EFFECT OF REPETITIVE STIMULATION ON THE CONTRACTILE RESPONSE OF RABBIT URINARY-BLADDER SUBJECTED TO IN-VITRO HYPOXIA OR IN-VITRO ISCHEMIA FOLLOWED BY REOXYGENATION

Purpose: We studied the effects of hypoxia followed by reoxygenation a nd an in vitro model of ischemia (hypoxia + substrate [glucose] depriv ation) followed by reperfusion (reoxygenation + substrate replacement) on the contractile response of rabbit urinary bladder strips to nonre petitive and repetitive field stimulation (FS), and correlated the res ults with the rate of lipid peroxidation. We view repetitive FS as a m odel for hyperreflexia. Methods: The effects of repetitive and nonrepe titive FS on the contractile responses of isolated strips of rabbit bl adder to FS, carbachol, and KCl were determined in the presence of 3 d ifferent incubation media: O-2 + glucose (normal physiological medium) ; N-2 + glucose (in vitro hypoxia), and N-2 - glucose (tin vitro ische mia). Then, all strips were incubated for 1 h in normal physiological medium ('reperfusion') followed by a final stimulation; the resultant contractile responses were correlated with the level of lipid peroxida tion as determined by malonedialdehyde (MDA) concentration. Results: R epetitive stimulation, a model of hyperreflexia, significantly increas ed the rate of development of contractile dysfunction in bladder tissu e strips incubated in all 3 media as compared to nonrepetitive stimula tion, which caused no degradation of the contractile response in norma l physiological medium. The rate of development of contractile dysfunc tion was significantly greater in bladder tissue strips incubated in t he in vitro ischemia medium (N-2 - glucose) than in strips incubated i n the hypoxia medium (N-2 + glucose); which, in turn, was significantl y greater than in those incubated in the normal physiological medium ( O-2 + glucose). Repetitive stimulation ('hyperreflexia') during all 3 incubation conditions resulted in increased [MDA] after reoxygenation or 'reperfusion'. Incubation in in vitro ischemia buffer (N-2 - glucos e) followed by 1 h reoxygenation + substrate replacement stimulated li pid peroxidation to a significantly greater extent than did incubation in hypoxia buffer (N-2 + glucose) followed by 1 h of reoxygenation; t he level of lipid peroxidation, [MDA], paralleled the magnitude of the contractile dysfunctions present. Independent of the incubation mediu m, the magnitude of FS-induced contractile dysfunction after reoxygena tion or 'reperfusion' was significantly greater than the magnitude of dysfunction in response to carbachol or KCl. Conclusions: The results demonstrate that the rate of contractile failure induced by in vitro i schemia is greater than that induced by in vitro hypoxia, and that the contractile response to FS is significantly more sensitive to both hy poxia and in vitro ischemia than is the contractile response to either carbachol or KCl. Repetitive stimulation ('hyperreflexia') increases the rate of contractile failure under all conditions tested, and the m agnitude of the contractile failure may be due, in part, to the genera tion of free radicals and subsequent stimulation of lipid peroxidation upon reoxygenation or 'reperfusion'.