Extracellular and intracellular pH with changes of temperature in the dogfish Scyliorhinus stellaris

Acid-base regulation in the air-breathing swamp eel (Monopterus albus) at different temperatures

Vertebrates reduce arterial blood pH (pHa) when body temperature increases. In water-breathers this response occurs primarily by reducing plasma HCO3− levels with small changes in the partial pressure of CO2 (PCO2). In contrast, air-breathers mediate the decrease in pHa by increasing arterial PCO2 (PaCO2) at constant plasma HCO3− by reducing lung ventilation relative to metabolic CO2 production. Much less is known in bimodal breathers that utilize both water and air. Here, we characterize the influence of temperature on arterial acid-base balance and intracellular pH (pHi) in the bimodal breathing swamp eel, Monopterus albus. This teleost uses the buccopharyngeal cavity for gas exchange and has very reduced gills. When exposed to ecologically relevant temperatures (20, 25, 30 and 35°C) for 24 and 48h, pHa decreased by -0.025 pH units/°C (U/°C) in association with an increased PaCO2, but without changes in plasma [HCO3−]. Intracellular pH (pHi) was also reduced with increased temperature. The slope of pHi of liver and muscle was -0.014 and -0.019 U/°C, while the heart muscle showed a smaller reduction (-0.008U/°C). When exposed to hypercapnia (7 or 14 mmHg) at either 25 or 35°C, Monopterus albus elevated plasma [HCO3−] and therefore seemed to defend the new pHa set-point, demonstrating an adjusted control of acid-base balance with temperature. Overall, the effects of temperature on acid-base balance in Monopterus albus resemble air-breathing amniotes, and we discuss the possibility that this pattern of acid-base balance results from a progressive transition in CO2 excretion from water to air as temperature rises.

Hagfish: Champions of CO2 tolerance question the origins of vertebrate gill function

The gill is widely accepted to have played a key role in the adaptive radiation of early vertebrates by supplanting the skin as the dominant site of gas exchange. However, in the most basal extant craniates, the hagfishes, gills play only a minor role in gas exchange. In contrast, we found hagfish gills to be associated with a tremendous capacity for acid-base regulation. Indeed, Pacific hagfish exposed acutely to severe sustained hypercarbia tolerated among the most severe blood acidoses ever reported (1.2 pH unit reduction) and subsequently exhibited the greatest degree of acid-base compensation ever observed in an aquatic chordate. This was accomplished through an unprecedented increase in plasma [HCO₃⁻] (>75 mM) in exchange for [Cl⁻]. We thus propose that the first physiological function of the ancestral gill was acid-base regulation, and that the gill was later co-opted for its central role in gas exchange in more derived aquatic vertebrates.

Optical mapping of the electrical activity of isolated adult zebrafish hearts: acute effects of temperature

The zebrafish (Danio rerio) has emerged as an important model for developmental cardiovascular (CV) biology; however, little is known about the cardiac function of the adult zebrafish enabling it to be used as a model of teleost CV biology. Here, we describe electrophysiological parameters, such as heart rate (HR), action potential duration (APD), and atrioventricular (AV) delay, in the zebrafish heart over a range of physiological temperatures (18-28°C). Hearts were isolated and incubated in a potentiometric dye, RH-237, enabling electrical activity assessment in several distinct regions of the heart simultaneously. Integration of a rapid thermoelectric cooling system facilitated the investigation of acute changes in temperature on critical electrophysiological parameters in the zebrafish heart. While intrinsic HR varied considerably between fish, the ex vivo preparation exhibited impressively stable HRs and sinus rhythm for more than 5 h, with a mean HR of 158 ± 9 bpm (means ± SE; n = 20) at 28°C. Atrial and ventricular APDs at 50% repolarization (APD50) were 33 ± 1 ms and 98 ± 2 ms, respectively. Excitation originated in the atrium, and there was an AV delay of 61 ± 3 ms prior to activation of the ventricle at 28°C. APD and AV delay varied between hearts beating at unique HRs; however, APD and AV delay did not appear to be statistically dependent on intrinsic basal HR, likely due to the innate beat-to-beat variability within each heart. As hearts were cooled to 18°C (by 1°C increments), HR decreased by ~40%, and atrial and ventricular APD50 increased by a factor of ~3 and 2, respectively. The increase in APD with cooling was disproportionate at different levels of repolarization, indicating unique temperature sensitivities for ion currents at different phases of the action potential. The effect of temperature was more apparent at lower levels of repolarization and, as a whole, the atrial APD was the cardiac parameter most affected by acute temperature change. In conclusion, this study describes a preparation enabling the in-depth analysis of transmembrane potential dynamics in whole zebrafish hearts. Because the zebrafish offers some critical advantages over the murine model for cardiac electrophysiology, optical mapping studies utilizing zebrafish offer insightful information into the understanding and treatment of human cardiac arrhythmias, as well as serving as a model for other teleosts.

