Smooth muscle caveolae differentially regulate specific agonist induced bladder contractions

Abstract

Aims: Caveolae are cholesterol-rich plasmalemmal microdomains that serve as sites for sequestration of signaling proteins and thus may facilitate, organize, and integrate responses to extracellular stimuli. While previous studies in the bladder have demonstrated alterations in caveolae with particular physiologic or pathologic conditions, little attention has been focused on the functional signi¢cance of these organelles. Therefore, the purpose of this study was to investigate the role of caveolae in the modulation of receptor-mediated signal transduction and determine the presence and localization of caveolin proteins in bladder tissue. Methods: Contractile responses to physiologic agonists were measured in rat bladder tissue before and after disruption of caveolae achieved by depleting membrane cholesterol with methyl-b-cyclodextrin. Stimulation with agonists was repeated after caveolae were restored as a result of cholesterol replenishment. RT-PCR, immmunohistochemistry, and Western blotting were used to determine the expression and localization of caveolin mRNA and proteins. Results: Following caveolae disruption, contractile responses to angiotensin II and serotonin were attenuated, whereas responses to bradykinin and phenylephrine were augmented. Cholesterol replenishment restored responses towards baseline. Carbachol and KCl induced contractions were not a¡ected by caveolae disruption. Ultrastructure analysis con¢rmed loss of caveolae following cholesterol depletion with cyclodextrin and caveolae restoration following cholesterol replacement. Gene and protein expression of caveolin-1, -2, and -3 was detected in bladder tissue. Immunoreactivity for all three caveolins was observed in smooth muscle cells throughout the bladder. Conclusions: The functional e¡ects of cholesterol depletion on speci¢c agonist-induced contractile events and the expression of all three caveolins in bladder smooth muscle support a central role for caveolae in regulation of selective G-protein-coupled receptor signaling pathways in bladder smooth muscle. Thus, caveolae serve to di¡erentially regulate bladder smooth muscle by a stimulus-dependent potentiation or inhibition of bladder contraction.

Regional distribution and molecular interaction of caveolins in bladder smooth muscle

“…As the functional attributes of caveolae are probably determined by the complex relationship among the different combinations of caveolin proteins represented in caveolae, we comprehensively examined the caveolin isoform distribution and interaction to begin to unravel the role The present data show an isoform-specifi c and region-dependent distribution of caveolin gene and protein expression in BSM. Consistent with previous observations [ 14,18 ] , widespread Cav-1 expression was detected throughout the bladder wall. Interestingly, regional differences in Cav-2 immunoreactivity did not parallel that of Cav-1, but showed graded attenuation of intensity in BSM across layers from submucosa to serosa.…”

“…Therefore, the role of these proteins in cell signalling has generally been studied only in cell types in which either Cav-1 or Cav-3 predominate. We and others have previously shown that urinary BSM, unlike non-SM cells and certain other SM systems, expresses all three caveolin isoforms [ 13,14,17 ] . As the functional attributes of caveolae are probably determined by the complex relationship among the different combinations of caveolin proteins represented in caveolae, we comprehensively examined the caveolin isoform distribution and interaction to begin to unravel the role The present data show an isoform-specifi c and region-dependent distribution of caveolin gene and protein expression in BSM.…”

“…Previous studies have shown that all three caveolins [ 13,14 ] , are expressed in BSM. However, the spatial distribution and relative expression of these proteins are unclear.…”

The effect of hypercholesterolemia on the contractions of detrusor strips in response to neural stimulation in rats

“…The cholesterolenriched domains of the cell membrane (6) play several important roles in cellular processes (7,8). They also provide a platform for G-protein, tyrosine kinase, and GTPase-mediated signaling in the detrusor membrane (9,10). Alterations in plasma cholesterol levels are closely related to membrane cholesterol, which in turn modulates signaling through the detrusor smooth muscle membrane (8)(9)(10)(11).…”

“…They also provide a platform for G-protein, tyrosine kinase, and GTPase-mediated signaling in the detrusor membrane (9,10). Alterations in plasma cholesterol levels are closely related to membrane cholesterol, which in turn modulates signaling through the detrusor smooth muscle membrane (8)(9)(10)(11). Based on this knowledge, it is natural to expect alterations of urinary bladder functions in response to changing plasma cholesterol.…”