Leucine enkephalin antagonizes norepinephrine-induced 45Ca++ accumulation in rat atria

Exposure of rat atrial slices to 10 −5 M norepinephrine (NE) for 10 minutes increases 45 Ca ++ accumulation from 1.64 ± 0.10 to 2.23 ± 0.06 nmol/mg tissue. In the presence of leucine enkephalin (10 −8 M), NE-stimulated 45 Ca ++ uptake is reduced to 1.44 ± 0.10 nmol/mg tissue. The effect of leu-enkephalin is reversed in the presence of 10 −7 M naloxone, NE-stimulated 45 Ca ++ uptake being increased to 2.17 ± 0.15 nmol/mg tissue. The results support a direct interaction of leuenkephalin with β-agonist-stimulated Ca ++ flux in rat atria, and correlate with the previously reported enkephalin antagonism of NE-induced positive chronotropy in the same tissue.