The organization of the salivary gland microcirculation

Salivary biomarkers--an update

Saliva is the most easily available and accessible body fluid, which makes it one of the most sought after tools in diagnostic pathology. Markers expressed in saliva can be used for diagnosis and concurrent patient follow-up of diseases ranging from hereditary disorders to infections, as well as malignancies and also analysis of therapeutic levels of drugs. The emergence of hand-held devices has made possible the elucidation of point-of-care methods and research in the field has resulted in the assimilation of knowledge useful for clinical applications.

CLINICAL RELEVANCE

This article describes some of the latest research in saliva diagnostics and provides some information on the various hand-held devices available.

An investigation of the vascular organisation of the canine submandibular gland

It is known that parasympathetic nerve stimulation elevates venous pressure in the dog submandibular gland, and that the venous pressure wave is transformed to that of the arterial pulse. The vascular arrangements and histological characteristics of the dog submandibular gland were therefore examined to establish which anatomical structures are responsible for the change in venous pressure during salivation induced by parasympathetic stimulation. The acinar and ductal circulations were found to be arranged in parallel and arteriovenous anastomoses were identified in both. Microsphere injection studies demonstrated the opening of arteriovenous anastomoses in actively secreting glands. Smooth muscle cells were rarely found in venous blood vessels but venous valves were abundant in both circulations. Dense connective tissue was observed to enclose the ductal system and its accompanying structures (blood vessels, lymphatic vessels and nerves); it was most abundant in the hilum and diminished aborally. The mechanism responsible for elevating venous pressure during parasympathetic salivation is thus probably related to opening of the arteriovenous anastomoses; the increase in the amount of surrounding dense connective tissue in a central direction may facilitate the preservation of the transmitted arterial pressure and pulse in the venous system.

Regulation of regional cerebral blood flow by cholinergic fibers originating in the basal forebrain

We review mainly recent studies on vasodilative regulation of cortex and hippocampus by central cholinergic nerves originating in the basal forebrain. We also briefly review the influence of other central noradrenergic fibers originating in the locus ceruleus, serotonergic fibers originating in the dorsal raphe nucleus, dopaminergic fibers originating in the substantia nigra, and peripheral sympathetic and parasympathetic nerve fibers upon regulation of regional cerebral blood flow. Local metabolites have long been considered to play an important physiological role in regulating regional cerebral blood flow. However, the evidence reviewed here emphasizes that the regulation of regional cerebral blood flow by these central cholinergic nerves is independent of regional metabolism. We propose through this review that although studies investigating neural regulation of cortical and hippocampal blood flow by cholinergic fibers originating in the basal forebrain have added much to the understanding of regulation of regional cerebral blood flow further studies are needed to determine the physiological relevance of regional cerebral blood flow in relation to higher nervous functions such as memory, learning, and personality, and changes in these cognitive functions with aging and pathology such as Alzheimer's disease.

Morphology and histochemistry of rabbit submandibular glands

The rabbit submandibular glands are of the heterocrine type. The most distal part of the ductal system consists of secretory endpieces, which are constructed mainly of two morphologically and histochemically different and distinct structures: distally the seromucous acini and proximally the serous tubuli. The cells of the seromucous acini are strongly AB pH 2.5 positive, AB pH 1.0 negative and PAS negative, indicating production of acid glycoproteins--mainly sialomucin. The cells of the serous tubuli are AB pH 2.5 negative and strongly PAS positive, indicating the presence of neutral glycoproteins. Glycol methacrylate embedded sections (Historesin) stained with a modified Trichrome stain were superior to standard paraffin sections regarding resolution and clarity of morphological details. Quantitative studies of intralobular structures on AB-PAS-stained paraffin sections compared with Trichrome-stained glycol methacrylate sections gave corresponding results. The latter method is well suited for study of structural changes in submandibular glands, as in assessing the effect of ionizing irradiation.

Transcapillary transport during secretion by the rabbit submandibular salivary gland

Fluid exchange in the rabbit submandibular gland has been studied with a view to improving understanding of the mechanisms underlying transcapillary transport during salivation. Using maximally vasodilated glands perfused in vitro, we found that acetylcholine had no significant effect on either hydraulic conductance (filtration coefficient, Lp) or reflection coefficient to albumin (sigma alb) of the gland microvessels. In vivo, parasympathetic nerve stimulation increased blood flow 20-fold, interstitial fluid production (total fluid efflux - saliva flow) increased approximately 10-fold, while filtration fraction remained constant. At the same time, isogravimetric capillary pressure increased about six-fold. There are several possible explanations for these findings, and it is argued that they are likely to include capillary recruitment. Thus, during salivation, parasympathetic nerve stimulation does not lead to a change in capillary permeability as such, but rather produces a relaxation of resistance vessels and a fall in precapillary resistance, accompanied by an increase in available capillary surface area. This leads to a rise in capillary blood flow and pressure, which in turn increase fluid filtration. These changes are self-limiting due to the resulting dilution of interstitial protein and rise in interstitial pressure, but during salivation, since the secretion is pumped out of the interstitium, this fluid transport has no net effect on transcapillary gradients.

