Effects of morphine and B-endorphin on Ca2+-ATPase activity of synaptic plasma membranes.

Morphine and B-endorphin were found to inhibit the high calcium -affinity Ca2+-ATPase of synaptic plasma membranes which is believed to be involved in the regulation of intracellular Ca2+ that triggers the release of neurotransmitters. The inhibition was blocked by naloxone and reversed by calmodulin. Kinetic analyses revealed that opioids decreased the affinity of Ca2+-ATPase for Ca2+ and calcium bound to the enzyme decreased the affinity of the enzyme for opioids.