MORPHINE AND (D-ALA2, NME-PHE4, GLY-OL)-ENKEPHALIN INCREASE THE INTRACELLULAR FREE CALCIUM IN ISOLATED RAT MYOCYTES-EFFECT OF NALOXONE OR PRETREATMENT WITH MORPHINE

The purpose of the present study was firstly to determine whether morphine and (D-Ala2, NMe-Phe4, Gly-ol)-enkephalin (DAGO), a highly selective mu-agonist, increased intracellular free calcium of rat myocytes and secondly to determine whether opioid receptors were involved. Two series of experiments were performed. In the first, the effect of morphine and DAGO on intracellular free calcium (Ca(i)) of cultured isolated myocytes was studied with a spectrophotometric method using fura2-AM as the fluorescent Ca2+ indicator. In the second, the effect of morphine on Ca(i) of isolated ventricular myocytes from rats which had received chronic daily injection of morphine for two weeks or myocytes which had been incubated in a solution with morphine for 12 hr was studied. It was found that both morphine at 100-250-mu-M and DAGO at 23-75-mu-M increased Ca(i) dose-dependently and that the effect was significantly antagonized by naloxone at a concentration of 50-mu-M, which itself did not cause any significant alteration in Ca(i). Pretreatment with morphine also abolished the morphine-induced increase in Ca(i) of isolated myocytes. The results suggest that morphine increases Ca(i) by directly activating the cardiac receptors (most likely mu-receptors) on the membrane of ventricular myocytes.