Contractions in human detrusor smooth muscle induced by hypo-osmolar solutions.

PURPOSE: The aim of this study was to investigate stretch activated channels in human detrusor using hypo-osmolar solutions to produce cell deformation. Stretch activated channels could provide another mechanism by which detrusor myocytes may be coupled. MATERIALS AND METHODS: Human detrusor removed at surgery was dissected into strips and also enzymatically digested and cultured. Strips (5x1x1 mm.) were mounted in an organ bath and perfused with gassed Tyrode's. Hypo-osmolar solutions were made by removal of NaCl. Gadolinium (Gd3+), a blocker of stretch activated channels (SACs), and diltiazem, an L-type Ca2+ channel antagonist were used at 10 microM concentrations. Mean data +/- S.E.M. are expressed as a percentage of maximal tension produced by 1 microM carbachol for each patient. Enzymatically disaggregated, human detrusor was cultured in flasks, passaged and placed on glass coverslips. Once confluent the cells were incubated with the Ca2+ sensitive fluorochrome Fura-2AM. Coverslips were placed in a bath on the stage of EPI-fluorescence microscope and solutions were perfused through the bath (5 ml. per minute, 35C, pH 7.4). Changes in fluorescence emission ratio (proportional to changes in cytosolic Ca2+) were measured. RESULTS: Hypo-osmolar solutions produced a tension increase in the strips and a Ca2+ influx in the cells. In the strips in paired experiments Gd3+ and diltiazem significantly reduced the response to hypo-osmolar solution (87%+/-16% v. 51%+/-12.5%, p = 0.003, n = 10 for Gd3+), and (69%+/-11% v. 37%+/-9%, p = 0.001, n = 9 for diltiazem). In Ca2+ free solution responses were significantly reduced (65%+/-10% v. 21%+/-8%, p = 0.001, n = 9). In the cells in paired experiments, 10 microM Gd3+ significantly reduced the elevation of cytosolic Ca2+ in response to hypo-osmolar solutions (median 0 v. 0.38 (62 cells, n = 7 bladders)), as did Ca2+ free hypo-osmolar solution (median 0 v. 0.44 (46 cells, n = 7)). 10 microM diltiazem (L-type Ca2+ channel antagonist) did not influence the response to hypo-osmolar solution (p = 0.14, median 0.5 v. 0.54 (31 cells, n = 4)). CONCLUSIONS: Hypo-osmolar solutions produced a tension increase in human detrusor that appears to be dependent on upon influx of Ca2+ through stretch activated channels (SACs), influx of Ca2+ through L-type Ca2+ channels and also on release of intracellular Ca2+.