title: In vitro electrophysiologic effects of morphine in rabbit ventricular myocytes.
abstract: BACKGROUND
  Morphine is widely used in patients undergoing surgical operations and is also reported to mediate cardioprotection of preconditioning. The current study determined effects of morphine at therapeutic to pharmacologic concentrations on cardiac action potential, L-type Ca2+ current (ICa.L), delayed rectifier K+ current (IK), and inward rectifier K+ current (IK1) in isolated rabbit ventricular myocytes.
  
  
  METHODS
  Ventricular myocytes were enzymatically isolated from rabbit hearts. Action potential and membrane currents were recorded in current and voltage clamp modes.
  
  
  RESULTS
  Morphine at concentrations from 0.01 to 1 microM significantly prolonged cardiac action potential, and at 0.1 and 1 microM slightly but significantly hyperpolarized the resting membrane potential. In addition, morphine at 0.1 microM significantly augmented ICa.L (at +10 mV) from 5.9 +/- 1.9 to 7.3 +/- 1.7 pA/pF (by 23%; P < 0.05 vs. control) and increased IK1 (at -60 mV) from 2.8 +/- 1.0 to 3.5 +/- 0.9 pA/pF (by 27%; P < 0.05 vs. control). Five microM naltrindole (a selective delta-opioid receptor antagonist) or 5 microM norbinaltorphimine (a selective kappa-opioid receptor antagonist) prevented the increase in ICa.L induced by morphine, but 5 microM CTOP (a selective mu-opioid receptor antagonist) did not. The three types of opioid antagonists did not affect the augmentation of IK1 by morphine. Morphine had no effect on IK.
  
  
  CONCLUSIONS
  These results indicate that morphine prolongs action potential duration by increasing ICa.L, an effect mediated by delta- and kappa-opioid receptors. It also hyperpolarizes cardiac resting membrane potential by increasing IK1, which is not mediated by opioid receptors.
authors.affiliation: Department of Physiology, The University of Hong Kong, Pokfulam, Hong Kong, SAR, China.
authors.forename: Guo-Sheng
authors.initials: GS
authors.lastname: Xiao
authors.forename: Jing-Jun
authors.initials: JJ
authors.lastname: Zhou
authors.forename: Guan-Ying
authors.initials: GY
authors.lastname: Wang
authors.forename: Chun-Mei
authors.initials: CM
authors.lastname: Cao
authors.forename: Gui-Rong
authors.initials: GR
authors.lastname: Li
authors.forename: Tak-Ming
authors.initials: TM
authors.lastname: Wong
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000378:metabolism; D020746:Calcium Channels, L-Type / Q000187:drug effects / Q000502:physiology; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000502:physiology; D009271:Naltrexone / Q000031:analogs & derivatives / Q000494:pharmacology; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology; D011817:Rabbits
chemical_list: D020746:Calcium Channels, L-Type; D015221:Potassium Channels; C051844:norbinaltorphimine; D009271:Naltrexone; D009020:Morphine; C055382:naltrindole; D002118:Calcium