Abstract

Acute desensitization of contraction and its relative mechanisms have been studied in smooth muscle cells isolated from guinea pig stomach. Desensitization was induced by pre-exposure of the cells to one of the excitatory neuropeptides linked to the phospholipase C intracellular cascade, i.e., cholecystokinin (CCK), gastrin-releasing peptide, and Substance P. Desensitization was homologous after a 30-s pre-exposure and heterologous if pre-exposure lasted for 5 min or longer. Homologous desensitization was studied in a more detailed way after pre-exposure to CCK. Preincubation with increasing concentrations of CCK (10 pM–1 μM) induced a progressive rightward shift of the dose-response curves associated with both a decrease in potency (ED₅₀ 4.5 pM–2.2 nM) and a maximum response that were not related to a modification of response kinetics. After brief pre-exposure to 1 nM CCK (D_max), an inhibition of contraction was observed in response to an identical dose of CCK (45.1 ± 8.6%), the decreased response being associated with an inhibition of inositol phosphates and [Ca⁺⁺]_imobilization. Both inositol trisphosphate (InsP₃)-induced contraction and [Ca⁺⁺]_i mobilization were inhibited to a lesser extent than CCK-induced responses. Any longer pre-exposure of cells to one of the above-mentioned neuropeptides caused heterologous desensitization, with an observed inhibition of contraction in response to all tested agonists (CCK, 60.3 ± 5.9%; gastrin-releasing peptide: 56.7 ± 3.5%; Substance P, 60.6 ± 6.5%). A similar decrease was observed in InsP₃-induced contractions resulting in a desensitization of the InsP₃ response as well. Full recovery of contractile responses appeared within 30 min from the end of preincubation, thus indicating that degradation of membrane receptors did not occur. Although pre-exposure of the cells to protein kinase C inhibitor GF109203X did not modify CCK-induced homologous desensitization, it blocked CCK-induced heterologous desensitization. This study demonstrates that excitatory phospholipase C-coupled enteric neuropeptides induce a time-dependent homologous as well as heterologous desensitization of smooth muscle contraction occurring at receptor and postreceptor levels.