EFFECTS OF OXYGENATION AND THE STRESS HORMONES ADRENALINE AND CORTISOL ON THE VISCOSITY OF BLOOD FROM THE TROUT ONCORHYNCHUS MYKISS

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1 The Journal of Experimental Biology 198, (1995) Printed in Great Britain The Company of Biologists Limited EFFECTS OF OXYGENATION AND THE STRESS HORMONES ADRENALINE AND CORTISOL ON THE VISCOSITY OF BLOOD FROM THE TROUT ONCORHYNCHUS MYKISS BODIL SØRENSEN AND ROY E. WEBER Department of Zoophysiology, Institute of Biological Sciences, Building 131, University of Aarhus, DK 8000 Aarhus C, Denmark Accepted 10 November 1994 Although the concentrations of the stress hormones adrenaline and cortisol in rainbow trout (Oncorhynchus mykiss) blood increase upon hypoxic exposure, the combined effects of these hormones and O 2 lack upon fish blood rheology have not been investigated. Deoxygenated blood taken by caudal puncture exhibited lower viscosities than oxygenated samples at low shear rates, whereas the opposite was true at high shear rates. However, blood from cannulated trout had similar viscosities in its deoxygenated and oxygenated states. In the deoxygenated state, addition of adrenaline lowered viscosity at low shear rates and increased it at high shear rates, resembling the effects of deoxygenation observed in blood taken by venepuncture. In oxygenated blood on the contrary, no marked Summary adrenaline effects were observed. In deoxygenated blood, addition of cortisol lowered viscosity at all measured shear rates compared with blood without cortisol. In oxygenated blood, however, no cortisol effects were observed. The viscosity effects observed in the presence of cortisol could not be attributed to concomitant changes in haematological variables, However, the effects in the presence of adrenaline manifested in deoxygenated cannula blood and in uncannulated blood without added hormones appear to result from parallel increases in haematocrit and cell volume. Key words: adrenaline, cortisol, fish blood, oxygenation, rheology, stress hormones, rainbow trout, Oncorhynchus mykiss. Introduction It is well known that O 2 transport from the gas-exchange organs to the tissues is proportional to the O 2 -carrying capacity, and thus the haematocrit (Hct, volume percentage of red cells), of the blood and is inversely proportional to flow resistance, so that the O 2 transport potential of blood will decrease above and below optimal Hct values (Snyder and Weathers, 1977; Wells and Weber, 1991; Gallaugher et al. 1992). Hct in fish may vary widely, changing in response to ambient hypoxia (Hughes et al. 1986; Soivo et al. 1974), stress and exercise (Wells and Weber, 1991), amongst other things (Weber, 1982). These variations may be mediated by catecholamines, which may mobilize red cells from the spleen (Gallaugher et al. 1992) or induce red cell swelling as a result of fluid shifts into the intracellular compartment. Catecholamine-induced swelling occurs through activation of a sodium/proton exchange pump, which decreases plasma ph and increases intracellular ph, thus raising blood O 2 -affinity via the Bohr effect (Nikinmaa, 1992). Blood is a non-newtonian fluid, its viscosity ( ) increasing with decreasing shear rate (see Figs 1 and 2) owing to the deformability of the red blood cell and changes in aggregation between cells (Chien, 1975). An increase in cell volume lowers the shear-dependence of blood viscosity (Wells and Weber, 1991) by decreasing cellular deformability and aggregation (Chien, 1970, 1975; La Celle and Weed, 1971). Changes in the shear-dependence also alter the overall resistance to flow and the resistance ratio between postcapillary and precapillary segments (Chien, 1969). The relationships between viscosity, shear rate, oxygenation state, swelling and stress hormones in fish red cells are still not clearly understood, and investigations on the rheological properties of fish blood have primarily focused separately on the effects of O 2 tension and hormones. The present paper reports the combined effects of adrenaline and cortisol and of oxygenation state on trout blood viscosity. Materials and methods Experimental animals Rainbow trout [Oncorhynchus mykiss (Walbaum)] of both Author for correspondence.

