The role of adenosine in the hypotensive actions of morphine.

The role of adenosine in the hypotensive action of morphine was examined in the pentobarbitone anaesthetized or pithed rat preparations. Adenosine (10-300 micrograms/kg) induced dose-dependent decreases in diastolic blood pressure in the anaesthetized rat preparation. These effects were attenuated by infusion of 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 50 micrograms/kg/min), 8-phenyltheophylline (8PT; 50 micrograms/kg) also induced dose-dependent decreases in diastolic blood pressure. Guanethidine (16 micrograms/kg/min), atropine (1 mg/kg), DPCPX and 8 PT reduced the effect of morphine on diastolic blood pressure, whilst DMPX (50 micrograms/kg/min) was inactive. In the pithed rat preparation morphine was inactive at doses up to 10 mg/kg. The results suggest that the hypotensive effect of morphine is mediated at least in part by the release of adenosine which then acts on centrally located adenosine receptors to induce changes in autonomic control of blood pressure.