Observations of valve-like structures and evidence for rectification of flow within the gill lamellae of the crab Carcinus maenas (Crustacea,Decapoda)

Summary The isolated gills of Carcinus maenas, perfused at pressure drops of 1–10 cm of water, exhibited flow rectification, the resistance to perfusion via the afferent vessel being many times lower than that for efferent perfusion. The asymmetry was greater at the lower end of this pressure range.The overall afferent branchial resistance for Carcinus of weight 65 g, and with no ventilatory component in the transmural pressure difference, was estimated to be 0.05 cm of water. l^–1 · sec. The corresponding overall reverse (efferent) branchial resistance was 0.36 cm of water · l^–1 · sec.LM, TEM and SEM examination of the gills indicated that haemolymph leaves each gill lamella via several discrete parallel efferent channels which drain different regions of the lamella, and that each efferent channel is nearly closed, at its junction with the efferent branchial vessel, by a diaphragm of loosely interwoven and very elongated cells. It is concluded that these cells may constitute efferent valves and that narrow apertures between them may contribute a major component to the branchial resistance and be primarily responsible for the rectification of flow. Relatively wide apertures lead directly from the afferent vessel into the lamellae and are not asociated with valves of any kind.The valves may be important in enabling changes in transmural pressure associated with ventilatory reversals to pump haemolymph unidirectionally through the lamellae. Similarly valves may allow the oscillating venous pressures associated with locomotor activity to improve gill perfusion during exercise.The elongated tails of the cells of the efferent valve contain numerous microtubules. The wider cell bodies contain the nucleus and many mitochondria. Unusual organelles composed of many short (about 0.25 m long) microtubules and often lying close to the nuclear membrane may be microtubule organising centres. It is speculated that, in addition to their simple mechanical function, the valve cells may play a more dynamic role in regulating flow of haemolymph through different lamellar routes, or that they may monitor composition, pressure or flow of the efferent lamellar circulation.     

Haemodynamic effects of adenosine on gills of the trout (Salmo gairdneri)

Summary The haemodynamic effects of adenosine on gills of the trout (Salmo gairdneri) were studied with in vitro and in vivo preparations.On the isolated head preparation, adenosine induced a decrease of the ventral aortic inflow and of the dorsal aortic outflow. Simultaneously the venous outflow increased. These effects were antagonized by theophylline. Adenosine induced a vasoconstriction in gill arches without filaments perfused by the afferent or the efferent branchial arteries. The efferent vessels were more sensitive to adenosine than afferent vessels. The whole systemic circulation of the isolated trunk did not show any response to adenosine. When adenosine was infused into the ventral aorta of living trout, the gill resistance to blood flow was greatly increased.These results suggest that adenosine is able to control the arterious and venous blood pathways in the trout gills by modulating their vascular resistance.     

Chloride extrusion in the isolated perfused teleost gill

Summary Chloride extrusion is examined in the isolated perfused gill of the pinfish,Lagodon rhomboides. In both sea water and Ringer's baths, the Cl efflux from the isolated gill is 45% that of the intact animal. The transepithelial electrical potential (TEP) across the isolated gill in sea water is equal to that in vivo, in Ringer's the gill TEP is slightly less than in vivo. Cl efflux is linearly dependent upon afferent flow of the perfusate. Furosemide, added to the perfusate inhibits 57% of the Cl efflux in gills bathed bilaterally by Ringer's. Ouabain causes a marked vasoconstriction and increase in afferent pressure. Removal of Na from the perfusate produces an inhibition of the Cl efflux that is not potential mediated. Net extrusion of Cl is inhibited in isolated gills bathed bilaterally by sodium free Ringer's.     

