Desensitization of Muscarinic Receptor-Coupled Inositol Phospholipid Hydrolysis in Human Detrusor Cultured Smooth Muscle Cells

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Abstract
Purpose: The aim of this study was to investigate the desensitization characteristics of muscarinic M 3 receptors in primary cultures of human detrusor smooth muscle cells. Materials and Methods: Cell cultures were prepared from cold cup pinch biopsies of the human detrusor muscle by explant culture methods. Accumulation of 3 H-inositol phosphates was measured on confluent monolayers as described previously in detail. Desensitization was achieved by preincubating the cells with carbachol or histamine for 5 to 60 minutes. Results: Carbachol induced a concentration-dependent increase in phosphoinositide turnover in naive cells, the response being rapid and evident after only a 30-second exposure to the agonist. Preincubation of the cells with carbachol produced a concentration-dependent decrease in the inositol phosphate response to a second challenge with carbachol. Preexposure to carbachol for only 5 minutes prior to a rechallenge reduced the mean size of response of the second stimulation to 49 percent of that observed in naive cells. Preexposure of the cells to histamine did not alter the response of the cells to a subsequent challenge with carbachol and vice versa. Conclusions: The muscarinic receptors retained by human detrusor smooth muscle cells in culture are susceptible to a desensitization of the carbachol-induced increase in phosphoinositide turnover observed in these cells. This desensitization is rapid, and the results indicate that it is homologous and does not occur via a postreceptor mechanism but at the level of the receptor itself.