A critical analysis of carbonic anhydrase function, respiratory gas exchange, and the acid-base control of secretion in the rectal gland of Squalus acanthias

We compared in vivo responses of rectal gland secretion to carbonic anhydrase (CA) inhibition (10(-4) mol l(-1) acetazolamide) in volume-loaded dogfish with in vitro responses in an isolated-perfused gland stimulated with 5 x 10(-6) mol l(-1) forskolin and removed from systemic influences. We also measured respiratory gas exchange in the perfused gland, described the acid-base status of the secreted fluid, and determined the relative importance of various extracellular and intracellular acid-base parameters in controlling rectal gland secretion in vitro. In vivo, acetazolamide inhibited Cl(-) secretion and decreased pHi in the rectal gland, but interpretation was confounded by an accompanying systemic respiratory acidosis, which would also have contributed to the inhibition. In the perfused gland, M(CO(2)) and M(O(2)) increased in linear relation to increases in Cl(-) secretion rate. CA inhibition (10(-4) mol l(-1) acetazolamide) had no effect on Cl(-) secretion rate or pHi in the perfused gland, in contrast to in vivo, but caused a transitory 30% inhibition of M(CO(2)) (relative to stable M(O(2))) and elevation in secretion P(CO(2)) effects, which peaked at 2 h and attenuated by 3.5-4 h. Secretion was inhibited by acidosis and stimulated by alkalosis; the relationship between relative Cl(-) secretion rate and pHe was almost identical to that seen in vivo. Experimental manipulations of perfusate pH, P(CO(2)) and HCO(3)(-) concentration, together with measurements of pHi, demonstrated that these responses were most strongly correlated with changes in pHe, and were not related to changes in P(CO(2)), extracellular HCO(3)(-), or intracellular HCO(3)(-) levels, though changes in pHi may also have played a role. The acid-base status of the secreted fluid varied with that of the perfusate, secretion pH remaining about 0.3-0.5 units lower, and changing in concert with pHe rather than pHi; secretion HCO(3)(-) concentrations remained low, even in the face of greatly elevated perfusate HCO(3)(-) concentrations. We conclude that pH effects on rectal gland secretion rate are adaptive, that CA functions to catalyze the hydration of CO(2), thereby maintaining a gradient for diffusive efflux of CO(2) from the working cells, and that differences in response to CA inhibition likely reflect the higher perfusion-to-secretion ratio in vitro than in vivo.

Evolutionary determinants of normal arterial plasma pH in ectothermic vertebrates

Mean values of normal arterial pH in different species of fish, amphibians and reptiles at 15 and 25°C, taken from the literature, are negatively correlated with arterial PCO2 and plasma [Na+]. At either temperature, the data accord with the hypothesis that extracellular acid–base homeostasis evolved to maintain an optimal pH at particular cell-surface sites that are similar in all species. These hypothetical sites bear fixed negative charges that attract H+, but which are partially screened by Na+; for the surface pH to be constant, the bulk interstitial pH should then vary inversely with [Na+], as is the case. At the same time, the bulk interstitial fluid must be more acid than arterial plasma by an amount that increases with decreasing arterial PCO2. With allowance made for additional screening by Ca2+ and Mg2+, the relevant cell-surface pH is probably approximately 6.2.

