Progression from homologous to heterologous desensitization of contraction in gastric smooth muscle cells

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 mu M) 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++](i) mobilization. Both inositol trisphosphate (InsP(3))-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 ail 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 InsP(3)-induced contractions resulting in a desensitization of the InsP(3) 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 revels.