Evaluation of neurohumoral substances in local regulation of blood flow

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Abstract
Studies on the terminal vascular bed of the rat mesocecum were designed to evaluate by direct microscopic visualization and quantitative measurement of microvascular diameters, the role of some neurohumoral substances in micro-circulatory regulation, viz., catecholamines, acetylcholine, and serotonin [5-hydroxytryptamine] (5-HT) by means of differential blockade or tissue amine depletion with known pharma-cologic antagonists. Alpha adrenergic blocking agents produced vaso-dilatation of many precapillaries, metarterioles, arterioles, and some venules, thereby suppressing the phenomenon of vasomotion. Although adrenergic blockers prevented catecholamine-induced constriction, vascular reactivity to other constrictor agonists such as 5-HT, angio-tensin, or vasopressins was unaffected or potentiated in some cases. Chronic pretreatment with reserpine, guanethidine, or alpha methyl dopa produced selective microvascular dilation (10-30% increase in vessel sizes) and suppressed vasomotion. Topical local anesthetics induced relaxation of arterioles, metarterioles, and precapillaries and suppressed vasomotion. 5-HT blockers had no measurable effects. Locally applied atropine led to constriction (25-35% decrease in vessel sizes) of arterioles, metarterioles, and precapillaries. Most of the latter effects of atropine were exacerbated after selective a-adrenergic block. Endogenous norepinephrine plays a fundamental role as a naturally occurring tonic regulator of rat mesenteric blood flow and that some unknown vasoconstrictor(s) such as neuro-hypophyseal polypeptides modify the actions of catecholamines in the terminal bed. Evidence was presented to indicate that acetylcholine and epinephrine appear to play roles as selective microvascular dilators.