Acetylcholine and molecular components of its synthesis and release machinery in the urothelium

Previous studies provided indirect evidence for urothelial synthesis and release of acetylcholine (ACh). We aimed to determine directly the ACh content in the urothelium and to characterize the molecular components of its synthesis and release machinery.

The study was performed on mouse bladder and abraded urothelium, and human mucosal bladder biopsies. ACh content was measured by high-performance liquid chromatography-electrochemical. Reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemistry served to investigate expression of ACh-synthesizing enzymes-choline acetyltransferase (ChAT) and carnitine acetyltransferase (CarAT)-vesicular ACh transporter (VAChT), and polyspecific organic cation transporters (OCTs; isoforms 1-3). Transfected cells served to investigate whether the anticholinergic drug trospium chloride interferes with ACh-transporting OCTs.

ACh is present in the urothelium in a nanomolar range per gram of wet weight. RT-PCR data support the presence of CarAT but not ChAT. VAChT, used by neurons to shuffle ACh into synaptic vesicles, is detected in subepithelial cholinergic nerve fibres, but not by RT-PCR or immunohistochemistry in the urothelium. OCT1 and OCT3 are expressed by the urothelium. The quarternary ammonium base trospium chloride inhibits cation transport by OCTs with a potency rank order of OCT2 (IC(50)=0.67+/-0.42micromol/l)>OCT1 (IC(50)=6.2+/-2.1micromol/l)>OCT3 (IC(50)=871+/-177micromol/l).

This study demonstrates a urothelial non-neuronal cholinergic system that differs widely from that of neurons with respect to molecular components of the ACh synthesis and release machinery. Consequently, these two systems might be differentially targeted by pharmacologic approaches.