Cardiovascular responses to acute myocardial ischaemia in morphine-dependent rats

Morphine dependence protects rat kidney against ischaemia-reperfusion injury

Ischaemic preconditioning (IPC) protects the heart and kidneys against ischaemia-reperfusion (I/R) injury. It has been shown that opioid receptor activation can mimic cardiac IPC. In a kidney model of I/R, a single dose of morphine failed to mimic IPC. The aim of the present study was to determine the role of chronic morphine (dependence) in protection against renal I/R injury. Male Wistar rats were treated with increasing doses of morphine (20-30 mg/kg per day, s.c., for 5 days) to develop morphine dependence (MD). Three weeks before the I/R procedure, the right kidney was removed. Ischaemia-reperfusion injury was induced by clamping the left renal artery for 45 min, followed by 24 h reperfusion. Some MD rats were pretreated with naloxone (5 mg/kg, s.c.). Twenty-four hours later, creatinine and sodium concentrations were measured in serum and urine, then creatinine clearance (CCr) and the fractional excretion of sodium (FE(Na)) were calculated. Blood urea nitrogen (BUN) was measured only in serum samples. Kidneys were also assessed histologically for evidence of tissue injury. In the present study, MD decreased tissue injury (histological score), serum creatinine and BUN levels, increased CCr and decreased FE(Na) after I/R. Pretreatment with naloxone attenuated the protective effects of MD. Morphine dependence did not have any significant effect on urine volume. In conclusion, it seems that morphine dependence protects the kidney against I/R injury via opioid receptor-dependent pathways. Further studies are required to clearly determine the mechanisms involved.

Naloxone-induced cardiovascular depression in rats that had received chronic morphine-treatment

1. Cardiovascular changes in response to intravenous injection of naloxone were studied in pentobarbitone-anaesthetized rats which had been given morphine in their drinking water for 1-21 days. The mechanisms of the observed changes were investigated in intact animals and in isolated hearts and tail arteries. 2. In rats that had received chronic morphine-treatment, intravenous administration of naloxone caused immediate decreases in blood pressure, heart rate, left ventricular pressure and dLVP/dtmax which were followed by the occurrence of atrial or ventricular extrasystoles and other signs of opiate withdrawal such as faecal passage and muscle twitching. 3. The intensities of the naloxone-precipitated cardiovascular changes were directly related to the duration of chronic morphine pretreatment, reaching statistically significant levels on day 2 or 3 and maximal levels on day 7 or 14. This phenomenon disappeared on days 3 to 14 following opiate withdrawal in animals which had been treated previously with morphine for 21 days. 4. Either atropine or clonidine pretreatment significantly prevented the occurrence of faecal passage or muscle twitching during naloxone-precipitated opiate withdrawal. However, clonidine, but not atropine or yohimbine, abolished the decreases in various haemodynamic parameters. The occurrence of cardiac extrasystoles was not affected. 5. In isolated heart or tail artery preparations from chronically morphine-treated rats, naloxone administration did not elicit reactions which differed from those of the preparations from naive animals. These findings suggest that under pentobarbitone anaesthesia, the cardiovascular systems of rats that had received chronic morphine treatment exhibit inhibitory, instead of excitatory, reactions to naloxone-precipitated opiate withdrawal.

Plasma, cardiac tissue and brain morphine concentrations in acute and chronic morphine-treated rats

1. The plasma, cardiac tissues and brain morphine concentrations in rats after acute or chronic morphine treatment were measured by high-performance liquid chromatography. 2. Morphine concentrations in plasma and cardiac tissues were found to be significantly higher than those in brain after acute morphine injection. However, after chronic oral administration, morphine concentrations in plasma, cardiac tissues and brain were similar. 3. Brain concentrations of morphine following chronic administration were higher than those obtained after acute administration although concentrations in plasma and cardiac tissues were lower.

Ventricular noradrenaline concentrations in naïve and morphine-treated rats subjected to acute myocardial ischaemia

1 Ventricular noradrenaline concentrations in morphine-treated rats subjected to acute left coronary artery ligation were measured by high performance liquid chromatography with electrochemical detection. 2 In naïve rats, acute left coronary artery ligation induced a significant increase in right ventricular noradrenaline concentration at 5 min and significant decreases in left ventricular noradrenaline concentration at 3 and 10 min. 3 Acute morphine treatment did not significantly alter ventricular noradrenaline concentrations in rats subjected to acute coronary artery ligation. 4 Chronic morphine treatment caused significant declines in ventricular noradrenaline concentrations in rats subjected to acute coronary artery ligation. The reductions increased with duration of opiate treatment, and were reversed by opiate withdrawal. 5 These findings indicate that there is an increase in sympathetic activity during acute myocardial ischaemia. It is suggested that chronic morphine treatment may be able to retard this response, and consequently to lessen the occurrence of early ventricular arrhythmias resulting from acute myocardial ischaemia.