pH regulation in horizontal cells of the skate retina

To examine the mechanisms by which horizontal cells regulate intracellular pH (pHi), measurements were recorded from isolated cells enzymatically dissociated from the skate retina utilizing the pH-sensitive dye BCECF. In a HCO3--containing Ringer solution, steady-state pHi was 7.32+/-0.13 (mean+/-S.D., n=70). Recovery from acidification was examined using the NH4+ prepulse technique. When NH4+ was removed from the extracellular solution, pHi dropped rapidly to approximately 0.3 pH units below the initial baseline, and then recovered at an initial rate of approximately 0.072 pH units/min. During recovery of pHi after the acid load, the removal of Na+ or the addition of amiloride from a HCO3--free extracellular solution reduced the rate of recovery by 79%+/-11% and 69%+/-14%, respectively. In the presence of DIDS, which inhibits primarily anion transport, or during the removal of Na+, the recovery from acidification was reduced by 83%+/-10% and 70%+/-11%, respectively, as compared to the control value in HCO3--containing solution. These results suggest that the skate horizontal cell possesses a Na/H exchanger as well as a Na+-and HCO3--dependent mechanism for removal of excess acid. Removal of HCO3- or Cl- from the extracellular solution had little effect on pHi, but removing external Na+ induced a marked decrease in pHi that fell at an initial rate of approximately 0.3 pH units min-1. This rate of acidification was decreased by 58%+/-19% in the presence of DIDS (500 micron) and reduced by 28%+/-13% with the addition of amiloride (2 mm). Thus, Na- and HCO3-dependent transport was about 2-fold more active than Na/H exchange during low Na+-induced acidification. The intrinsic pH-buffer capacity, determined from the pHi change induced by incremental reductions in the [NH4+] of the extracellular solution, was 24.2 mm/pH unit at the horizontal cell's resting pHi. Moreover, pHi was relatively insensitive to changes in membrane potential; in experiments under whole-cell voltage clamp (-70 mV), intracellular pH remained constant during depolarizing voltage swings to -30 mV or +30 mV, as well as during hyperpolarizing pulses to -90 or -110 mV.

Control of ventilation in the hypercapnic skate Raja ocellata: II. Cerebrospinal fluid and intracellular pH in the brain and other tissues

This study examined the possible role(s) of central acid-base stimuli in the increase in ventilation induced by hypercapnia in the skate, a response that is not due to an O2 signal (Graham et al., Respir. Physiol., 1990, 80: 251-270). Skate were sampled for cerebrospinal fluid (CSF) acid-base status, intracellular pH of the brain (14C-DMO method), and pHi in other tissues throughout 24 h of exposure to PICO2 = 7.5 Torr. CSF PCO2 rapidly equilibrated with the elevated PaCO2. Despite the much lower non-HCO3- buffer capacity in the CSF, CSF pH was not depressed to the same extent as blood pHa. CSF pH was also regulated rapidly, returning to control levels by 8-10 h, whereas pHa remained significantly depressed at 24 h. Similarly, the pHis of the weakly buffered brain and heart ventricle were initially compensated more rapidly than those of more strongly buffered white muscle and red blood cells. However, brain pHi adjustment slowed markedly after 4 h and stabilized at only 70% compensation by 20-24 h, suggesting that brain intracellular acidosis may play a role in the long-term increase in ventilation. CSF and brain were the only compartments which did not exhibit an apparent compounding metabolic acidosis during the initial stages of hypercapnic exposure. While these results illustrate the primacy of central acid-base regulation, they do not support a role for CSF pH in the long-term elevation of ventilation in response to hypercapnia. Depressions in pHa and brain pHi appear the two most likely candidates for proximate stimuli.

