On the autonomic control of the lower urinary tract. Functional and morphological aspects in the normal and the inflamed urinary bladder

Abstract

The regulation of urinary bladder function involves interplay of signalling systems. Disturbances in any of the interactive systems may result in lower tract dysfunction, which is often apparent to the patient by urge to void or by incontinence, irrespectively of the cause to the lower tract syndrome. The main objective of this animal experimental thesis was to characterize receptor interactions in the normal rat urinary bladder and to examine how these may be changed in the inflamed bladder. In order to induce experimental cystitis, rats were pre-treated with cyclophosphamide, a cytostatic widely used in the treatment of neoplastic diseases that is bladder toxic and induces haemorrhagic cystitis. The interactions of muscarinic, purinergic and adrenergic receptor subtypes were examined in functional in vitro and in vivo studies, while the expression of the receptor subtypes as well as of nitric oxide synthases was studied by Western immunoblotting and by immunohistochemistry. Occasional observations were made on cultured urothelial cells by employing microphysiometry. The principal parasympathetic contractile response in the normal rat urinary bladder is elicited by acetylcholine and ATP, which relative contributions depend on the intensity of stimulation. In the thesis, the cholinergic response was found to be almost entirely mediated by muscarinic M3 receptors, in spite of the fact that muscarinic M2 receptors occur in much higher number. Nevertheless, even though the atropine-resistant purinergic response is a large part of the parasympathetic contraction, exogenous ATP evokes also a relaxation. In the thesis, the purinergic relaxation is shown to be mediated by P1 and P2Y (P2Y4) purinoceptors, and further, the P1 purinoceptor relaxation is shown to be modulated by the muscarinic M2 receptor. It is also demonstrated that the same mechanism of action of the muscarinic M2 receptor influences the beta-adrenoceptor bladder response. In the cyclophosphamide-induced cystitis, the beta-adrenoceptor-mediated relaxation is markedly inhibited, whereas up-regulated nitric oxide-mediated relaxations coupled to muscarinic receptor seem to occur. In cystitis, the expression of muscarinic M1 and M5 receptors are up-regulated in the bladder in general, but in particular in the urothelium. By employing muscarinic antagonists with different selectivity profiles, functional responses indicate that the muscarinic relaxatory responses are mediated indirectly by muscarinic M5 receptors, probably urothelially located; microphysiometric examinations of an urothelial cell line confirm occurrence of functional muscarinic receptors. In the suburothelial layer it was shown that endothelial nitric oxide synthase is densely expressed in the inflamed bladder but not in the normal. In conclusion, the release of acetylcholine during the micturition response elicits concomitantly to the contraction by the muscarinic M3 receptor, inhibitory effects by the M2 receptor on purinoceptors and adrenoceptors. In the inflamed bladder, sympathetic beta-adrenoceptor responses may be impaired, but indirect muscarinic M5 receptor-mediated relaxations increased. The plasticity in receptor expression as well as the numerous receptor interaction possibilities, make the interplay between signalling systems intricate, particularly when aiming at pharmacological treatment of dysfunction.