Sympathetic nervous control of blood flow in the gills of the Atlantic cod,Gadus morhua

Summary The effects of sympathetic nerve stimulation, adrenaline and isoprenaline on the inflow pressure and efferent arterial and venous flow rates were studied in a cod gill preparation perfused at constant flow rate.The dominant effect of adrenaline was a reduced inflow pressure, accompanied by an increase in arterial flow and a decrease in venous flow. Isoprenaline also decreased the inflow pressure, but the changes in both outflow rates were small or absent.Sympathetic nerve stimulation gave arterial and venous flow changes comparable to the adrenaline effects, but the inflow pressure increased during nerve stimulation. Propranolol has little effect on the nerve responses, but phentolamine abolished or reversed the increase in inflow pressure, and also decreased or abolished the changes in outflow rates.The possible sites of action of the sympathetic fibres, and the distribution of adrenoceptors in the effector tissue is discussed. It is concluded that the main effect of sympathetic nerve stimulation is -adrenoceptor mediated, involving constriction of the arterio-venous pathway. The-adrenoceptor mediated control of total branchial vascular resistance may largely depend on circulating catecholamines.     

Morphology and ultrastructure of the gills of terrestrial crabs (Crustacea,Gecarcinidae and Grapsidae): Adaptations for air-breathing

Summary The terrestrial crabsGeograpsus grayi, Geograpsus crinipes, Cardisoma hirtipes andGecarcoidea natalis have a reduced number of gills and show a reduced planar gill surface (SA) compared to aquatic species. Gill lamellae are stiffened and thickened (increasing blood/gas (BG) diffusion distances) and nodules maintain wide spacing between lamellae. Haemolymph is directed through the gill lamellae by rows of pillar cells and in the afferent region an intralamellar septum splits the haemolymph into two parallel networks. Gaps in the lines of pillar cells allow movement of haemolymph between adjacent channels. The afferent vessel distributes haemolymph to the lamella via a number of direct channels including the marginal canal and in large gills with the aid of a long, forked sinus which supplies the ventral and central regions of the lamellae. The marginal canal functions in both distribution and collection of haemolymph; the role varies with species. Potential flow-control sites were identified at the junctions between afferent and efferent areas and where the efferent channels enter the efferent branchial vessel. Each gill receives a branch from the sternal artery which supplies all the lamellae. Transport epithelia is the principal cell type in the gills of all species examined though its location varies between species, either being confined to certain gills or specific parts of the lamellae.The gill lamellae of air-breathing crabs are clearly modified to breathe air (stiffening and presence of nodules), though the overall contribution of the gills to gas exchange has been reduced (smaller SA and longer BG diffusion distances). The role of the gills in air-breathing crabs thus appears to have switched from one of an efficient aquatic gas-exchanger (thin with large surface area) and transport tissue, to one that is predominantly set up for ion-regulation.Abbreviations a afferent branchial vessel - ac afferent channels - art arteriole - ass artifactual subcuticular space - bl basal lamina - c cuticle - col collagen - ct connective tissue - e efferent branchial vessel - ec efferent channels - epi epithelium - f folds - g Glycogen - h haemolymph - hc haemocyte - is intralamellar septum - m marginal canal - mi mitochondria - mt microtubules - n nucleus - p pillar cell - s shaft of efferent vessel - sd septate desmosome     

Gills of antarctic fish

We review the literature on the way the structure of icefish gills relates the physiology of these haemoglobin-less fishes. Vascular casting confirmed earlier reports that the only special feature of the gills is the large size of the blood vessels, especially the prominent and continuous marginal channels Isolated perfused gill arches were used to study the effects of changes in afferent and efferent pressure on gill resistance and tritiated water influx in Chionobathyscus dewitti. Increasing perfusion rate did not change gill resistance, but there were moderate proportional increases in water influx. Reducing efferent pressure increased gill resistance but did not affect water influx. In both C. dewitti and Cryodraco antarcticus gills perfused at constant flow rate, noradrenaline produced concentration-dependent decreases in gill resistance and, with high concentrations, increases in water influx. Fixation while perfusion continued was used to compare blood space dimensions in control, noradrenaline-treated and unperfused gills. Noradrenaline caused large increases in the thickness of the lamellar blood space and increased lamellar height, despite a greatly reduced afferent pressure. This suggests that modulation of pillar cell active tension might be involved in control of lamellar perfusion. The possible relationship between gill water fluxes and lamellar recruitment is discussed.     