Regulation of ventilation and acid-base status in the elasmobranch Scyliorhinus stellaris during hyperoxia-induced hypercapnia

Exposure of the elasmobranch Scyliorhinus stellaris to environmental hyperoxia (PO2 of 500 mm Hg) resulted in a considerable rise of arterial PCO2 from 1.9 mm Hg during normoxia to about 11 mm Hg after 6 days as an expression of the primarily oxygen-oriented regulation of gill ventilation. In contrast to the typical pattern during environmental hypercapnia, however, arterial plasma pH was hardly affected by the considerable hyperoxia-induced hypercapnia. At elevated arterial PO2 values (200-300 mm Hg) gill ventilation was apparently not adjusted exclusively for the oxygen demands of the organism, but was matched to the requirements of acid-base regulation such that the rise in PCO2 could be compensated for by a net gain of bicarbonate-equivalent ions from the environment. This fine adjustment of gill ventilation to the bicarbonate-equivalent uptake rate extended the process of adaptation to about 6 days and resulted in an almost complete pH compensation during the entire process of PCO2 increase. These data suggest that during conditions of reduced oxygen-related respiratory drive the regulation of gill ventilation is primarily dependent upon the acid-base parameters.

The apparent pK of carbonic acid in rainbow trout blood plasma between 5 and 15 degrees C

Values for carbon dioxide solubility (alpha CO2) and the apparent first dissociation constant (pKapp) of carbonic acid in rainbow trout plasma were measured at 5, 10 and 15 degrees C so as to eliminate the uncertainties with continued use of mammalian values extrapolated from the much higher temperatures of their determination. Estimates of pKapp were based on the in vivo measurement criteria most commonly used (i.e. whole blood pH, PCO2 and the CCO2 of true plasma separated from red cells at room temperature). Apparent pK varied inversely with pH, the dpKapp/dpH slopes at 10 and 15 degrees C (-0.075 and -0.080, respectively) being significantly elevated with respect to that at 5 degrees C (-0.004). At constant pH, dpKapp/dTemp varied between -0.0160 (pH 7.4) and -0.0208 (pH 8.0), both of which are higher than theoretically and experimentally based literature data on separated plasma. When we repeated our pKapp determinations (using identical methods) on rainbow trout separated plasma, we obtained dpKapp/dT slopes ranging from -0.009 to 0.0110, similar to all previous determinations. In attempts to account for the discrepancies between our whole blood and plasma based pKapp estimates, we found that the pH of whole blood was always lower than that of its isothermally separated true plasma (0.015 units lower at 15 degrees C) and that this difference became magnified at lower temperatures (0.033 units lower at 5 degrees C). Also, if cool blood was allowed to warm towards room temperature before and/or during anaerobic centrifugation for true plasma, CO2 was found to leave the red cells and result in a higher plasma total CO2 content relative to the amount contained in the original blood plasma (0.40 mM for a 15 degree C dT of separation). We conclude that use of pKapp values obtained from gasometric determinations on separated plasma is not appropriate for PCO2 or [HCO3-] calculation by the Henderson-Hasselbalch equation when the practice is to measure the whole blood pH and the total CO2 content of true plasma separated at temperatures other than that of the original blood plasma.

Effects of temperature change on acid-base regulation in skipjack tuna (Katsuwonus pelamis) blood

The effects of temperature change (in vitro) on acid-base balance of skipjack tuna blood were investigated. By examining the relationship between blood pH and temperature (in vitro) under conditions of constant CO2 tension (open system), it was observed that dpH/dT = -0.013 U/degrees C. This value falls well within the range of in vivo values reported for other ectothermic vertebrates, and is only slightly different than results obtained in vitro under conditions of constant CO2 content (closed system; dpH/dT = -0.0165 U/degrees C). It is concluded that changes in pH following temperature changes can be accounted for solely by the passive, in vitro behaviour of the chemical buffer system found in the blood, so that active regulatory mechanisms of pH adjustment need not be postulated for skipjack tuna.

Effect of temperature on intracellular pH in crayfish neurons and muscle fibers

Intracellular pH microelectrodes were used to determine the effects of temperature (13-26 degrees C) on the in vitro regulation of intracellular acid-base status of neurons and muscle fibers of the crayfish Astacus leptodactylus. The values of the temperature coefficients delta pH/delta T (pH unit/degrees C) were -0.019 and -0.026 for muscles and neurons, respectively, values which are close to the temperature coefficient (-0.019) of the pK' of protein imidazole buffer groups. When temperature varies, the dissociation ratio of imidazole groups is thus maintained by the cellular regulation of cytoplasmic pH. According to the alphastat regulation hypothesis, this constancy would minimize the temperature effects on enzymic systems.

