Calcium stimulates opioid receptor agonism in rat cardiac sarcolemma

We have found that calcium stimulates, in a dose-dependent manner, the binding of opioid to kappa and delta receptors in sarcolemma from rat's ventricles isolated by hypotonic lithium bromide shock. Opioid binding was measured using [3H]-diprenorphine as a radioligand. The delta-selective agonist [D-Ala2, D-Leu5]-enkephalin and the kappa-selective agonist U-50, 488H both inhibited control and Ca(2+)-stimulated [3H]-diprenorphine binding to the sarcolemma, whereas [D-Ala2,MePhe4,Gly-(ol)5]-enkephalin was ineffective. The stimulatory effect of calcium increased the maximal binding capacity without affecting the affinity of the receptor for the ligand.