2 954 B. SØRENSEN AND R. E. WEBER sexes, weighing 380±30 g, were obtained from fish farms in Jutland and kept in aerated tapwater (>90 % air saturation) in 1 m 1 m 1 m glassfibre tanks at 15±1 C and a 12h:12 h light:dark photoperiod. The fish were given maintenance rations of commercially available trout pellets and were allowed to acclimate to laboratory conditions for at least 10 days before use. Blood sampling and surgery Blood was collected either by venepuncture from the caudal blood vessels or, to avoid the effects of handling stress, via implanted cannulae. Blood sampling by caudal venepuncture was normally carried out within 1 min after lifting the fish out of the water. The fish were cannulated in the dorsal aorta after anaesthesia in tapwater containing 0.1 g l 1 4-aminobenzoate, using PE-50 polyethylene tubing (Portex) filled with a 0.9 % NaCl solution containing 125 i.u. of sodium heparin (Soivio et al. 1972, 1975; Tetens and Lykkeboe, 1985). The operation was completed within 15 min from induction of anaesthesia and was carried out at least 48 h before blood sampling. Blood samples of 6 8 ml were drawn into heparinized syringes and kept on ice until use. Viscosity measurements Viscosity measurements were carried out using whole blood, since the mechanical properties of red cells may be altered by removal of plasma proteins, which are implicated in rouleaux formation and aggregation (Fåhraeus, 1921). Viscosity measurements were carried out at 15 C using an LVT-DVII cone/plate viscometer with a 0.8 cone spindle (Brookfield Engineering Laboratory, Stoughton, MA), which has eight rotational speeds ranging from 0.3 to 60 revs min 1 (corresponding to shear rates of s 1 ) and requires a sample volume of ml. Calibration checks were carried out using two Brookfield viscosity standards (9.2 and 98.3 cp at 25 C) that correspond to the values measured in blood. Viscosity readings were normally recorded first at the highest shear rate after a 4 min stabilization period, and thereafter at successively lower shear rates. This minimises the effects of aggregation and erythrocyte sedimentation that may occur if the reverse order is followed (International Committee for Standardization in Haematology, ISCH, 1986). Viscosity readings at low shear rates (45 and 22.5 s 1 ) were occasionally erroneously low, which may be due to the above-mentioned phenomena or to cohesion between the blood cells. In such cases, the shear rate was returned to the highest level (450 s 1 ). If the reading returned to within 5 % of the earlier recorded value, shear rate was once more reduced to the value where the anomalous value had been observed; if not, measurements on the sample in question were discontinued. Care was taken to avoid trapping air bubbles between the cone and plate, which may cause erroneously high viscosity readings. In the present investigations, some measurement series lasted up to 6 h after blood had been drawn. Before measuring viscosities, 1.2 ml blood samples were equilibrated at predetermined gas tensions in Kutofix tonometers (Eschweiler and Co., Kiel) for 60 min. Gas mixtures were prepared by two serially coupled Wösthoff (Bochum, Germany) gas-mixing pumps. Oxygenated and deoxygenated blood were obtained by mixing % CO 2 with, respectively, room air or pure (99.98 %) N 2. To observe the effects of hypoxia, an additional gas mixture of % air and 99.5 % N 2 was used. To prevent oxygenation of deoxygenated samples during measurements, the cup of the viscometer was encased in an acrylic glove box that was flushed for at least 2 h with N 2 before measurements started. This reduced the O 2 content in the glove box to less than 1 % of that in air, as predicted by a simple exponential equation. O 2 saturations measured in the deoxygenated samples before and after viscosity measurement in the glove box were 0.02±0.04 % (N=10) and 1.44±3.03 % (N=9), respectively. O 2 tensions in oxygenated blood samples before and after viscosity measurement were 138±11 mmhg (18.4± kpa; N=8) and 119±7 mmhg (15.9±0.9 kpa; N=7), respectively. Unless otherwise mentioned, measurements on deoxygenated subsamples were carried out before those on oxygenated samples. Hormonal effects To investigate hormonal effects, blood from individual fish was divided into four subsamples, for measuring viscosity in the oxygenated and deoxygenated states, each in the presence and absence of hormone. The hormones were obtained from the