The bladder must refrain from premature contraction during filling. This project investigates an intrinsic mechanism of detrusor muscles in mouse and humans that senses volume changes through the elongation of smooth muscle fibers. The mechanism is due to the expression and function of stretch-dependent K+ channels. Previous studies have shown that stretch of detrusor smooth muscle cells (SMCs) activates outward current that stabilizes the excitability of these cells. Blockers of the stretch-activated K+ channels cause significant activation of transient contractions (TCs) in the bladder wall during filling, and TCs are a primary stimulant of sensory information conveyed by pelvic afferents. We found that Kcnk2, which encodes the dominant stretch-dependent conductance TREK1 in detrusor SMCs, is down-regulated after loss of female hormones following ovariectomy. We noted a concomitant increase in TCs when TREK1 levels decreased. We also found that estrogen can rescue nearly normal bladder function after ovariectomy. Such findings do not completely agree with the literature regarding the effects of hormone replacement therapy on overactive bladder, and we will explore possible reasons for this discrepancy. One explanation could be that the timing of estrogen replacement could be critical for efficacy. Thus, we have proposed experiments to study the effects of the delay between ovariectomy and estrogen replacement on the ability of estrogen to restore Kcnk2 and TREK1 expression and normal bladder responses to filling. We have also found in preliminary experiments that there is a time- dependent loss of estrogen receptor ? (ER?) after ovariectomy. Thus, we will investigate the time-dependence of the loss of estrogen receptors and the correlation between receptor loss and reduction in the effectiveness of estrogen replacement. Another idea to be investigated is that the balance between ER? and ER? might change after ovariectomy, causing not only loss of effectiveness of estrogen replacement but also shifting the responses to estrogen from helpful to eleterious to normal bladder function. This question will be explored using animals with SMC-specific knockouts of ER? and ER?. The project will use several state-of-the-art molecular and physiological assays to provide rigorous associations between expression of TREK1 and possibly TREK2 channels and bladder function. The consequences of changes in ER? and ER? expression and estrogen regulation of Kcnk2 and Kcnk10 will provide new information about the role of stretch-dependent K+ conductances in bladder function and suggest a mechanism for the development of overactive bladder after menopause.
Estrogen affects gene expression in many parts of the body. We have found in mice that loss of estrogen after removal of the ovaries reduces expression of key ion channels that are part of an intrinsic mechanism that stabilizes contractile activity of the bladder during filling. Generation of premature contractile activity can lead to symptoms of overactive bladder. This project will investigate how estrogen regulates this mechanism and how it might be rescued when hormone levels decrease after menopause.
