Damron D S, Van Wagoner D R, Moravec C S, Bond M
Department of Cardiovascular Biology, Cleveland Clinic Foundation, Ohio 44195-5069.
J Biol Chem. 1993 Dec 25;268(36):27335-44.
The release of arachidonic acid by phospholipases in response to cell surface receptor activation may be an important step in the initiation of inotropic events in cardiac muscle. Endothelin has been shown to activate phospholipase A2 and release arachidonic acid in isolated rat hearts. Endothelin also has a positive inotropic effect in cardiac muscle, suggesting that endothelin increases Ca2+ influx or the amount of Ca2+ released from the sarcoplasmic reticulum. We used suspensions of adult rat ventricular myocytes loaded with fura-2/AM to compare the effects of arachidonic acid and endothelin on Ca2+ transients evoked by extracellular ATP. We showed recently (Damron, D.S., and Bond, M. (1993) Circ. Res. 72, 376-386) that pretreatment of cardiac myocytes with arachidonic acid significantly potentiated the amplitude of the ATP-triggered Ca2+ transient. We now report that endothelin also enhances the ATP-triggered Ca2+ transient and that the effect of the combination of maximal doses of endothelin and arachidonic acid is additive. Neither endothelin nor arachidonic acid was found to affect the size of the sarcoplasmic reticulum Ca2+ store. The potentiating effects of both arachidonic acid and endothelin were sensitive to inhibitors of protein kinase C. Endothelin was also found to stimulate phospholipase C but not phospholipase A2. Application of arachidonic acid to individual cardiac muscle cells resulted in inhibition of the transient outward K+ current, whereas application of endothelin inhibited the delayed rectifier current. These effects of arachidonic acid and endothelin were additive, and both effects could be blocked by the protein kinase C inhibitor, staurosporine. Similarly, staurosporine inhibited endothelin-induced increases in isometric contractions in ventricular papillary muscle. We conclude that arachidonic acid and endothelin may be involved in the modulation of inotropic activity in cardiac muscle by means of protein kinase C-dependent inhibition of two distinct K+ channels. This would result in a prolongation of action potential duration and thus an increase in Ca2+ influx across the sarcolemma.
磷脂酶响应细胞表面受体激活而释放花生四烯酸，这可能是心肌收缩性事件起始过程中的重要一步。内皮素已被证明可激活磷脂酶A2并在离体大鼠心脏中释放花生四烯酸。内皮素对心肌也有正性肌力作用，提示内皮素可增加Ca2+内流或肌浆网释放的Ca2+量。我们使用负载fura-2/AM的成年大鼠心室肌细胞悬液，比较花生四烯酸和内皮素对细胞外ATP诱发的Ca2+瞬变的影响。我们最近发现（Damron, D.S., and Bond, M. (1993) Circ. Res. 72, 376 - 386），用花生四烯酸预处理心肌细胞可显著增强ATP触发的Ca2+瞬变的幅度。我们现在报告，内皮素也可增强ATP触发的Ca2+瞬变，且最大剂量的内皮素和花生四烯酸联合应用的效果是相加的。未发现内皮素和花生四烯酸会影响肌浆网Ca2+储存的大小。花生四烯酸和内皮素的增强作用均对蛋白激酶C抑制剂敏感。还发现内皮素可刺激磷脂酶C，但不刺激磷脂酶A2。将花生四烯酸应用于单个心肌细胞会导致瞬时外向K+电流受到抑制，而应用内皮素则会抑制延迟整流电流。花生四烯酸和内皮素的这些作用是相加的，且两种作用均可被蛋白激酶C抑制剂星形孢菌素阻断。同样，星形孢菌素可抑制内皮素诱导的心室乳头肌等长收缩增加。我们得出结论，花生四烯酸和内皮素可能通过蛋白激酶C依赖性抑制两种不同的K+通道参与心肌收缩性活动的调节。这将导致动作电位持续时间延长，从而增加跨肌膜的Ca2+内流。