dispensary of Aarhus University Hospital. Final concentrations in blood were: mol l 1 cortisol (Solucortef, hydrocortisone succinate) and mol l 1 adrenaline (adrenaline tartrate). These concentrations, which exceed physiological values under stress situations, were chosen to ensure that effects, if present, were detected. Haematological effects To examine the effect of haematocrit on viscosity, blood samples were gently centrifuged [4 min at 1000 revs min 1 (350 g) in a Sigma 3 MK centrifuge], plasma was removed or added and the cells were resuspended. Control experiments revealed no consistent effect of centrifugation and resuspension of the cells. In order to analyse the effects of non-adrenergic swelling, whole blood was centrifuged and a volume of plasma corresponding to 25 % of that of the whole blood was removed. The plasma removed was dialysed against distilled water to remove salts. The resulting protein suspension was then concentrated by suspending the dialysis bags in air in a refrigerator, after which distilled water was added to restore the original plasma volume. The ion-free plasma was then returned to the blood sample after the blood had been equilibrated for 45min with an oxygen-rich (air + % CO 2 ) gas mixture, and the whole sample was equilibrated for a further 15 min. Hct was measured by the standard glass capillary procedure after 5 min of centrifugation at revs min 1. No

3 Trout blood viscosity 955 correction was made for trapped plasma. [Hb] and mean cellular haemoglobin concentration, MCHC, were determined as previously described (Wells and Weber, 1990), except that the cells were lysed in mol l 1 Tris buffer, ph 7.5. Red blood cell counts were obtained using an improved Neubauer haemocytometer. ph and O 2 measurements The ph was measured with a BMS2 Mk2 blood micro system coupled to a PHM 73 ph meter (Radiometer, Copenhagen). O 2 tensions were measured with a Radiometer E5046 O 2 electrode fitted in a D616 thermostat cell. O 2 contents of the blood samples were determined as described by Tucker (1967). The electrodes were calibrated before each measurement. Statistics Student s t-test was used for statistical analyses of the data. Results As expected, blood viscosity increased with Hct (Fig. 1; Table 1). In this study, the relationship was linear at the Hct values measured (0 36 %), but the linear correlation coefficient decreased with shear rate. Interestingly, the Hctdependence of viscosity tended to be lower in deoxygenated A 11.3 s s 1 45 s 1 90 s s s 1 Table 1. Dependence of viscosity (y) on haematocrit (x) in oxygenated and deoxygenated cannula blood measured at different shear rates Shear rate (s 1 ) Oxygenated blood Deoxygenated blood 450 y=0.083x+2.22, r=0.95 y=0.082x+2.08, r= y=0.106x+2.82, r=0.87 y=0.094x+2.93, r= y=0.164x+3.72, r=0.86 y=0.123x+3.98, r= y=0.264x+4.30, r=0.87 y=0.164x+5.73, r= y=0.335x+5.78, r=0.76 y=0.295x+6.09, r= y=0.928x+20.74, r=0.79 y=76x+20.86, r=0.66 r, linear regression coefficient. than in oxygenated blood at low shear rates, but the differences were not significant (Table 1). The viscosity of trout blood taken by caudal venepuncture (Fig. 2) increased with decreasing shear rate. As indicated for a single series of measurements (Fig. 2) shear-dependence was lower in deoxygenated blood than in oxygenated blood. No consistent variations in blood viscosity with season were observed. Since individual differences in blood properties, e.g. Hct, could mask the effects of oxygenation or hormones, blood viscosity effects were assessed from viscosity ratios obtained for the same blood sample under different conditions (see Figs 3 6). For trout blood taken by venepuncture, the deoxygenated/oxygenated viscosity ratios were below 1 at low shear rates (below 100 s 1 ), but increased to values greater than 1 at high shear rates (225 and 450 s 1, Fig. 3A). For blood taken by venepuncture, hypoxygenated/oxygenated ratios (data not shown) were similar to deoxygenated/oxygenated ratios. In contrast, blood from the cannulated specimens had similar viscosities in the deoxygenated and oxygenated states, giving ratios of approximately 1 (Fig. 3B). Viscosity (cp) B Viscosity (cp) Oxygenated Deoxygenated Haematocrit (%) Fig. 1. Effects of haematocrit and shear rate on viscosity in (A) oxygenated and (B) deoxygenated trout cannula blood Fig. 2. Blood viscosity at different shear rates in oxygenated and deoxygenated trout blood obtained by caudal venepuncture.

