Intravital investigation of rat mesenteric small artery tone and blood flow.

KEY POINTS: Substantial information on rat mesenteric small artery physiology and pharmacology based on in vitro experiments is available. Little is known about the relevance of this for artery function in vivo. We here present an intravital model where rat mesenteric small artery diameters are studied under isolated and controlled conditions in situ with simultaneous measurement of blood flow. The responses of the isolated arteries vary with the anaesthetic used, and they are quantitatively but not qualitatively different from the responses seen in vitro. ABSTRACT: Functional characteristics of rat mesenteric small arteries (internal diameter ∼150-200 μm) have been extensively studied in vitro using isometric and isobaric myographs. In vivo, precapillary arterioles (internal diameter < 50 μm) have been studied, but only a few studies have investigated the function of mesenteric small arteries. We here present a novel approach for intravital studies of rat mesenteric small artery segments (∼5 mm long) isolated in a chamber. The agonist-induced changes in arterial diameter and blood flow were studied using video imaging and laser speckle analysis in rats anaesthetized by isoflurane, pentobarbital, ketamine-xylazine, or by a combination of fentanyl, fluanison and midazolam (rodent mixture). The arteries had spontaneous tone. Noradrenaline added to the chamber constricted the artery in the chamber but not the downstream arteries in the intestinal wall. The constriction was smaller when rats were anaesthetized by rodent mixture in comparison with other anaesthetics, where responses were qualitatively similar to those reported in vitro. The contraction was associated with reduction of blood flow, but no flow reduction was seen in the downstream arteries in the intestinal wall. The magnitude of different endothelium-dependent relaxation pathways was dependent on the anaesthesia. Vasomotion was present under all forms of anaesthesia with characteristics similar to in vitro. We have established an intravital method for studying the tone and flow in rat mesenteric arteries. The reactivity of the arteries was qualitatively similar to the responses previously obtained under in vitro conditions, but the choice of anaesthetic affects the magnitude of responses.