Non-adrenergic, non-cholinergic, non-purinergic contractions of the urothelium/lamina propria of the pig bladder

Acetylcholine, and to a lesser extent ATP, mediates neurogenic contractions of bladder smooth muscle. Recently, the urothelium and lamina propria have also been shown to have contractile properties, but the neurotransmitters involved in mediating responses to nerve stimulation have not been investigated. Isolated strips of porcine urothelium with lamina propria were electrically field stimulated and contractions recorded. Drugs interfering with neurotransmission were then employed to identify which neurotransmitters mediated responses. Strips of urothelium/lamina propria developed spontaneous contractions with a frequency of 3.5 ± 0.1 cycles min ^-1 and amplitude of 0.84 ± 0.06 g. Electrical field stimulation at 5, 10, and 20 Hz resulted in frequency-related contractions (1.13 ± 0.36 g, 1.59 ± 0.46 g and 2.20 ± 0.53 g, respectively, n = 13), and these were reduced in the presence of tetrodotoxin (1 μm) by 77 ± 20% at 5 Hz, 79 ± 7% at 10 Hz and 74 ± 12% at 20 Hz (all P < 0.01), indicating they were predominantly neurogenic in nature. Neither the muscarinic antagonist atropine (10 μm), the adrenergic neurone blocker guanethidine (10 μm) nor desensitization of the purinergic receptors with α,β-methylene ATP (10 μm) affected the contractile amplitude. Similarly, responses were not affected by the nitric oxide synthase inhibitor l-NNA (100 μm) or drugs that interfere with peptide neurotransmission (capsaicin, NK2 antagonist GR159897, protease inhibitors). In conclusion, electrical depolarization of the nerves present in the porcine urothelium/lamina propria results in frequency-dependent contractions, which are predominantly neurogenic in nature. These contractions are resistant to drugs that inhibit the adrenergic, cholinergic and purinergic systems. The neurotransmitter involved in the responses of this tissue is therefore unknown but does not appear to be a peptide.