An ultrastructural comparison of neuromuscular relationships in blood vessels with functional and ?non-functional? neuromuscular transmission

Abstract

The neuromuscular relationships of the guinea-pig renal artery, which has previously been shown to be noradrenergically innervated but ‘non-functional’, i.e. not responsive to nerve stimulationin vitro, is compared with those of two arteries where functional neuromuscular transmission occurs: the carotid artery, where responses to nerve stimulation are slow and of medium amplitude, and the mesenteric artery, where the responses to nerve stimulation are relatively fast and of large amplitude. Substantial differences of ultrastructure were demonstrated. In the renal artery, there were no nerve varicosities closer than 400 nm to the smooth muscle layer, all varicosities had connective tissue interposed between them and the muscle wall, and there were significantly fewer varicosities (6 ± 3.1 mm⁻¹) compared with the carotid (33 ± 8.3 mm⁻¹;P < 0.05) or mesenteric (105 ± 15.8 mm⁻¹;P < 0.001) arteries. In the carotid artery, a small group of varicosities (17%), with little interposed connective tissue, had an average neuromuscular gap of 1.5 μm, whilst the remainder were separated by an average of 4.3 μm and by cellular and other connective tissue elements from the nearest smooth muscle cell. In the mesenteric artery, about half of the nerve varicosities were closer than 400 nm to the muscle layer and 25% of the varicosities had no connective tissue except basal lamina interposed in the neuromuscular space. Differences were also shown in the ultrastructural appearance of the nerves: analysis of the neuronal vesicles showed that the majority of varicosities were noradrenergic in all three vessels, with non-adrenergic varicosities forming 6% of the total in the carotid artery and 32% of the total in the mesenteric artery. Noradrenaline content was greater in the mesenteric artery (0.37 ± 0.052 ng mm⁻²) compared with the carotid (0.17 ± 0.017 ng mm⁻²;P < 0.01) and renal (0.11 ± 0.024 ng mm⁻²;P < 0.01) arteries. These differences appear to correlate with the differences of neuromuscular activity in the three arteries.