Original Article

Naloxone increases vascular responsiveness in chronic morphine treated rats and facilitates intracellular signaling in cultured vascular cells

Authors: Yu Wang, Jian-Guo Xu, Ying-Qun Yu, Hong-Yi Wang, Bo Jiang, Xiao-Yu Li


Aim: To probe the changes of vascular responsiveness and intracellular signaling during opiate withdrawal syndrome.
Methods: Morphine withdrawal syndrome in rats was precipitated by iv naloxone following daily injection of increasing dose of morphine for 2 weeks, the changes in mean arterial blood pressure (MBP) caused by acetylcholine (ACh) were recorded. Isolated mesenteric vascular beds were perfused with Kreb's solution containing different concentration of drugs. The cytoplasmic Ca2+ ([Ca2+]i) in Fura 2-loaded cultured bovine aortic endothelial cells (aec) and smooth muscle cells (smc) were assayed. The positive immunoreaction to the phosphorylated cyclic AMP responsive element-binding protein (phospho-CREB) in cultured smc was calculated.
Results: Naloxone 2 mg/kg iv following chronic morphine treatment precipitated severe opiate withdrawal signs in company with blunted hypotensive effect of Ach that was the same as chronic morphine treated rats before withdrawal. In the mesenteric vascular beds from chronic morphine treated rats, the EC50 of pressor effect of norepinephrine (NE) was decreased from (2.06 +/- 0.38) to (1.14 +/- 0.21) micromol/L (n = 8, P < 0.01) after the perfusion solution containing morphine 20 micromol/L was replaced by Kreb's containing naloxone 25 micromol/L. Furthermore, NE-induced perfusion pressure increases were completely prevented by Kreb's contained morphine 40 micromol/L. Morphine acutely applied to control smc produced some variable and naloxone-reversible [Ca2+]i changes, but naloxone did not. However, naloxone increased [Ca2+]i in two thirds of smc preincubated with morphine 0.1 or 0.5 mmol/L for 48 h from (97 +/- 20) to (167 +/- 29) nmol/L (n = 9, P < 0.01) and from (106 +/- 19) to (225 +/- 48) nmol/L (n = 10, P < 0.01), respectively, and it also increased the ratio of positive immunoreaction to phospho-CREB from (7.7 +/- 3.2) % to (19.6 +/- 4.7) % (n = 6, P < 0.01) in smc preincubated with morphine 0.5 mmol/L. In addition, naloxone decreased [Ca2+]i from (146 +/- 34) to (78 +/- 24) nmol/L in one third morphine-preincubated aec (n = 10, P < 0.01).
Conclusion: That naloxone enhances vascular responsiveness to NE in chronic morphine treatment rats may be relevant to [Ca2+]i transient facilitation in company with cAMP-dependent phosphorylation enhancement.

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