Neuronal release and function of nitric oxide in smooth muscle

Abstract

The aim of the present thesis was to study nitric oxide (NO) as an effector molecule in autonomic neurotransmission. The nitrergic transmitter was examined with regard to its pharmacological profile, stimuli of release, release pattern, and effects in gastrointestinal and urinary tract smooth muscle.

The pharmacological profile and chemical identity of the nitrergic transmitter in guinea-pig intestinal smooth muscle was examined in a bioassay system. It was shown that the bioactivity of the nitrergic transmitter was indistinguishable from NO in the bioassay system whereas putative endogenous NO-donors showed different pharmacological profiles. The nitrergic transmitter is thus likely to be NO itself.

Inhibitory neuroeffector responses in guinea-pig colon were markedly reduced by the NO synthase inhibitor L-NNA in vitro while contractile responses were enhanced. Spontaneous as well as vagally induced contractions in the stomach and colon of anaesthetised rabbits were markedly enhanced by i.v. infusion of the NO synthase inhibitor L-NAME. These data suggest that endogenous NO is an important inhibitory mediator of autonomic neuroeffector transmission in gastric and colonic smooth muscle.

Nerve-induced NO/NO2- release in the isolated guinea-pig colon was quantified by chemiluminescence technique in order to identify receptor stimuli for neuronal NO release. It was shown that a considerable part of the nerve-induced NO release likely results from M1 muscarinic receptor activation. Moreover the concentration of NO oxidation products increased in the rabbit gastric and colonic wall during vagal nerve stimulation, as detected by in vivo microdialysis and capillary electrophoresis. This increase was inhibited by i.v. infusion ofL-NAME, thus suggesting vagal nerve activity as being a stimulus for endogenous NO formation in the gastrointestinal tract.

The release pattern of nerve-induced NO release in the guinea-pig myenteric plexus and in the isolated rabbit hypgastric nerve trunk was visualised by a novel technique using a photon 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 nerve cells in the hypogastric nerve trunk. These findings indicate that NO is diffusely released from the whole nerve cell upon stimulation.

NO synthase activity was present in the human renal pelvis, and exogenous NO inhibited rhythmic and tonic contractions in the isolated renal pelvis and ureter indicating a role for endogenous NO in regulation of peristaltic activity in the human upper urinary tract. NO synthase activity was present also in the human detrusor muscle, bladder neck and prostatic urethra. The enzyme activity as well as inhibitory nitrergic neuroeffector responses in vitro were most prominent in the outflow region, thus indicating a role for endogenous NO i the micturition reflex in man.

In conclusion, these findings suggest that NO itself, constitutes an important inhibitory effector molecule of autonomic neurotransmission in gastrointestinal and urinary tract smooth muscle. It is diffusely released from the whole nitrergic neuron, and vagal nerve activity as well as M1 muscarinic receptor activation are important stimuli for NO release.