The linear cable theory as a model of gill blood flow

The vascular organization of the teleost gill suggests that blood flow distribution from the filamental artery to the respiratory lamellae is governed by relationships analogous to the cable conduction properties of a nerve axon. The space constant (λ) by definition is the distance along the gill filament at which the in-series resistance of the afferent filament artery equals the in-parallel resistance of the afferent lamellar arteriolar, lamellar, efferent lamellar arteriolar (ALA-L-ELA) segments. Constriction of the afferent filamental artery or uniform dilation of the ALA-L-ELA will decrease λ. As λ decreases, flow through the proximal (basal) lamellae greatly increases at the expense of distal lamellar perfusion. When λ increases in a system of finite length the flow profile must account for reflected pressures within the main vessel. The λ calculated from corrosion casts of gill vasculature is $\text{1}\text{4}$ to $\text{1}\text{2}$ the filament length. This favors blood flow through the proximal lamellae and when cardiac output increases, the proportion of cardiac output perfusing the proximal areas increases at the expense of distal lamellar blood flow. To offset these changes it is proposed that increased distal lamellar perfusion is achieved by simultaneous vasodilatation of distal and constriction of proximal ALA-L-ELA segments and dilation of the afferent filamental artery.     

Gill morphometrics in relation to gas transfer and ram ventilation in high‐energy demand teleosts: Scombrids and billfishes

This comparative study of the gill morphometrics in scombrids (tunas, bonitos, and mackerels) and billfishes (marlins, swordfish) examines features of gill design related to high rates of gas transfer and the high‐pressure branchial flow associated with fast, continuous swimming. Tunas have the largest relative gill surface areas of any fish group, and although the gill areas of non‐tuna scombrids and billfishes are smaller than those of tunas, they are also disproportionally larger than those of most other teleosts. The morphometric features contributing to the large gill surface areas of these high‐energy demand teleosts include: 1) a relative increase in the number and length of gill filaments that have, 2) a high lamellar frequency (i.e., the number of lamellae per length of filament), and 3) lamellae that are long and low in profile (height), which allows a greater number of filaments to be tightly packed into the branchial cavity. Augmentation of gill area through these morphometric changes represents a departure from the general mechanism of area enhancement utilized by most teleosts, which lengthen filaments and increase the size of the lamellae. The gill design of scombrids and billfishes reflects the combined requirements for ram ventilation and elevated energetic demands. The high lamellar frequencies and long lamellae increase branchial resistance to water flow which slows and streamlines the ram ventilatory stream. In general, scombrid and billfish gill surface areas correlate with metabolic requirements and this character may serve to predict the energetic demands of fish species for which direct measurement is not possible. The branching of the gill filaments documented for the swordfish in this study appears to increase its gill surface area above that of other billfishes and may allow it to penetrate oxygen‐poor waters at depth. J. Morphol. 2010. © 2009 Wiley‐Liss, Inc.     

A morphological study on posterior gills of the mangrove crab Ucides cordatus

Morphological and histological studies on posterior gills of the mangrove crab Ucides cordatus showed that the 5th gill (of 7) has a larger surface area and a greater number of lamellae compared to the 6th gill. Regular separation of gill lamellae, important when the gill is in air, is maintained by enlargements of the marginal canals. Conical, spine-like structures along the efferent vessel of both 5th and 6th gills were also observed. In addition, pillar cells, a discontinuous lamellar septum and a hypobranchial artery were observed. The presence of valve-like structures near the efferent vessel was also indicated. These structures, together with the pillar cells, may have a role in directing the hemolymph flow towards certain gills during particular physiological states. Localization of osmoregulatory epithelia in the lamellae of both gills was inferred from dimethylaminostyrylethylpyridiniumiodine staining. Apparently gills 5 and 6 have osmoregulatory epithelial cell patches of similar area, corresponding to 43% and 38% of the total lamellae area, respectively. However, their localization is quite different. Gill number 5 osmoregulatory patches seem to be restricted to the afferent region of the lamella whereas in gill number 6, they are more dispersed over the entire lamella. These differences may be related to the particular functional characteristics of these gills.     

