Temperature-dependent effects of morphine on the isolated right atrium

The aim of the present study was to examine whether changes in temperature alter the effects of morphine on isolated right atria. Morphine dose-response curves at different temperatures (30, 37 or 40 degrees C) from right atria of the rat were obtained. The maximal negative chronotropic effect induced by morphine was 11 +/- 3% at 37 degrees C and 20 +/- 1% at 30 degrees C. However, when the temperature was increased to 40 degrees C, morphine induced a positive chronotropic effect. The increase in auricular automaticity induced by morphine at 40 degrees C was antagonized in the presence of naloxone (5 x 10(-6) mol/l). These data demonstrated that: (1) hyperthermia modifies the effects of morphine on isolated right atria and (2) at 40 degrees C morphine induces a positive chronotropic effect by activation of opioid receptors.