Calcium ions, morphine tolerance and noradrenergic transmission in the mouse vas deferens.

The response of the isolated vas deferens of the mouse to electrical stimulation is inhibited by morphine and levorphanol via an opiate receptor, the inhibition decreasing with increasing stimulation frequency (0.2-16 Hz). Tolerance to the locomotor stimulant effect of morphine was induced over 48 h using a slow release preparation. Vasa from mice similarly treated with the slow release preparation showed a shift to the right of the morphine and levorphanol twitch inhibition curves. The reduction in the fractional release of [3H]noradrenaline by morphine and levorphanol was less in vasa from morphine-pretreated mice. Altering the Krebs solution by reducing the Ca2+ or Na+ or adding Mg2+ increased the effect of opiate agonists in vasa from naive and morphine-tolerant mice. Therefore, tolerance to morphine has not changed the ability of these ions to modulate opiate responses.