Simultaneous recording of mechanical and intracellular electrical activity in human urinary bladder smooth muscle

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

Electrical properties of detrusor smooth muscles from the pig and human urinary bladder

(1) The electrophysiological properties of detrusor smooth muscles have been studied almost exclusively in small mammals and the relevance of the information to the human bladder has been questioned. In the present study, electrical properties of detrusor smooth muscles of the pig and human were investigated using intracellular recording techniques. (2) Bladder smooth muscles of the pig and human exhibited nifedipine (10 microm)-sensitive spontaneous action potentials, and their frequency was highly sensitive to membrane polarization. (3) During bursts of action potentials, each action potential was followed by a fast after-hyperpolarization (fast AHP). Charybdotoxin (CTX, 50 nm) increased the amplitude and duration of action potentials but failed to inhibit the fast AHPs, while apamin (0.1 microm) blocked the fast AHPs and induced action potential complexes, which were followed by slow AHPs. 4-Aminopyridine (4-AP, 1 mm) suppressed the slow AHP and increased action potential frequency. (4) In the human bladder, transmural stimuli initiated inhibitory junction potential-like hyperpolarizations, which were followed by action potential discharges. The hyperpolarizations were blocked by atropine (1 microm) and by apamin (0.1 microm) but not by CTX (50 nm). In the pig bladder, transmural stimuli evoked excitatory junction potentials (EJPs), which triggered action potentials. After desensitizing P2x receptors with alpha,beta methylene-ATP (10 microm), nerve-evoked responses were similar to those of human bladder. (5) These results indicate that detrusor smooth muscles of the pig share many features of electrical properties with those of the human. In addition to large conductance (BK) and small conductance (SK) Ca2+-activated K+ channels, voltage-dependent K+ (VK) channels may play an important role in the regulation of electrical activity of detrusor smooth muscles.