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

Blood Flow and Interstitial Fluid Pressure in the Rat Submandibular Gland during Changes in Perfusion

The submandibular gland is a cell-rich encapsulated organ with high transport of fluid through the interstitial space during salivation. We hypothesized that the gland is a low-compliant tissue, i.e., that a modest increase in fluid volume will produce a rise in interstitial fluid pressure (IFP) counteracting fluid filtration into the interstitium. To test this hypothesis, we measured IFP with micropipettes and glandular blood flow (GBF) with a laser-Doppler flowmeter during changes in perfusion. Clamping of the carotid artery or the jugular vein, or electrical stimulation of the sympathetic or parasympathetic nerve to the gland, induced changes in perfusion. Baseline IFP averaged 3.5 ± 0.5 mm Hg. Clamping of the artery reduced IFP and GBF (−56.5 ± 8.4% and −53.1 ± 6.4%, respectively), whereas clamping of the vein decreased GBF (−21.6 ± 14.3%) and increased IFP (141.2 ± 27.4%). Sympathetic nerve stimulation reduced both parameters (−86.9 ± 16.5% and −74.4 ± 7.0%, respectively). In contrast, stimulation of the parasympathetic nerve elicited an increase in GBF (133.2 ± 5.9%) and in IFP (173.3 ± 41.4%). Thus, changes in vascular volume led to concomitant changes in IFP consistent with low tissue compliance, a phenomenon of importance for fluid volume regulation.

The influence of hypoxia on transvascular leakage in the isolated rat heart: quantitative and ultrastructural studies

1. The multiple indicator dilution method was used to study the transvascular movement of gamma-globulin, bovine serum albumin, insulin and cyanocobalamin in the isolated rat heart. 2. Perfusion of the heart with well-oxygenated solution for 75 min (constant flow) did not produce a significant change either in the total area under the dilution curve or the 'leakage index' (an arbitrary measure of transvascular flux) for all the tracers. 3. Perfusion of the heart with hypoxic solution produced a significant increase in leakage of gamma-globulin of 38.6 +/- 18, 48.5 +/- 17.6, 60.5 +/- 24 and 58 +/- 20% after 15, 30, 45 and 60 min, respectively, compared with the well-oxygenated equilibration period. Permeability- surface area products (PS) for the smaller diffusible solutes, therefore, could not be estimated. 4. The flux of albumin, insulin and cyanocobalamin in response to hypoxia was similar to that of gamma-globulin. 5. Ultrastructural examination of well-oxygenated hearts revealed that Monastral Blue-labelled albumin remained within the lumen and that endothelial integrity remained intact. 6. Conversely biopsies from hypoxic hearts showed that the labelled albumin had passed to the interstitium through gaps (approximately 3 microns) in venular endothelium. 7. The results showed that, in intact hearts, hypoxia produced gaps in the endothelium of venules and that these gaps could be the possible route for transvascular leakage of macromolecules.

Secretory oedema in diabetic submandibular glands during parasympathetic nerve stimulation: relationship to microvascular abnormalities in streptozotocin-treated rats

1. Submandibular secretion during parasympathetic stimulation (5 Hz) was examined in streptozotocin-diabetic and age-matched control rats. 2. At 3 weeks, but not 3 and 6 months, flow rate was initially greater than in controls, but it declined rapidly after 30 min. 3. The reduction in flow rate was associated with oedema of the gland. 4. At 3 months, graded stimulation revealed a tendency to oedema at frequencies of 10 Hz and above. 5. Morphologically, submandibular capillary density was increased in diabetic rats. 6. Thus, in diabetes the submandibular gland appears less able to withstand continuous parasympathetic stimulation, due in part to an increase in tissue capillary area.

Filtration coefficient and osmotic reflection coefficient to albumin in rabbit submandibular gland capillaries

