The effects of forced activity on circulating catecholamines and pH and water content of erythrocytes in the toad

The effects of beta-adrenergic stimulation of teleost erythrocytes have been well documented. Addition of beta-adrenergic agonists to teleost erythrocytes in vitro results in an increase in erythrocyte pH and water content (Nikinmaa, 1982; Nikinmaa & Huestis, 1984; Cossins & Richardson, 1985; Heming, Randall & Mazeaud, 1986b). It has also been demonstrated in vivo that an increase in circulating catecholamine levels results in an increase in erythrocyte pH and water content (Nikinmaa, Cech & McEnroe, 1984; Primmett, Randall, Mazeaud & Boutilier, 1986).

There are, however, only a very limited number of studies that document the effect of beta-adrenergic stimulation on both the pH and water content of nucleated erythrocytes from other species of vertebrates. Nikinmaa & Huestis (1984) have shown that addition of isoproterenol to avian erythrocytes in vitro causes an increase in cellular water content only at very high extracellular potassium concentration and does not affect erythrocyte pH. Rudolph & Greengard (1980) have documented cell swelling in amphibian erythrocytes in vitro after addition of isoproterenol, but only in the presence of a phosphodiesterase inhibitor. To our knowledge, the effect of beta-adrenergic stimulation on erythrocyte pH has not been documented in amphibians. We have examined, therefore, the effects of catecholamines on the pH and water content in erythrocytes from the toad, Bufo marinus, both in vivo and in vitro. In the in vivo experiments, we exercised the toads in order to determine if erythrocyte pH and, therefore, the characteristics of haemoglobin:oxygen binding, are modulated during an exercise-induced acidosis in an amphibian, as in teleost fish.

Adult Bufo marinus of both sexes (250-500 g) obtained from Charles D. Sullivan Co. Inc. (Nashville, TN, USA) were used for all experiments. The animals were kept in large fibreglass aquaria with 2–4 cm dechlorinated tap water at the experimental temperature (22°C) for at least a week prior to the experiments.

Toads were anaesthetized in neutralized MS-222 ( 1 · 5 g I⁻¹ ) and chronically cannulated in the femoral artery (using the method of Boutilier, Randall, Shelton & Toews, 1979). Following surgery, the animals were transferred to light-proof boxes with 500 ml of dechlorinated tap water and allowed to recover overnight.

Following overnight recovery, a 1-ml control blood sample was removed from the femoral cannula and immediately analysed for pH and haematocrit. The sample was then centrifuged and the plasma was removed for catecholamine analysis. The packed red cells were immediately frozen in liquid nitrogen for later analysis of intracellular pH. The animal was then subjected to 30 min of vigorous exercise. This involved the manual manipulation of the animal by the experimenter so that continuous righting movements were elicited. The goal of this procedure was to induce catecholamine release. Upon completion of this exercise period, the animal was returned to the chamber and another blood sample was immediately taken. Blood samples were also taken 0·5, 1 and 4h after exercise and analysed as described above for the control sample.

An additional set of experiments was conducted to determine any relationship between changes in haematocrit and changes in the erythrocyte water content. These two parameters were measured in five animals subjected to the previously described protocol with the exception that only two samples (control and Oh post-exercise) were taken.