4 956 B. SØRENSEN AND R. E. WEBER A A ηdeoxy/ηoxy B ηadr/η adr B Fig. 3. Ratio of viscosities in deoxygenated and oxygenated blood ( deoxy/ oxy) taken by (A) venepuncture (N=10) and (B) implanted cannula (N=7), as a function of shear rate. In this and later figures, the daggers and asterisks denote significance of difference: P<0.1; P<0.05; P<0.01; P<0.005; P<0.001; P<0.0005; P< Fig. 4. Ratio of viscosity in the presence and absence of mol l 1 adrenaline ( adr/ adr) for (A) oxygenated samples (N=5) and (B) deoxygenated samples (N=7) of blood taken by cannula. Other details as in Fig. 3. Addition of adrenaline had no effect or only a slight effect on the viscosity of oxygenated cannula blood, the values of adr / adr tending to be below 1 at low shear rates and above 1 at high shear rates (Fig. 4A). In the deoxygenated cannula blood (Fig. 4B), these effects were more pronounced, resembling the deoxygenated/oxygenated pattern found in blood from uncannulated trout (cf. Fig. 3A). In oxygenated cannula blood, cortisol exerted no effect on viscosity compared with values in blood without cortisol (Fig. 5A). In deoxygenated blood, however, cortisol lowered viscosity at all measured shear rates (Fig. 5B). Besides possible direct effects, hormones may influence the rheological properties of blood through changes in secondary variables, e.g. Hct, MCHC, mean cell volume, MCV (Trevan, 1918; Nygaard et al. 1935; Chien, 1975; Fletcher and Haedrich, 1987; Chiocchia and Motais, 1989), blood ph (Wells et al. 1963; Rand et al. 1968; Giombi and Burnard, 1970) and ATP concentrations (Nakao et al. 1960; Weed et al. 1969; La Celle and Weed, 1971). The haematological data for the blood samples used to study the effects of hormones are shown in Tables 2 and 3. Viscosity ratios for osmotically swollen compared with control cells (where Hct values were 21.15±2.23 and 14.58±0.85, respectively; Fig. 6) showed similar variation with shear rate to the the deoxygenated/oxygenated ratios in blood taken by venepuncture (Fig. 3A; Table 2) and the deoxygenated viscosity ratios in the presence and absence of adrenaline (Fig. 4B; Table 3), i.e. higher ratios at high shear rates than at low shear rates. This reflects the lower sheardependence in swollen cells (as previously observed by Wells et al. 1991). The possibility that the viscosity of drawn blood may change with time was investigated by repeating viscosity measurements under the same conditions or by changing the sequence of measurements carried out on subsamples (oxygenated and deoxygenated samples, with and without hormones). These results showed unchanged viscosity ratios in blood from the same pool within the measurement period, although the absolute viscosity readings showed small increases after 4 6 h. Discussion This study appears to be the first focusing on the combined effects of stress hormones and oxygenation on the viscosity of fish blood. Handling stress (caudal blood sampling) and in vitro adrenaline administration markedly increased Hct in deoxygenated trout blood (Tables 2 and 3). The change was correlated with decreased MCHC, increased MCV and

5 Trout blood viscosity 957 A ηswollen/ηcontrol ηcort/n cort B Fig. 6. Ratio of viscosities in oxygenated osmotically swollen and control cells ( swollen/ control) from blood taken by cannula (N=3). Other details as in Fig. 3. Fig. 5. Ratio of viscosities in the presence and absence of mol l 1 cortisol ( cort/ cort) for (A) oxygenated samples (N=5) and (B) deoxygenated samples (N=6) of trout blood taken by cannula. Other details as in Fig. 3. unchanged blood [Hb], which reflect the red cell swelling associated with adrenergic stimulation of trout red cells by activation of the Na + /H + exchanger (Nikinmaa, 1992). Adrenaline decreased the viscosity at low shear rates, but increased it at high shear rates in deoxygenated blood, without exerting pronounced effects in oxygenated blood (Fig. 4). Cortisol lowered the viscosity of deoxygenated blood at all shear rates, without affecting viscosity in oxygenated blood (Fig. 5). As noted previously (Wells et al. 1991), it is not known whether the adrenergic