1. The isolated perfused submandibular salivary gland of the rabbit has been used in order to make estimates of the filtration coefficient (Kf) and reflection coefficient (sigma d) of the capillary wall to albumin. 2. An isogravimetric preparation was used and in paired experiments the value for Kf obtained in glands perfused with albumin-Krebs solution, 0.96 +/- 0.086 (mean +/- S.E. of mean) ml min-1 mmHg-1 100 g-1, was not significantly different from that in blood-perfused glands, 0.90 +/- 0.15. 3. On analysing the data for reflection coefficient, it was concluded that the above values underestimated Kf by about 30%; using corrected values for Kf, osmotic reflection coefficients were determined from the weight changes following a sudden change in the oncotic pressure of the perfusate. The value for sigma d to albumin lay between 0.79 and 1.0, the lower value being obtained after the Kf correction. 4. The high hydraulic conductivity, combined with sieving properties comparable to those in continuous capillaries, is consistent with other data on fenestrated capillaries. 5. Finally, it was observed that, while the Kf value calculated from the initial flux rate was similar whether measured during fluid efflux from or influx into the microvasculature, on returning to the initial conditions after raising osmotic pressure, efflux was now more rapid than influx. This phenomenon is discussed in relation to readjustment of Starling forces and the possible existence of an asymmetric double membrane in the capillary, interstitium system and cells of the salivary gland.

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.

Restricted diffusion of CrEDTA and cyanocobalamine across the exchange vessels in rat hindquarters

The degree of diffusional restriction of skeletal muscle capillary walls to small solutes was estimated from the permeability surface area products (PS) of CrEDTA (MW = 341) and cyanocobalamine (MW = 1355), using computerized 'on-line' recordings of venous single injection indicator dilution curves. Experiments were performed on isolated perfused maximally vasodilated rat hindquarters during largely isogravimetric conditions and the arrangements allowed for measurements of capillary filtration coefficients (CFC). Extraction of tracer varied markedly as a function of transit time and, furthermore, PS increased with increasing flows, both these phenomena indicating tissue and flow heterogeneity. At maximal flows the disturbing influence of heterogeneity will be minimal and hence the diffusion capacities obtained by extrapolating PS area to infinite flows, so called PS tot values, were considered to give the best estimation of the 'true' capillary diffusion capacities. The value of PS tot was 12.9 +/- 0.5 for CrEDTA and 5.1 +/- 0.3 ml min-1 per 100 g for vitamin B12. The calculated PS tot ratio of 2.59 +/- 0.11 indicates restricted diffusion through equivalent pores of radius 53 A, whereas the ratio of the free diffusion coefficients for these solutes is 1.79. Using PS peak for the calculations (totally neglecting heterogeneity) the pore radius was, however, markedly overestimated. Thus, for a PS-ratio of 1.89 +/- 0.04 for CrEDTA vs. B12 an equivalent pore radius of 300 A was calculated. Also, using PS area (only partly correcting for heterogeneity) overestimated the pore radius (70 A) from a mean PS-ratio of 2.33 +/- 0.05. It was concluded that the equivalent pore radius in rat hindquarter microvascular walls is 53 A or even smaller in essential agreement with data from osmotic transient experiments in the same preparation (r approximately 40 A).

Exchange area and surface properties of the microvasculature of the rabbit submandibular gland following duct ligation

The exchange area of the submandibular salivary gland microvasculature has been measured to allow the value of microvascular permeability (P) to hydrophilic solutes to be calculated from previous measurements of permeability-surface area (PS) products. Glands whose ducts had been ligated for 2 weeks and the contralateral control glands were perfusion-fixed with a modified Karnovsky's fixative after perfusion with a solution containing cationized ferritin, and examined with transmission electron microscopy. Stereological techniques were used to estimate the surface area of the exchange vessels on random thin sections from four control and four duct-ligated glands. The mean exchange surface area in control glands was 512 cm2 g-1 and 336 cm2 g-1 in duct-ligated glands. The fenestral density was calculated to be 0.57% of the exchange surface in control glands and 0.30% in duct-ligated tissue. Molecules of cationized ferritin appeared bound to the luminal surface of the microvascular endothelium, including the surface of the fenestrae to a depth of about 25 nm in both control and ligated glands. These experiments have shown that the exchange surface area of the fenestrated endothelium of the submandibular salivary gland is comparable to that in cardiac muscle but the permeability (P) to small solutes is about 10 times greater. Following ligation of the salivary gland duct, solute permeability falls and an explanation of this, based on the reduced surface area and the nature of the permeability-flow relationship for small solutes is offered.

