Evidence of gap junctions in the stable nonobstructed human bladder

Gap junctions are thought to synchronize muscle cell actions by promoting intercellular communications. Since it is still a matter of debate whether smooth muscle cells in the bladder are electrically coupled, we investigated the occurrence of gap junctions and the gap junction forming protein connexin 45 in normal human detrusor.

Detrusor smooth muscle was obtained during radical cystectomy in 6 patients. For transmission electron microscopy studies samples were high pressure frozen and cryo-fixed. Additional frozen samples were processed for freeze fracture/freeze fracture immunolabeling analyses or for reverse transcriptase-polymerase chain reaction using primers for connexin 45. Furthermore, a set of samples was fixed with Bouin's solution, embedded in paraffin and used for immunohistochemistry with a polyclonal antibody against connexin 45 as well as for in situ hybridization studies with digoxigenin labeled connexin 45 riboprobes.

Ultrastructural and freeze fracture studies showed gap junctions at detrusor smooth muscle cells. However, these gap junctions appeared to be small and irregularly shaped. Reverse transcriptase-polymerase chain reaction and in situ hybridization showed that connexin 45 mRNA was expressed in the detrusor muscularis. Furthermore, we detected prominent connexin 45 immunoreactions at detrusor smooth muscle cells. Freeze fracture immunolabeling techniques revealed connexin 45 signals at small gap junctional plaques.

To our knowledge this study is the first to provide morphological, molecular biological and immunohistochemical evidence that smooth muscle cells of stable human detrusor are electrically coupled through gap junctions. These results may contribute to new concepts in the understanding of bladder physiology.