Neuronal release and function of nitric oxide in smooth muscle

Abstract: NEURONAL RELEASE AND FUNCTION OF NITRIC OXIDE IN SMOOTH MUSCLE by HENRIK H. IVERSENFrom the Department of Physiology and Pharmacology, Division of Physiology I Karolinska Institute, 17177 Stockholm, Sweden The aim of the present thesis was to study nitric oxide (NO) as an effector molecule inautonomic neurotransmission. The nitrergic transmitter was examined with regard to itspharmacological profile, stimuli of release, release pattern, and effects in gastrointestinal andurinary tract smooth muscle. The pharmacological profile and chemical identity of the nitrergic tramsmitter inguinea-pig intestinal smooth muscle was examined in a bioassay system. It was shown that thebioactivity of the nitrergic transmitter was indistinguishable from NO in the bioassay systemwhereas putative endogenous NO-donors showed different pharmacological profiles. Thenitrergic transmitter is thus likely to be NO itself Inhibitory neuroeffector responses in guinea-pig colon were markedly reduced by theNO synthase inhibitor L-NNA in vitro while contractile responses were enhanced. Spontaneousas well as vagally induced contractions in the stomach and colon of anaesthetised rabbits weremarkedly enhanced by i.v. infusion of the NO synthase inhibitor L-NAME. These data suggestthat endogenous NO is an important inhibitory mediator of autonomic neuroeffectortransmission in gastric and colonic smooth muscle. Nerve-induced NO/NO2- release in the isolated guinea-pig colon was quantified bychemiluminescence technique in order to identify receptor stimuli for neuronal NO release. Itwas shown that a considerable part of the nerve-induced NO release likely results from M1muscarinic receptor activation. Moreover the concentration of NO oxidation products increasedin the rabbit gastric and colonic wall during vagal nerve stimulation, as detected by in vivomicrodialysis and capillary electrophoresis. This increase was inhibited by i.v. infusion ofL-NAME, thus suggesting vagal nerve activity as being a stimulus for endogenous NOformation in the gastrointestinal tract. The release pattern of nerve-induced NO release in the guinea-pig myenteric plexusand in the isolated rabbit hypgastric nerve trunk was visualised by a novel technique using aphoton counting camera coupled to a microscope. The light signal, which was NO synthase-dependent, was evenly distributed in the myenteric plexus and was present along whole nervecells in the hypogastric nerve trunk. These findings indicate that NO is diffusely released fromthe whole nerve cell upon stimulation. NO synthase activity was present in the human renal pelvis, and exogenous NOinhibited rhythmic and tonic contractions in the isolated renal pelvis and ureter indicating a rolefor endogenous NO in regulation of peristaltic activity in the human upper urinary tract. NOsynthase activity was present also in the human detrusor muscle, bladder neck and prostaticurethra. The enzyme activity as well as inhibitory nitrergic neuroeffector responses in vitrowere most prominent in the outflow region, thus indicating a role for endogenous NO i themicturition reflex im man. In conclusion, these findings suggest that NO itself, constitutes an important inhibitoryeffector molecule of autonomic neurotransmission in gastrointestinal and urinary tract smoothmuscle. It is diffusely released from the whole nitrergic neuron, and vagal nerve activity as wellas M1 muscarinic receptor activation are important stimuli for NO release.Key words: Autonomic neurotransmission, gastrointestinal tract, nitric oxide, nitric oxidesynthase, smooth muscle, urinary tract. STOCKHOLM 1996 ISBN 91-628-1961-5