Branchial ion exchange and acid-base regulation after strenuous exercise in rainbow trout (Salmo gairdneri)

Specimens of rainbow trout (Salmo gairdneri) were electrically stimulated to exhausting activity in a closed water recirculation system and the changes in dorsal aortic plasma pH, PCO2, PO2, O2 content, [Na+], [Cl-], [K+], [Lactate-] and Ht were measured during a 24 h recovery period. Net transfer of H+, Na+, Cl- and ammonia between fish and environment were determined by measurement of the concentration changes in the recirculating water. Strenuous exercise resulted in a severe lactacidosis which was corrected by transient net transfer of H+ ions to the environmental water within 4 h, about 6-8 h before the lactate was metabolically removed. The net transfer of H+ ions was achieved in part by branchial HCO3-/Cl- ion exchanges, but to a larger extent by branchial exchange of H+ and/or NH4+ against Na+. The excretion of ammonia, which was considerably enhanced during the first 4 h after exercise, was at least partially due to non-ionic diffusion across the gill epithelium. The observed elevation in ammonia excretion was probably the result of an exercise-induced increase in nitrogen metabolism rather than of production of ammonia for the purpose of acid-base regulation.

The influence of temperature on extracellular and intracellular pH in the American eel, Anguilla Rostrata (Le Sueur)

The effects of ambient temperature (5-20 degrees C) on the pH of the extracellular and intracellular compartments of the Americal eel (Anguilla rostrata) were measured. Venous blood pH was sampled from an indwelling catheter in the caudal vein, and intracellular pH of liver, heart, white muscle, and red muscle was estimated by the distribution ratio of 14C-5,5 dimethyloxazolidine-2,4-dione (DMO). The absolute value of pH varies in a manner blood greater than liver greater than red muscle greater than heart and white muscle. The values for the change in pH with temperature (delta pH/delta T degrees C) are: blood, -0.0076; liver, -0.0177; heart, -0.0203; white muscle, -0.0090; red muscle, -0.0033. Delta pH/delta T degrees C values for blood, white muscle and red muscle are not statistically different, and are far from the -0.018 value predicted by the alphastat hypothesis. In contrast, delta pH/delta T degrees C values for liver and heart are statistically different from blood and match those expected for alphastat regulation. The results are discussed in the context of the alphastat hypothesis and metabolic 'torpor' in the American eel.

Acid-base regulation in response to environmental hypercapnia in two aquatic salamanders, Siren lacertina and Amphiuma means

The partial pressure of CO2 (PCO2) in certain areas of the aquatic habitat of the salamanders Siren lacertina and Amphiuma means frequently rises to values of up to 60 mm Hg. This ambient hypercapnia occurs due to hindrance of gas exchange between water and air caused by dense water-surface vegetation. In order to investigate the acid-base regulation in response to the respiratory acidosis, which wound be expected to result from the high CO2 conductance of the amphibian skin, specimens of both species were subjected to water PCO2 of 47 mm Hg while having free access to normocapnic air in a closed water recirculation system. Arterial PCO2 rose considerably from 12 to 35 mm Hg in Siren and from 17 to 36 mm Hg in Amphiuma. The resultant fall in plasma pH remained uncompensated, whereas intracellular pH of white muscle and heart muscle of Siren were little affected owing to elevated intracellular bicarbonate concentrations. The bicarbonate accumulated in the intracellular compartments was in part produced by intracellular and extracellular nonbicarbonate buffering, and in part gained from the environment in exchange for Cl- ions. Elevated water bicarbonate concentration or bicarbonate infusion into Siren had no effect on the acid-base regulation. These data suggest that the availability of bicarbonate is not a limiting factor for extracellular compensation of increased PCO2, but that the threshold of the bicarbonate-regulating structures is simply not readjusted in hypercapnia. This type of regulation may have evolved as a result of the specific environmental conditions of these animals and may be considered as an energetically efficient way of maintaining a constant milieu for the pH-sensitive intracellular structures.