Gill Morphometrics of the Lampreys Lampetra fluviatilis (L.) and Lampetra planeri (Bloch)

Morphometric measurements have been made on various gill components of different stages in the life cycle of the anadromous parasitic lamprey, Lampetra fluviatilis, and its nonparasitic derivative Lampetra planeri. The total gill area, expressed in terms of body weight, of both larval (1462–2717 mm² g^–1) and adult (1402–2337 mm² g^–1) L. fluviatilis are greater than those previously recorded in the rather meagre literature on lamprey gill measurements and are comparable with those found in the most active teleosts. The gills of the two Lampetra species are apparently identical in the larval stages and those of metamorphosing and adult L. planeri are similar to those of metamorphosing L. fluviatilis. Although the pharyngeal arrangement of lampreys differs greatly from that of teleosts, there are many features of the gills indicative of convergence between the two groups. Thus, in a given stage in the life cycle of lampreys, the secondary lamellae on either side of the filaments also alternate, become more widely spaced as the filament length increases and increase in area as the body weight becomes greater. Furthermore, the fractional cumulative increase in secondary lamellae area along a line following the presumed direction of water flow is also represented by a sigmoid curve. While at metamorphosis the pharynx becomes considerably modified to accommodate the change from a unidirectional to a tidal respiratory water flow, the total gill areas of the ammocoete are similar to those of metamorphosing stages which have attained adult characteristics. However, there are clearly differences in some of the components that influence and contribute towards the total gill area. Thus, in terms of body weight, the number and total length of the filaments and the total number of secondary lamellae, together with the number of secondary lamellae found on a given distance of filament, are greater in late metamorphosing stages, while the reverse is true for the average bilateral area of the secondary lamellae which is considerably greater in ammocoetes.     

Endothelin redistributes blood flow through the lamellae of rainbow trout gills

The lamellae of the fish gill are the primary sites for oxygen uptake from the water. Here, only two very thin layers of cells separate the blood from the water. Therefore, energetically costly ion-fluxes will also occur between blood and water, and it has been hypothesised that the blood flow within the lamellae can be regulated through vasoconstriction, but evidence for this has been lacking. Through direct observations of the lamellae of rainbow trout (Oncorhynchus mykiss) in vivo, using epi-illumination microscopy, we show here that an endothelium-derived vasoactive peptide, endothelin-1 (ET-1, 0.2 μg kg⁻¹ or 1.0 μg kg⁻¹), is able to completely constrict the vascular sheet in the lamellae, probably by inducing contraction of pillar cells. This coincided with a dose-dependent increase in ventral aortic blood pressure (rising from 6.6 kPa to 12.0 kPa in response to the high ET-1 dose). However, blood continued to flow through the marginal channel that circumvents each lamella. Thus, ET-1 caused an intralamellar blood shift from the lamellar sheet towards the marginal channels. Vasoconstriction in the lamellae is likely to provide the fish with a mechanism for matching its respiratory surface area with its respiratory needs, thereby minimising ion-fluxes. Accepted: 8 September 1998     

Adrenergic innervation of the gills of brown and rainbow trout,Salmo trutta and S. gairdneri

The adrenergic innervation of structures in the gills of brown and rainbow trout was studied with catecholamine fluorescence histochemistry. In the arterio-arterial vascular pathway, there was an innervation of the afferent and efferent lamellar arterioles, but the afferent and efferent filamental arteries and the secondary lamellae were devoid of any fluorescent nerve fibres. In S. trutta only, there was an additional innervation of the afferent and efferent branchial arteries and the base of the efferent filamental artery. The innervation of the arterio-venous vascular pathway was similar in both trout species. Many fluorescent nerve fibres were found on nutritive arterioles in the gill arch and interbranchial septum, and in the core of each filament between the surface epithelium and the wall of the filament venous sinus. No fluorescent nerve fibres were observed at the origins of the capillaries arising from the efferent filamental artery. The sympathetic nerve supply is provided to the gills mainly through the posttrematic nerve, with an occasional small contribution through the pretrematic nerve. The presence of adrenergic nerves in the gills is discussed in relation to the regulation of blood flow through the arterio-arterial and arterio-venous pathways.     

