Opioid Effects on Contractility, Ca2+-transients and Intracellular pH in Cultured Cardiac Myocytes

Catherine Ela, Yonathan Hasin, Yael Eilam

Research output: Contribution to journalArticlepeer-review

32 Scopus citations


Morphine, the opioid-agonist, and the antagonist 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+],-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 drug: (a) An increase in [Ca2+]-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+]-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), increases Ca2+ influx leading to increased Ca2+-transients and (2) decreased intracellular pH leading to reduced myofibril responsiveness to Ca2+.

Original languageEnglish
Pages (from-to)599-613
Number of pages15
JournalJournal of Molecular and Cellular Cardiology
Issue number5
StatePublished - May 1993
Externally publishedYes


  • Cardiac myocytes
  • Contractility
  • Cytosolic-calcium
  • G-proteins
  • Intracellular-pH
  • Maloxone
  • Morphine


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