The organization of the salivary gland microcirculation

Abstract

The microvasculature of the rabbit submandibular salivary gland was investigated employing in vivo microscopy, blood flow measurements, latex casts, microsphere injections and examination of fixed sections of the gland. Two principal microcirculations were distinguished in the living gland, one supplying the acini and the other the intralobular ducts. Parasympathetic nerve stimulation (2, 5 or 10 s-1) elicited different responses in each of the 2 microcirculations. Flow in the capillaries around the acini slowed initially before increasing. Flow in the intralobular duct capillaries increased soon after beginning stimulation. In some experiments whole gland flow and microvascular flow were monitored simultaneously. While gland flow increased at the same time as flow in the acinar capillaries was decreasing and as flow in the intralobular duct capillaries increased. Flow in acinar capillaries increased about 5 s after glandular flow started to increase. These observations could be explained if either the vascular beds of the acini and the intralobular ducts were arranged in parallel or if arteriovenous anastomoses were to shunt the acinar circulation. No such anastomoses were found in latex casts made of the gland vasculature, and microspheres injected into the artery supplying the gland were not found in the venous effluent. The intraglandular distribution of microspheres was measured in histological sections of the injected glands to give an estimate of the distribution of blood flow between the duct and acinar microcirculations. At rest and during maintained stimulation about 55% of the blood flow passed through the intralobular duct microcirculations; during the initial 15 s of stimulation this proportion was increased to over 70%. The duct and acinar microcirculations are arranged in parallel. There are differences in the way the vasodilatation is mediated in these circulations. Arterio-venous anastomoses play no significant role in this gland.