An electron microscopic study on the innervation of the gill filaments of a lamprey,Lampetra japonica

This paper reports observations on the innervation of gill filaments of the lamprey, Lampetra japonica. Nerve fibers run on each side of the afferent filament artery (AFA nerve) and in the connective tissue compartment along the efferent filament artery (EFA nerve). The AFA nerve supplies vasomotor fibers to the afferent filament artery and arteriovenous anastomoses and special visceral motor fibers to branchial muscle fibers (musculus compressor branchialis circularis). Nerve endings of the vasomotor fibers contain large, cored vesicles (60–180 nm in diameter) with a variable number of small, clear vesicles (30–70 μm in diameter), whereas those of the visceral motor fibers have many small, clear vesicles with few large, cored vesicles. The EFA nerve supplies vasomotor fibers to the efferent filament artery. Their endings, containing mixtures of predominantly large, cored vesicles and small, clear vesicles make close synaptic contacts with reticular cells. The latter in turn are connected with each other or with smooth muscle cells in the wall of the efferent filament artery by nexuses. No nerves are found in the axial plate between the afferent and efferent filament arteries nor in the secondary lamellae of individual gill filaments. No afferent nerve supply to the gill filament has been found.     

Vascular organization of the head and respiratory organs of the air-breathing catfish,Heteropneustes fossilis

Light and scanning electron microscopy of vascular replicas from the facultative air-breathing fish Heteropneustes fossilis show modifications in the macrocirculation of the respiratory organs and systemic circulation, whereas, gill microcirculation is similar to that found in typical water-breathing fish. Three and sometimes four ventral aortae arise directly from the bulbus. The most ventral vessel supplies the first pair of arches. Dorsal to this another aorta supplies the second gill arches, and a third, dorsal to, and larger than the other two, supplies the third and fourth arches and the air sacs. Occasionally a small vessel that may be the remnant of a primitive aortic arch arises from the first ventral aorta and proceeds directly to the mandibular region without perfusing gill tissue. The air sac is perfused by a large-diameter extension of the afferent branchial artery of the fourth gill arch and its circulation is in parallel with the gill arches. Blood drains from the air sac into the fourth arch epibranchial artery. A number of arteries also provide direct communication between the efferent air sac artery and the dorsal aorta. All four gill arches are well developed and contain respiratory (lamellar) and nonrespiratory (interlamellar and nutrient) networks common to gills of water-breathing fish. Air sac lamellae are reduced in size. The outer 30% of the air sac lamellar sinusoids are organized into thoroughfare channels; the remaining vasculature, normally embedded in the air sac parenchyma, is discontinuous. A gill-type interlamellar vasculature is lacking in the air sac circulation. Despite the elaborate development of the ventral aortae, there is little other anatomical evidence to suggest that gill and air sac outflow are separated and that dorsal aortic oxygen tensions are maintained when the gills are in a hypoxic environment. Physiological adjustments to hypoxic water conditions probably include temporal regulation of gill and air sac perfusion to be effective, if indeed they are so.     

