Inhibition of peptidases potentiates enkephalin-stimulated contraction of gastric muscle cells

Cell surface peptidases degrade enkephalins and thereby restrict the number of molecules available to activate receptors. The effects of peptidase inhibitors on degradation of enkephalins and on enkephalin-stimulated contraction of gastric smooth muscle cells were examined. Muscle cells dispersed from the guinea pig stomach degraded [Tyr¹-³H][Leu⁵]enkephalin (41.6 ± 9.0% degradation at 60 min incubation, mean ± SD, n = 4 animals). Amastatin (10 μM, an aminopeptidase inhibitor) inhibited degradation by 72.1 ± 1.5%. The residual peptidase activity was inhibited by phosphoramidon (1 μM, an endopeptidase EC 3.4.24.11 inhibitor) by 58.0 ± 11.0%. [Tyr¹-¹²⁵I][Met⁵]enkephalin was similarly degraded. Phosphoramidon (1 μM) inhibited the degradation of the aminopeptidase-resistant peptide [Tyr¹-³H][D-Ala²][Leu⁵] enkephalin by >95%. [Met⁵]enkephalin, incubated with cells for 30 s, stimulated contraction [50% maximal contraction (EC₅₀) 120 ± 50 nM, n = 6]. Pretreatment of cells with phosphoramidon alone, amastatin alone, or phosphoramidon plus amastatin, caused 20-fold (EC₅₀ 6.5 ± 1.1 nM), 2-fold (EC₅₀ 63 ± 23 nM), and 100-fold (EC₅₀ 1.1 ± 0.3 nM) increase in potency of [Met⁵]enkephalin, respectively. The results show that endopeptidase EC 3.4.24.11 and aminopeptidases contribute to degradation of enkephalins by gastric muscle cells. The rapidity and magnitude of the potentiating effects of the inhibitors on enkephalin-stimulated contraction suggest a close physical relationship between the peptidases and the enkephalin receptors.