心房中Kv1.5钾离子通道基因的表达。糖皮质激素可能通过改变Kv1.5钾离子通道的表达来影响心室肌细胞的兴奋性以及临床常用药物的疗效。