Vascular resistance and vasoactivity of gills and pulmonary artery of the salamander,Ambystoma tigrinum

Summary In order to understand the blood flow patterns and their regulation in the gills and pulmonary artery ofAmbystoma tigrinum, the vascular resistance and vasoactivity of the two major branchial perfusion pathways and a vascular plexus in the pulmonary artery were investigated using an isolated-tissue perfusion method. Acetylcholine and epinephrine were both pressor agents in all three vascular segments. Angiotensin II also constricted the branchial respiratory vasculature. Norephinephrine was primarily a vasodilator in the branchial respiratory vasculature, however, it had no effect on the shunt vessels of the gill or the pulmonary arterial plexus. Both gill circulations were insensitive to alterations in CO₂ and pH. Anoxia produced a slight vasodilation of the branchial respiratory vessels but had no effect on the shunt vasculature. Mild hypoxia had no effect on either branchial circulations. The results suggest that: (1) blood flow through the respiratory section of the gill may vary between 8 and 47% of total gill flow, (2) the major perfusion pathway to the lung is probably from the efferent artery of the third gill through the ductus arteriosus and then into the pulmonary artery, (3) O₂, CO₂ and pH exert no local control of branchial perfusion, (4) both cholinergic and adrenergic regulation of branchial and proximal pulmonary arterial vascular resistance is possible, (5) a rise in circulating norepinephrine should increase blood flow to the respiratory section of the gill.Abbreviations AII angiotensin II - ACh acetylcholine - EPi epinephrine - NE norepinephrine     

Functional morphology of the gills of the shortfin mako,Isurus oxyrinchus,a lamnid shark

This study examines the functional gill morphology of the shortfin mako, Isurus oxyrinchus, to determine the extent to which its gill structure is convergent with that of tunas for specializations required to increase gas exchange and withstand the forceful branchial flow induced by ram ventilation. Mako gill structure is also compared to that of the blue shark, Prionace glauca, an epipelagic species with lower metabolic requirements and a reduced dependence on fast, continuous swimming to ventilate the gills. The gill surface area of the mako is about one‐half that of a comparably sized tuna, but more than twice that of the blue shark and other nonlamnid shark species. Mako gills are also distinguished from those of other sharks by shorter diffusion distances and a more fully developed diagonal blood‐flow pattern through the gill lamellae, which is similar to that found in tunas. Although the mako lacks the filament and lamellar fusions of tunas and other ram‐ventilating teleosts, its gill filaments are stiffened by the elasmobranch interbranchial septum, and the lamellae appear to be stabilized by one to two vascular sacs that protrude from the lamellar surface and abut sacs of adjacent lamellae. Vasoactive agents and changes in vascular pressure potentially influence sac size, consequently effecting lamellar rigidity and both the volume and speed of water through the interlamellar channels. However, vascular sacs also occur in the blue shark, and no other structural elements of the mako gill appear specialized for ram ventilation. Rather, the basic elasmobranch gill design and pattern of branchial circulation are both conserved. Despite specializations that increase mako gill area and efficacy relative to other sharks, the basic features of the elasmobranch gill design appear to have limited selection for a larger gill surface area, and this may ultimately constrain mako aerobic performance in comparison to tunas. J. Morphol. 271:937–948, 2010. © 2010 Wiley‐Liss, Inc.     

The structure of the gill of the trout,Salmo gairdneri (Richardson)

Summary A light and electron microscopic study was made of the structure of the gill arch, filament and secondary lamella of Salmo gairdneri R. Blood pathways through the gill were traced from serial histological sections, and from the examination of ink perfused tissue and perspex casts formed following resin injection of the circulatory system.The epithelium covering the gill consists of unspecialized, dark, chloride and mucous cells. The distribution of specialized cells appears to be related to gill function. The basement membrane underlying the epithelium consists of three layers, the inner collagen layer being continuous with the connective tissue core of the gills.Blood supply to the secondary lamellar respiratory surface is via branchial, filament and secondary lamellar arteries. Blood spaces of the secondary lamellae are delimited by pillar cells containing what appears to be contractile material. The marginal channel of each lamella is bounded distally by cells of endothelial origin. A network of lymph spaces within the filaments connects with efferent branchial arteries. Nutritionary capillaries within the filaments connect with afferent branchial arteries. No shunts between afferent and efferent filament arteries were found.Data from this study and previous physiological and histopathological studies suggest a mechanism for the control of blood flow to suit the respiratory requirements of the fish. This mechanism involves a system of recruitment of additional respiratory units and changes in overall blood flow patterns.This work formed part of a thesis submitted for the degree of Doctor of Philosophy in 1971 and for which M. M. was in receipt of a studentship from the Natural Environmental Research Council. The authors are grateful for the support given by research grants from the M.R.C (P.T.) and the N.E.R.C. (M.M.), and to Prof. G. M. Hughes in whose department the work was carried out.     

