The decompensated detrusor IV: experimental bladder outlet obstruction and its functional correlation to the expression of the ryanodine and voltage operated calcium channels

Abstract

Purpose: Calcium ion homeostasis has a significant role in smooth muscle function. Its regulation requires complex storage and release mechanisms via ion pumps and channels located within intracellular storage sites (sarcoplasmic reticulum) and at the plasma membrane. A prominent component of the sarcoplasmic reticulum is the ryanodine sensitive ion channel which releases calcium from the sarcoplasmic reticulum into the cytosol. At the level of the plasma membrane the voltage operated calcium channel (dihydropyridine sensitive) serves to allow an influx of extracellular calcium. Our prior studies have shown a loss of sarcoplasmic endoplasmic reticulum Ca++Mg++ATPase expression following outlet obstruction. In this study we correlate ryanodine and voltage operated calcium channel protein expression with bladder function following partial outlet obstruction.

Materials and methods: Standardized partial bladder outlet obstructions were created in adult New Zealand white rabbits, which were divided into control, sham operated and obstructed groups. Muscle strip studies further subcategorized the obstructed group into compensated (force greater than 50% of control) and decompensated (force less than 50% of control) and were correlated with in vivo determinations of voiding frequency and voided volumes. Microsomal membrane protein fractions were prepared from the same bladder tissue and were used for Western blot analysis using specific monoclonal antibodies.

Results: Increased voiding frequency and decreased voided volumes correlated with the definitions of compensated and decompensated. The Western blots revealed a near disappearance of ryanodine expression in the decompensated group with minimal changes in the expression of the voltage operated calcium channel.

Conclusions: Bladder performance as measured in vivo and in vitro after outlet obstruction is influenced in part by smooth muscle cell ability to maintain calcium homeostasis via the sarcoplasmic reticulum. Bladder decompensation is highly associated with a loss of sarcoplasmic reticulum function with lesser changes taking place in those calcium regulatory proteins at the plasma membrane.