Ela C, Hasin Y, Eilam Y
Department of Bacteriology, Hebrew University-Hadassah Medical School, Jerusalem, Israel.
J Mol Cell Cardiol. 1993 May;25(5):599-613. doi: 10.1006/jmcc.1993.1069.
Morphine, the opioid-agonist, and the antagonists naloxone and levallorphan exerted direct effects on spontaneously-contracting cultures of cardiac myocytes from neonatal rats. Naloxone and levallorphan induced an increase in the amplitude of systolic cell motion (ASM) and in the size of [Ca2+]i-transients, measured as indo-1 fluorescence ratio (IFR), whereas morphine caused an increase in IFR with no change in ASM. Both morphine and naloxone caused a transient increase in 45Ca2+ influx into the cardiomyocytes. Analysis of the relationship between changes in ASM and IFR indicated dual action of the drugs: (a) An increase in [Ca2+]i-transients elicited by morphine and the antagonists, apparently resulting from a transient increase of Ca2+ influx. (b) Altered myofibril responsiveness to Ca2+; the agonists decreased it, and the antagonists increased it. Intracellular pHi measurements in cardiomyocytes loaded with the fluorescent indicator BCECF revealed that morphine caused acidosis and the antagonists caused alkalosis. These pH changes were inhibited by pertussis-toxin, protein kinase inhibitor K323a, phorbol-ester and ethylisopropyl-amiloride, indicating pathways mediated by GTP-binding proteins and altered activities of protein kinase C and Na+/H+ exchanger. Preincubation with pertussis toxin prior to the addition of morphine prevented the decrease in the myofibril responsiveness to Ca2+ as well as the decrease in pHi but did not affect the increase in [Ca2+]i-transients and the increase in the rate of Ca2+ influx. As a result, addition of morphine after preincubation with pertussis toxin caused a positive inotropic effect. Our results indicate that morphine acts by two different pathways distinguishable by their sensitivity to pertussis toxin (1), increased Ca2+ influx leading to increased Ca(2+)-transients and (2) decreased intracellular pH leading to reduced myofibril responsiveness to Ca2+.
阿片类激动剂吗啡以及拮抗剂纳洛酮和左洛啡烷对新生大鼠心肌细胞的自发收缩培养物产生直接影响。纳洛酮和左洛啡烷可使收缩期细胞运动幅度（ASM）以及以indo-1荧光比率（IFR）衡量的[Ca2+]i瞬变幅度增大，而吗啡可使IFR升高，ASM无变化。吗啡和纳洛酮均可使45Ca2+流入心肌细胞的量短暂增加。对ASM和IFR变化之间关系的分析表明这些药物具有双重作用：（a）吗啡和拮抗剂引起的[Ca2+]i瞬变增加，显然是由于Ca2+流入量短暂增加所致。（b）肌原纤维对Ca2+的反应性改变；激动剂使其降低，拮抗剂使其升高。用荧光指示剂BCECF加载的心肌细胞内pHi测量结果显示，吗啡引起酸中毒，拮抗剂引起碱中毒。这些pH变化受到百日咳毒素、蛋白激酶抑制剂K323a、佛波酯和乙基异丙基氨氯吡咪的抑制，表明这些途径由GTP结合蛋白介导，并涉及蛋白激酶C和Na+/H+交换体活性的改变。在添加吗啡之前先用百日咳毒素预孵育可防止肌原纤维对Ca2+的反应性降低以及pHi降低，但不影响[Ca2+]i瞬变增加和Ca2+流入速率增加。因此，在用百日咳毒素预孵育后添加吗啡会产生正性肌力作用。我们的结果表明，吗啡通过两种不同途径起作用，这两种途径可通过它们对百日咳毒素的敏感性来区分：（1）Ca2+流入增加导致Ca(2+)瞬变增加；（2）细胞内pH降低导致肌原纤维对Ca2+的反应性降低。