effects on the viscous properties of blood are a direct result of hormonal stimulation or whether they result from changes in haematological variables. Blood viscosity increased with Hct (Fig. 1; Table 1). At all shear rates measured in this study where Hct values were below 40%, the relationship was linear, as previously observed at low Hct (Trevan, 1918; Nygaard et al. 1935; Snyder and Weathers, 1977; Pankhurst et al. 1992), but the linear correlation coefficient decreased at low shear rates (Table 1). Since increased Hct raises viscosity at all shear rates, the adrenaline-induced decrease in viscosity observed at low shear rates (Fig. 3) cannot be attributed to an associated increase in Hct. Instead, the data indicate that the adrenergic effect on viscosity is due to red cell swelling per se. The effects of adrenaline on the viscosity of deoxygenated cannula blood (Fig. 4B) correlate with increases in Hct and MCV (Table 3). This reflects cell swelling, which is known to decrease aggregation, deformability and shear-dependence (Chien, 1975; La Celle and Weed, 1971). The deoxygenated/oxygenated viscosity ratio in uncannulated trout blood (Fig. 3A) may similarly result from adrenaline release Table 2. Haematological data for trout blood obtained either by caudal puncture or by cannula Caudal puncture Cannula Deoxygenated Oxygenated P Deoxygenated Oxygenated P Haematocrit (%) 28.34±3.13 (10) 22.68±2.06 (9) < ±5.05 (8) 16.14±4.73 (8) [Haemoglobin] (mmol l 1 ) 1.15±0.32 (6) 1.23±0.45 (6) 0.92±0.30 (8) 0.92±0.30 (8) 10 6 red blood cell count ( l 1 ) 0.85±0.13 (5) 0.85±0.13 (5) 7±0.17 (8) 6±0.17 (8) MCHC (mmol l 1 ) 4.33±0.73 (6) 5.39±1.45 (6) < ±1.39 (8) 5.70±1.40 (8) <0.005 Mean cell volume (nm 3 ) 327±31 (5) 265±34 (5) < ±43 (8) 291±37 (8) ph 7.67±0.06 (4) 7.68±0.09 (4) 7.79±0.10 (8) 7.69±0.15 (8) <0.005 Data are means ± S.D. (N). Blood samples were equilibrated with % CO 2 in either air (oxygenated blood) or pure (99.98 %) N 2 (deoxygenated blood). MCHC, mean cellular haemoglobin concentration.

6 958 B. SØRENSEN AND R. E. WEBER Table 3. Haematological data for blood from cannulated trout in the absence and presence of added adrenaline and cortisol Oxygenated Deoxygenated Control Treated P Control Treated P Adrenaline Haematocrit (%) 18.96±3.00 (4) 18.67±5.12 (5) 21.28±3.01 (7) 26.70±3.96 (7) < [Haemoglobin] (mmol l 1 ) 1.16±0.25 (5) 1.16±0.25 (5) 1.25±0.33 (7) 1.25±0.19 (7) 10 6 red blood cell count ( l 1 ) 6±0.17 (5) 9±0.16 (5) 0.74±0.17 (7) 0.73±0.14 (7) MCHC (mmol l 1 ) 7.10±1.72 (4) 6.21±0 (5) < ±4 (7) 4.68±1.19 (7) < MCV (nm 3 ) 298±43 (4) 318±53 (5) < ±18 (6) 370±46 (7) <0.05 ph 7.79±0.14 (5) 7.67±0.14 (5) < ±0.10 (7) 7.78±0.13 (7) <0.05 Cortisol Haematocrit (%) 16.35±1.97 (5) 16.24±2.05 (5) 16.58±1.95 (6) 16.40±1.88 (6) [Haemoglobin] (mmol l 1 ) 0.93±0.25 (5) 0.93±0.25 (5) 0.96±0.24 (6) 0.96±0.24 (6) 10 6 red blood cell count ( l 1 ) 0.60±0.11 (5) 0.63±0.07 (5) 0.60±0.10 (6) 0.60±0.10 (6) MCHC (mmol l 1 ) 5.67±1.47 (5) 5.73±4 (5) 5.82±6 (6) 5.85±0 (6) MCV (nm 3 ) 281±66 (5) 257±21 (5) 276±21 (6) 275±29 (6) ph 7.66±0.05 (5) 7.71±0.08 (5) < ±0.05 (6) 7.80±0.03 (6) Other details as in Table 2. during the stressful blood-sampling procedure, which increases Hct and MCV more in deoxygenated than in oxygenated blood (Table 2). This is supported by the observation that the same responses (increased viscosity ratio at high shear rates) were seen upon administration of adrenaline to deoxygenated cannula blood and in osmotically swollen cells (compare Figs 4B and 6). The effects of osmotic swelling (Fig. 6) agree with the concept that swelling reduces aggregation, and thus viscosity, at low shear rates, but increases viscosity at high shear rates, because of the lower deformability of swollen red cells (Chien, 1970, 1975; La Celle and Weed, 1971). The effects of adrenaline on rheology are varied and appear to depend on species and on the exact measurement conditions. For trout, Wells et al. (1991) found that adrenaline decreased blood viscosity, particularly at low shear rate where the adr / adr ratio was 0.65, although it increased to approximately 1 at 450 s 1, and Wells and