In this series of experiments, toads were cannulated as described in the previous section and allowed to recover overnight. The next day, blood was collected from several cannulated toads and pooled. Samples (2·5 ml) of blood were then transferred to glass tonometers and equilibrated with a humidified 5 % CO₂/95 % air mixture delivered by Wösthoff gas mixing pumps. This gas mixture resulted in blood pH values close to those of animals immediately following exhaustive exercise (see Fig. 1). Following a 90-min equilibration period, a 1-ml blood sample was taken from each tonometer and centrifuged; 0·7ml in one tube for the determination of plasma and erythrocyte pH and the remaining 0-3 ml in another tube for the determination of cell water content. The pH of the plasma (pH_e) was measured immediately and the remaining plasma from both sample tubes was discarded. The packed erythrocytes from the first sample tube were then frozen in liquid nitrogen for later determination of erythrocyte pH while those from the second sample tube were saved for the determination of cell water content. Baroin, Garcia-Romeu, Lamarre & Motáis (1984) have demonstrated that rainbow trout erythrocytes exhibit significant beta-adrenergic sensitivity following 3h of equilibration in vitro at 15 °C and therefore it was assumed that the equilibration time (90 min) would not alter the beta-adrenergic sensitivity of the cells. At this point, each tonometer received either ICT³ mol I⁻¹ isoproterenol (final concentration) or an equivalent volume (50 of Mackenzie’s amphibian saline. Isoproterenol was chosen since the regulation of pH in teleost erythrocytes-appears to be a beta-adrenergic effect (Nikinmaa et al. 1984; Primmett et al. 1986) and beta-adrenergic effects are most potently stimulated by isoproterenol (Lefkowitz, 1976). The concentration of 10^−smoll⁻¹ was used in order to saturate the beta-adrenergic receptors. The sampling procedure was then repeated following another 30 min of equilibration. It should be noted that in representative cases (N = 3), plasma catecholamine measurements were made at this point in the experiment. In these cases, plasma adrenaline and noradrenaline levels from the control tonometers were very similar to those of resting animals in vivo.

Measurements of pH were made using a Radiometer PHM 72 acid-base analyser and associated micro-pH unit (Radiometer, Copenhagen, Denmark). Erythrocyte pH measurements were made using the freeze-thaw method of Zeidler & Kim (1977). Plasma adrenaline and noradrenaline levels were determined by high pressure liquid chromatography (Spectra Physics, model SP8700) with electrochemical detection (Woodward, 1982; Primmett et al. 1986). The water content of the erythrocytes was determined from the wet and dry mass of the packed erythrocytes according to the method of Nikinmaa & Huestis (1984).

The significance of the results was assessed using a paired Student’s t-test. Differences were accepted as being significantly different at the P< 0·05 level.

Control (resting) levels of adrenaline and noradrenaline for Bufo marinus measured in the present study were slightly lower than resting levels recorded for Bufo arenarum (Donoso & Segura, 1965). Exhaustive exercise in the toad was associated with large increases in circulating levels of adrenaline and noradrenaline (Table 1). While there are no values available from the literature for catecholamine levels following activity in amphibians, the magnitude of these increases was similar to that found in the rainbow trout (Primmett et al. 1986) following exhaustive exercise.

The effect of the exercise period on the haematocrit of the toad is shown in Table 2. The haematocrit was significantly elevated following exercise and did not return to control values until 1 h into the recovery period. McDonald, Boutilier & Toews (1980) documented similar increases in haematocrit in Bufo marinus following forced activity. In Rana catesbeiana, it has also been shown that infusion of adrenaline or noradrenaline (Mbangkollo & deRoos, 1983; Herman, 1977) or a period of handling (Mbangkollo & deRoos, 1983) will cause an increase in the haematocrit.

There are several factors which may have contributed to this increase in haemato-crit. In teleosts, increased haematocrit levels following exhaustive exercise are due to erythrocyte swelling as well as an increase in the circulating number of erythrocytes relative to the plasma volume (Nikinmaa et al. 1984; Primmett et al. 1986). In Bufo, the absence of erythrocyte swelling in the presence of increased catecholamine levels in vivo (Table 2) is interesting in view of the study of Rudolph & Greengard (1980) in which catecholamines were found to stimulate swelling in frog erythrocytes in vitro. The ion exchanges involved in the adrenergic swelling response in frog erythrocytes (Palfrey & Greengard, 1981) resemble those in teleosts (Baroin et al. 1984; Nikinmaa & Huestis, 1984; Heming et al. 1986b). However, swelling has only been documented in frog erythrocytes in vitro if the cells are adrenergicalla stimulated in the presence of a phosphodiesterase inhibitor. The present experiments indicate that these ion exchange processes are not capable of raising the erythrocyte water content in the toad during beta-adrenergic stimulation in vivo. This may be due to changes in other factors that are capable in vivo of influencing erythrocyte water content, such as plasma osmotic pressure. However, Table 3 shows that the beta-adrenergic agonist isoproterenol caused no increase in the cell water content of toad erythrocytes in vitro. In fact, there was a small decrease in cell water content in vitro at 120 min both in erythrocytes treated with saline and with isoproterenol. Together these results seem to indicate that beta-adrenergic stimulation of toad erythrocytes does not result in changes in water content under physiological conditions. The increase in haematocrit in the present experiments is, therefore, probably due to an increase in the circulating number of erythrocytes relative to the plasma volume. According to Boutilier, Emilio & Shelton (1986), uptake of plasma water by osmotically enriched muscle cells may account for a portion of the haematocrit increase. Erythrocyte recruitment may also have been a contributing factor, since Nilsson & Grove (1974) have demonstrated that constriction of the spleen may be induced in teleosts by perfusion of the splenic artery with adrenaline or noradrenaline.