The role of cyclic nucleotides and related compounds in nerve-mediated vasodilatation in the cat submandibular gland

1 Intra-arterial administration of a number of purine compounds to the cat submandibular salivary gland led to an increased blood flow. The threshold concentration of the most potent vasodilators, adenosine 5'-triphosphate (ATP) and adenosine 5'-diphosphate (ADP) was about 2 mumol/l. Adenosine and guanosine 5'-triphosphate (GTP) required about 25 mumol/l, adenosine 3',5'-cyclic monophosphate (cyclic AMP) 40 mumol/l, guanosine 5'-diphosphate (GDP) 125 mumol/l and dibutyryl guanosine 3',5' cyclic monophosphate (db cyclic GMP) 400 mumol/l. Dibutyryl cyclic AMP and cyclic GMP were ineffective. 2 The cyclic nucleotide phosphodiesterase inhibitors, theophylline, papaverine, quinine and 3-isobutyl-1-methylxanthine (IBMX), all acted as vasodilators. 3 When intra-arterial infusion of theophylline or IBMX was combined with sympathetic nerve stimulation, the vasodilatation observed after the stimulus ceased was significantly potentiated. 4 Theophylline and IBMX also potentiated the vasodilatation accompanying parasympathetic nerve stimulation and this response persisted after atropine. 5 These results are discussed in relation to the possible mediators of sympathetic and parasympathetic vasodilatation in the gland.

Transcapillary exchange in the cat salivary gland during secretion, bradykinin infusion and after chronic duct ligation

1. Capillary permeability-surface area products for 86Rb, [51Cr]EDTA (mol. wt. 357), [57Co]cyanocobalamin (mol. wt. 1353) and [125I]insulin (approximate mol. wt. 6000) have been measured using the single-circulation, multiple-tracer dilution technique in the in situ perfused submandibular salivary gland during parasympathetic nerve stimulation, close-arterial bradykinin infusion and following chronic duct ligation. 2. In glands with a natural blood supply, permeability-surface area for 86Rb and [51Cr]EDTA increased during parasympathetic stimulation, but this was shown to be related to the concomitant increase in blood flow rather than to a change in capillary permeability or in surface area. 3. In glands perfused at constant flow, parasympathetic stimulation led to a decrease in permeability-surface area for EDTA (-19.1 +/- 5.2%, mean +/- S.E., n = 5, P less than 0.05) cyanocobalamin (-12.3 +/- 6.0, n = 12, P less than 0.05), and insulin (-15.3 +/- 4.8, n = 11, P less than 0.02). It is suggested that this may be the result of a redistribution of flow from the acinar microcirculation to a less permeable ductal vasculature. 4. Bradykinin infusion had no significant effect on permeability-surface area for EDTA and cyanocobalamin in perfused glands. 5. In perfused glands, ligation of the submandibular duct for 3--12 days reduced permeability-surface area (ml.min-1.g-1) for [51Cr]EDTA from 5.26 +/- 0.60 (mean +/- S.E., n = 9) to 4.20 +/- 0.12 (n = 4, P less than 0.30), [57Co]cyanocobalamin from 3.22 +/- 0.12 (n = 48) to 2.02 +/- 0.08 (n = 15, P less than 0.001) and [125I]insulin from 1.52 +/- 0.07 (n = 39) to 0.72 +/- 0.23 (n = 11, P less than 0.001).

Microvascular filling pattern in rat liver sinusoids during vagal stimulation

1. The terminal microcirculation in the transilluminated ventral margin of the rat liver was observed and recorded by a video-microscope system. The volumetric flow rate in a liver sinusoid was calculated from the observed diameter of the sinusoid and the intra-sinusoid erythrocyte flow velocity. 2. The topographic distribution of liver sinusoids within an arbitrary boundary of a microscopic field of terminal liver microcirculation was observed and the total inflow and outflow in the field were determined. 3. Both vagus nerves at the lower end of the oesophagus were stimulated at supramaximal voltage. Vagal stimulation dilated the calibre of liver sinusoids and paradoxically diminished the erythrocyte flow velocity in each individual liver sinusoid, but the total volumetric flows in a microscopic field remained unchanged. 4. Vagal stimulation also increased the number of liver sinusoids in a microscopic field by opening previously closed liver sinusoids. This recruitment contributed two-thirds of the total increase of the sinusoidal capacity while the other one third was the result of distension of existing liver sinusoids.