Techniques and applications of extracellular space determination in mammalian tissues

This review summarizes the ways in which the extracellular space (ECS) may be estimated in mammalian tissues, and briefly describes some of the uses to which the EC confinement of certain molecules (markers or tracers) may be put in the elucidation of physiological functions. The introductory section is followed by a description of the more commonly used marker molecules and their functional characteristics, and of factors likely to lead to the spurious over- or under-estimation of the ECS. Certain alternative methods are also described, in particular those based on morphological and electrical criteria which seek to demonstrate small, functionally important, changes in the size of specialized regions of the ECS (e.g. lateral cellular interspaces) without necessarily being required to provide a quantitatively precise estimate of their size. Section III describes the results of measurements of ECS in various mammalian tissues (muscle, gastro-intestinal tract, nervous tissue, crystalline lens, placenta, lung and kidney) and some applications of EC markers to investigation of cellular function (e.g. uptake of metabolic substrates and epithelial transport) and, in outline, characterization of capillary permeability. The available literature in this field is very extensive, and in the interests of brevity the reader is, where appropriate, referred to previous reviews covering specialized aspects of ECS determination and related topics.

Permeability of fenestrated capillaries in the isolated pig pancreas, with effects of bradykinin and histamine, as studied by simultaneous registration of filtration and diffusion capacities

Combining an isogravimetric technique and a colorimetric 'on-line' method (Rippe & Stage 1978), filtration capacity (CFC) and diffusion capacity (PS) were simultaneously measured in the maximally vasodilated 'fenestrated' capillary bed of isolated, artificially perfused pancreatic glands in 12 juvenile pigs. Both CFC and PS for Cr-EDTA were about 20 times greater than in the 'continuous' capillary bed of skeletal muscle. With perfusate flow rates of 250 ml/min x 100 g during isogravimetry, PS-Cr-EDTA averaged 110 +/- 10.0 (S.E.) ml/min X 100 g, and diffusion limitation occurred first at flow rates above 300 ml/min X 100 g. CFC was independent of flow rate and averaged 0.641 +/- 0.027 ml/min X 100 g X mmHg. The parallel augmentation of PS-Cr-EDTA and CFC in the fenestrated capillary bed compared with continuous ones seems to reflect both a higher number of capillaries per unit tissue and an increased number of 'small pores' per unit capillary surface, whilst the 'large pore system' appears to be similar. Following bradykinin or histamine infusion, results were similar to those for continuous capillaries (e.g. Rippe, Kamiya & Folkow 1978). Thus, without further vasodilatation CFC increased 3-fold While PS-Cr-EDTA increased only some 25%, and subsequent isoprenaline infusion reversed these effects. Previous studies on continuous capillaries indicate that histamine-type agents act by opening additional 'large pores' in the venular exchange sections (cf. Rippe & Grega (1978, Svensjö 1978), while beta-adrenergic agonists block this effect. The results further suggest that the fenestrae are not involved in these bradykinin-histamine effects, but rather function as a high-density, small pore population.

Permeability of the fenestrated capillaries in the cat submandibular gland to lipid-insoluble molecules

1. Permeability-surface area products for the fenestrated capillaries in the perfused cat submandibular gland have been measured for graded lipid-insoluble molecules using the single-passage, multiple-tracer dilution technique. 2. The permeability-surface area for [57Co]cyanocobalamin (mol. wt. 1353) increased as the perfusion flow was increased, but reached a constant value of 4.11 +/- 0.25 ml.min-1.g-1 (mean +/- S.E., n = 9) at flows above 8 ml. min-1.g-1. For [125I]insulin (approximate mol. wt. 6000) it was 1.80 +/- 0.13 ml.min-1.g-1 (mean +/- S.E., n = 9) and apparently diffusion-limited at all the high flow rates studied. A similar permeability-surface area product was measured for [14C]inulin (mol. wt. 5500): 1.76 +/- 0.10 (mean +/- S.E., n = 4). 3. Permeability-surface area values for cyanocobalamin and insulin in the salivary gland are respectively about 20 and 200 times larger than the estimates reported for the continuous capillaries of cardiac and skeletal muscle. 4. The permeability-surface area (PS) ratio [57Co]cyanocobalamin/[125I]insulin (2.33 +/- 0.15, mean +/- S.E., n = 9) was significantly greater than the apparent ratio of their free diffusion coefficients (1.76), suggesting restricted diffusion of insulin relative to cyanocobalamin across the capillary endothelium. 5. Permeability-surface area products for the smaller molecular weight tracers (22Na, 86Rb and 51Cr-EDTA (mol. wt. 357)) increased continuously with perfusion rate, indicating flow-limited solute exchange. The PS ratio of Rb/EDTA was close to unity whereas the corresponding free diffusion ratio is 3.85. 6. The high permeability-surface area values measured were thought to be associated with the fenestrae which appeared to act as high concentrations of 'small pores' rather than as 'large pores'.