Tempol Reverses the Negative Effects of Morphine on Arterial Blood-Gas Chemistry and Tissue Oxygen Saturation in Freely-Moving Rats

Abstract

We have reported that pretreatment with the clinically approved superoxide dismutase mimetic, Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl), blunts the cardiorespiratory depressant responses elicited by a subsequent injection of fentanyl, in halothane-anesthetized rats. The objective of the present study was to determine whether Tempol is able to reverse the effects of morphine on arterial blood-gas (ABG) chemistry in freely-moving Sprague Dawley rats. The intravenous injection of morphine (10 mg/kg) elicited substantial decreases in pH, pO₂ and sO₂ that were accompanied by substantial increases in pCO₂ and Alveolar-arterial gradient, which results in diminished gas-exchange within the lungs. Intravenous injection of a 60 mg/kg dose of Tempol 15 min after the injection of morphine caused minor improvements in pO₂ and pCO₂ but not in other ABG parameters. In contrast, the 100 mg/kg dose of Tempol caused an immediate and sustained reversal of the negative effects of morphine on arterial blood pH, pCO₂, pO₂, sO₂ and Alveolar-arterial gradient. In other rats, we used pulse oximetry to determine that the 100 mg/kg dose of Tempol, but not the 60 mg/kg dose elicited a rapid and sustained reversal of the negative effects of morphine (10 mg/kg, IV) on tissue O₂ saturation (SpO₂). The injection of morphine caused a relatively minor fall in mean arterial blood pressure that was somewhat exacerbated by Tempol. These findings demonstrate that Tempol can reverse the negative effects of morphine on ABG chemistry in freely-moving rats paving the way of structure-activity and mechanisms of action studies with the host of Tempol analogues that are commercially available.

Keywords: arterial blood pressure; blood-gas chemistry; morphine; rats; tempol; tissue oxygen saturation.

FIGURE 1

Changes in pH, pCO₂, pO₂ and sO₂ elicited by a bolus injection of morphine (10 mg/kg, IV) and subsequent injection of vehicle (saline) or Tempol (, 60 mg/kg, IV; Tempol 60) in freely-moving rats. The data are presented as mean ± SEM. The numbers of rats in the vehicle and Tempol 60 groups were 7 and 3, respectively. *p < 0.05, significant change from baseline (time 0 value). ^† p < 0.05, Tempol-induced response versus vehicle-induced responses.

FIGURE 2

Changes in Alveolar-arterial (A-a) gradient elicited by the bolus injection of morphine (10 mg/kg, IV) and subsequent injection of vehicle (saline) or Tempol at doses of 60 mg/kg, IV (Tempol 60) (top panel) or 100 mg/kg, IV (Tempol 100) (bottom panel) in freely-moving rats. The data are presented as mean ± SEM. The numbers of rats in vehicle, Tempol 60 and Tempol 100 groups were 7, 3 and 7, respectively. *p < 0.05, significant change from baseline (time 0 value). ^† p < 0.05, Tempol-induced response versus vehicle-induced responses.

FIGURE 3

Changes in pH, pCO₂, pO₂ and sO₂ elicited by the bolus injection of morphine (10 mg/kg, IV) and the subsequent injection of vehicle (saline) or Tempol (100 mg/kg, IV; Tempol 100) in freely-moving rats. The data are presented as mean ± SEM. The numbers of rats in the vehicle and Tempol 100 groups were 7 and 7, respectively. *p < 0.05, significant change from baseline (time 0 value). ^† p < 0.05, Tempol-induced response versus vehicle-induced responses.

FIGURE 4

Changes in tissue oxygen saturation (SpO₂) elicited by the bolus injection of morphine (10 mg/kg, IV) and the subsequent injection of vehicle (saline), Tempol at 60 mg/kg, IV (Tempol 60) or Tempol at 100 mg/kg, IV (Tempol 100) in freely-moving rats. The data are presented as mean ± SEM. The numbers of rats in the vehicle, Tempol 60 and Tempol 100 groups were 9, 3 and 10, respectively.

FIGURE 5

Changes in mean arterial blood pressure (MAP) and heart rate elicited by bolus injection of morphine (10 mg/kg, IV) and the subsequent injection of vehicle (saline), Tempol at 60 mg/kg, IV (Tempol 60) or Tempol at 100 mg/kg, IV (Tempol 100) in freely-moving rats. Data are shown as mean ± SEM. The numbers of rats in vehicle, Tempol 60 and Tempol 100 groups were 9, 3 and 10, respectively.