Weber (1991) observed a decrease in shear-dependence in blood from exercised and anaesthetised specimens where the cells were swollen. Chiocchia and Motais (1989) found that in vitro adrenergic stimulation increased the deformability of washed trout red cells. However, Hughes and Albers (1988) found that adrenaline decreased the filtration rate of whole blood equilibrated with low O 2 and high CO 2 tensions in carp blood. In washed rat and human red cells, adrenaline similarly lowered the filterability (Rasmussen et al. 1975), which accords with the results of Pfafferott et al. (1986), who showed that noradrenaline decreased the deformability of human washed cells. Rasmussen et al. (1975) consider that the effects of adrenaline may be mediated by changes in the cell membrane, in cell shape or in cell volume. It should, however, be borne in mind that mammalian red cells do not exhibit catecholamine-induced alkalization and swelling (Nikinmaa, 1992). In contrast to the red cell swelling induced by catecholamines (Nikinmaa, 1992), swelling and MCV changes were not observed with cortisol (Table 3). Since cortisol had no major influence on the haematological variables measured in either oxygenated or deoxygenated blood (Table 3), its viscosity effects cannot be attributed to concomitant changes in Hct, MCHC, MCV or blood ph. The slight increase in the ph of deoxygenated blood observed in the presence of cortisol (from 7.78 to 7.80) is unlikely to have had a significant effect since a greater ph increase in oxygenated blood induced by cortisol (from 7.66 to 7.71) was not associated with a lower viscosity (Table 3; Fig. 5A). The decreased viscosity after cortisol administration may be due to changes in the red cell membranes or to a lowered cellular ATP concentration, which has been observed in cortisol-stimulated blood from the fish Pagrus auratus (Bollard et al. 1993) and in deoxygenated trout blood (O. B. Nielsen and G. Lykkeboe, unpublished data). Lowering the ATP concentration decreases the deformability of human red cells (Weed et al. 1969). However, Wells and Weber (1991) found no evidence for effects of lowered ATP concentrations on the viscosity of trout blood. The present results indicate that, in rainbow trout, red blood cell swelling in the presence of adrenaline reduces the viscosity of deoxygenated blood at low shear rates. Physiologically, this effect may be more important than the accompanying increase in viscosity of hypoxic blood at high shear rates, given the low flow rates of blood in the veins and the small blood vessels (La Celle and Weed, 1971). Sirs (1993) suggests that fish with low blood pressures have an improved blood flow with less flexible cells caused by a reduction in the Fåhreaus Lindqvist phenomenon, i.e. decreasing viscosity with decreasing vessel diameter, since less flexible cells do not move towards the central core to the same degree as flexible ones. The present results suggest that viscosity changes continuously in parallel with arterio-venous changes in oxygenation and red cell volume.

7 Trout blood viscosity 959 We thank the Danish Natural Science Research Council and the E. and K. Petersen Fund for financial support. References BOLLARD, B. A., PANKHURST, N. W. AND WELLS, R. M. G. (1993). Effects of artificially elevated plasma cortisol levels in the teleost fish Pagrus auratus (Sparidae). Comp. Biochem. Physiol. 106A, CHIEN, S. (1969). Blood rheology and its relation to flow resistance and transcapillary exchange, with special reference to shock. Adv. Microcirc. 2, CHIEN, S. (1970). Shear dependence of effective cell volume as a determinant of blood viscosity. Science 168, CHIEN, S. (1975). Biophysical behavior of red cells in suspensions. In The Red Blood Cell, vol. 2 (ed. D. Mac and N. Surgenor), pp New York: Academic Press. CHIOCCHIA, G. AND MOTAIS, R. (1989). Effect of catecholamines on deformability of red cells from trout: Relative roles of cyclic AMP and cell volume. J. Physiol., Lond. 412, FÅHREAUS, R. (1921). The suspension stability of the blood. 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8 960 B. SØRENSEN AND R. E. WEBER