Adrenergic nervous control of resistance and capacitance vessels. Studies on isolated blood vessels from the rat.

A systematic comparison of neurogenic responses from consecutive sections of the cardiovascular system has not yet been performed under the well-defined conditions that an in-vitro system can offer. The present investigation aimed at analysing the neuroeffector properties of different isolated vessels, mounted in a myograph, where isotonic and isometric responses to transmural nerve stimulation could be determined. The vessels investigated were the abdominal aorta and caval vein, the superior mesenteric artery and vein, and 200-micron arteries and corresponding veins from the mesenteric arcades, the latter representing true resistance and capacitance vessels, respectively. Further, the vascular adrenergic innervation was visualized by Hillarp-Falck's formaldehyde fluorescence technique as well as immunohistochemical methods. In all vessels the innervation was confined to the adventitio-medial border. The responses to single neurogenic impulses differed markedly between the vessels, being distinct and rapid in the small, slow in the intermediate vessels, and absent in the aorta and the caval vein. This was in contrast to the responses to direct electrical activation of the muscle, which were quite rapid in all vessels. The organisation of the neuromuscular transmission thus has a marked influence on the effector response. The maximal neurogenic responses parallelled the innervation density, being greatest in the small arteries and least in the abdominal aorta. The frequency for half-maximal response appeared to depend also on other factors, such as velocity of contraction and relaxation of the smooth muscle. Of the small vessels, the veins were relatively more activated by low frequencies. Stimulation with an irregular impulse pattern, derived from human sympathetic nerve discharge, had greater effects than the corresponding constant-frequency stimulation on the resistance arteries; the small veins responded equally to either pattern. This difference could be related to the observed frequency-response relations around the average frequency used. The effects of inhibition of neuronal amine uptake on neurogenic responses and on responses to exogenous noradrenaline differed considerably, particularly in the resistance arteries. Here the magnitude of the neurogenic responses were hardly affected by this inhibition, whereas responses to exogenous noradrenaline were strongly enhanced. On its own, blockade of prejunctional adrenergic alpha 2-receptors had little effect on either response, but blocking both alpha 2-receptors and reuptake clearly potentiated the neurogenic responses.(ABSTRACT TRUNCATED AT 400 WORDS)