Functional Organization of the Teleost Gill

The circulation of the gills has been studied in the perch, trout and eel combining the conventional histological methods and casting techniques. The existence of two blood pathways in each gill arch was confirmed. 1 — An arterio-arterial pathway assuming the respiratory function. It includes the afferent branchial artery and in each primary lamella the afferent primary artery, the secondary lamellae capillaries and the primary and branchial efferent arteries. 2 — An arterio-venous pathway arising from both the branchial artery, in the gill arch, and the primary arteries in each primary lamella. This pathway includes the central venous sinus of the primary lamella, several small veins and is finally connected with the branchial veins. 3 — The lack of connections between afferent primary arteries and cvs in the trout and the perch makes impossible a direct blood flow from the afferent to the efferent artery (shunt). In the eel connections between cvs and both afferent and efferent arteries do not mean that a shunt is operating according to the pressure gradient.     

Blood flow distribution with adrenergic and histaminergic antagonists

Superficial fibular nerve stimulation (SFNS) causes increased pre- and post-capillary resistances as well as increased capillary permeability in the dog hind paw. These responses indicate possible adrenergic and histaminergic interactions. The distribution of blood flow between capillaries and arteriovenous anastomoses (AVA) may depend on the relative effects of these neural inputs. Right hind paws of anesthetized heparinized dogs were vascularly and neurally isolated and perfused with controlled pressure. Blood flow distribution was calculated from the venous recovery of 85Sr-labeled microspheres (15 microns). The mean transit times of 131I-albumin and 85Sr-labeled microspheres were calculated. The effects of adrenergic and histaminergic antagonists with and without SFNS were determined. Phentolamine blocked the entire response to SFNS. Prazosin attenuated increases in total and AVA resistance. Yohimbine prevented increased total resistance, attenuated the AVA resistance increase, and revealed a decrease in capillary circuit resistance. Pyrilamine attenuated total resistance increase while SFNS increased capillary and AVA resistances. Metiamide had no effect on blood flow distribution with SFNS. The increase in AVA resistance with SFNS apparently resulted from a combination of alpha 1 and alpha 2 receptor stimulation but not histaminergic effects.     

Graded alterations of RBC aggregation influence in vivo blood flow resistance

Although the effects of red blood cell (RBC) aggregation on low-shear rate blood viscosity are well known, the effects on in vivo flow resistance are still not fully resolved. The present study was designed to explore the in vivo effects of RBC aggregation on flow resistance using a novel technique to enhance aggregation: cells are covalently coated with a block copolymer (Pluronic F-98) and then suspended in unaltered plasma. RBC aggregation was increased in graded steps by varying the Pluronic concentration during cell coating and was verified by microscopy and erythrocyte sedimentation rate (ESR), which increased by 200% at the highest Pluronic level. RBC suspensions were perfused through an isolated in situ guinea pig hindlimb preparation while the arterial perfusion pressure was held constant at 100 mmHg via a pressure servo-controlled pump. No significant effects of enhanced RBC aggregation were observed when studies were conducted in preparations with intact vascular control mechanisms. However, after inhibition of smooth muscle tone (using 10(-4) M papaverin), a significant change in flow resistance was observed in a RBC suspension with a 97% increase of ESR. Additional enhancements of RBC aggregation (i.e., 136 and 162% increases of ESR) decreased flow resistance almost to control values. This was followed by another significant increase in flow resistance during perfusion with RBC suspensions with a 200% increase of ESR. This triphasic effect of graded increases of RBC aggregation is most likely explained by an interplay of several hemodynamic mechanisms that are triggered by enhanced RBC aggregation.