The effect of forced activity on the whole blood and erythrocyte pH is illustrated in Fig. 1. The acidosis in whole blood following the activity period is similar to that found by McDonald et al. (1980) for Bufo marinus after forced activity. The erthrocyte pH also falls during this period, but the magnitude of this drop (0·270 pH units) is less than that of the extracellullar pH (0·429pH units). Thus, the pH difference across the erythrocyte membrane is 0-443 pH units at rest and is reduced to 0’284 pH units after exercise. Hladky & Rink (1977) explain that a reduction in pH reduces the net charge on the haemoglobin inside the erythrocyte. This would result in a reduction in the erythrocyte proton concentration (relative to the extracellular concentration) and, therefore, in the erythrocyte transmembrane pH gradient. The erythrocyte transmembrane pH gradient also decreases with decreasing pH in rainbow trout erythrocytes equilibrated in vitro in the absence of catecholamines (Heming et al. 1986a).

It has been demonstrated that while plasma pH falls after exhaustive exercise in teleosts, an increase in circulating catecholamine levels causes the erythrocyte pH to be maintained (Nikinmaa et al. 1984) or increased (Primmett et al. 1986). Beta-adrenergic agonists also cause an increase in the pH of teleost erythrocytes in vitro (Nikinmaa & Huestis, 1984; Cossins & Richardson, 1985; Heming et al. 1986b). Nevertheless, the erythrocyte pH fell significantly following activity (0·270 pH units; Fig. 1) in the present experiments, even though circulating catecholamine levels increased (Table 1). In addition, isoproterenol had no effect on the pH of erythrocytes equilibrated in vitro (Table 3). The effect of catecholamines on the pH of toad erythrocytes is therefore quite different from the effects which have been documented in teleosts. Note, however, that at the same extracellular pH, there is a difference in the magnitude of the pH gradient between the in vivo experiment at 0 h (Fig. 1) and the in vitro experiment (Table 3). This difference in the pH gradient could reflect some degree of intracellular pH regulation in vivo which is not observed in vitro. However, the difference can also be explained as a haemolysis effect on the liquid junction potential of the pH apparatus (Siggaard-Andersen, 1961; Boutilier, Iwama, Heming & Randall, 1985), since whole blood pH was measured in Fig. 1 whereas true plasma pH was measured in Table 3.

It is possible that the regulation of erythrocyte pH via adrenergic mechanisms is an adaptation which may be specific to water-breathing vertebrates. However, the difference in the adrenergic effects on the erythrocyte between the toad and teleost fish may also be explained if the subsequent effects on the oxygen-carrying capacity in the blood of each of these animals are considered. In teleost fish, a fall in the erythrocyte pH lowers the oxygen-carrying capacity of the blood because of the Root shift (Cameron, 1971; Nikinmaa et al. 1984; Boutilier, Iwama & Randall, 1986). However, anuran amphibians do not possess a Root shift (Bridges, Pelster & Sheid, 1985) and a fall in the erythrocyte pH would not lower the oxygen-carrying capacity in the blood of Bufo marinus as it does in teleosts. The functional significance of the adrenergic response in teleost erythrocytes may, therefore, be to offset the Root shift.

In conclusion, circulating catecholamine levels in the toad increase following forced activity. The activity period also caused an increase in haematocrit and plasma acidosis which have previously been described for Bufo marinus. There was an apparent absence, however, of any beta-adrenergic regulation of pH or water content of erythrocytes in the toad, Bufo marinus. This is in sharp contrast to the response of teleost fish erythrocytes to beta-adrenergic stimulation and may be due to the fact that amphibians do not have a Root shift.

Financial support for this study was provided by an NSERC operating grant to DJR and an NSERC graduate fellowship to BLT.