Temperature and blood acid-base status in the blue crab, Callinectes sapidus

The blood acid-base status of blue crabs. Callinectes sapidus, was assessed at temperatures ranging from 10 to 30 degrees C. Blood pH varied inversely with temperature, with a slope (dpH/tT) of - 0.012. The PaCO rose only slightly between 10 and 27 degrees C, from 2.09 torr to 2.91 torr, with the result that the total dissolved CO2 actually fell slightly due to the decrease in solubility. In vitro the blood behaved like a conventional Rosenthal system, with a rise in PCO2 at constant CT which was much greater than the observed in vivo increase. I vivo the CT of the blood decreased significantly in order to maintain the OH/H ratio. The ratio of ventilation to oxygen consumption (and presumably carbon dioxide production) was not significantly different at 10 and 27 degrees C, indicating that ventilation is not participating in the acid-base adjustment to temperature. The principal acid-base regulating mechanisms in the crab are probably ionic exchanges in the gills rather than ventilatory control of PaCO2.

Bicarbonate exchange between body compartments after changes of temperature in the larger spotted dogfish (Soyliorhinus stellaris)

Intracellular/extracellular and extracellular/sea-water bicarbonate exchanges were measured in Larger Spotted Dogfish (Sycliorhinus stellaris) exposed to 10 degrees C temperature step changes in a closed sea-water recirculation system. Changes of the bicarbonate concentration in blood plasma (= extracellular space) and in the recirculating sea-water were monitored for 36 h after the temperature change. Intracellular/extracellular transfer of bicarbonate was computed from bicarbonate changes in the extracellular space and sea-water. When the temperature was changed from 10 to 20 degrees C a signigicant transfer of bicarbonate was observed from the intracellular to the extracellular compartment and from the extracellular compartment to the sea-water. These transfers were reversed when the temperature was lowered from 20 to 10 degrees C. The exchange processes were practically completed after 18 h. The amount of bicarbonate exchanged between intracellular and extracellular compartments and sea-water was larger than predicted on the basis of in vitro buffer values of white, red and heart muscle, suggesting that additional tissues exchange significant amounts of bicarbonate with the extracellular space. It is concluded that physicochemical buffering is not sufficient to account for the observed adjustment of intracellular and extracellular pH and that bicarbonate exchange between body compartments and environment may be the most important regulatory mechamism, responsible for the final adjustment of acid-base balance in dogfish.

Intracellular acid-base state at a variable temperature in air-breathing vertebrates and its representation

When temperature changes are superimposed on changes in the control variables of acid-base state (PCO2, strong ion difference), the understanding of acid-base changes becomes difficult. A solution has recently been proposed for blood (Malan, 1977); it was based on the assumption that closed system conditions correspond to a minimal change in the overall acid-base state when temperature varies. The feasibility of extending these concepts to muscle intracellular acid-base vs temperature relationships is evaluated on the basis of a model study; the errors made by replacing closed conditions (which require knowledge of chemical composition) by more convenient approximations are estimated. A representation of both extracellular and intracellular acid-base data on a temperature-corrected bicarbonate-pH diagram is derived. It allows the interpretation of variable-temperature intracellular acid-base changes in terms of changes in control variables, 'respiratory (PCO2) or 'metabolic' (strong ion difference).

Temperature-induced variations of blood acid-base status in the lugworm, Arenicola marina (L.): II. In vivo study

Lugworms, Arenicola marina (L.), acclimatized at 16-17 degrees C, were acclimated at temperatures between 5.3 and 25.7 degrees C for 96 h. Whereas in vitro Arenicola blood behaves like a Rosenthal system, in vivo prebranchial blood does not: the higher the acclimation temperature, the lower the pHv and [HCO3]V, PVCO2, remaining practically constant. Nevertheless, the very low relative alkalinity of the blood in vivo ([OH-]/[H+] is less than 3), and the degree of dissociation of extra- and intracellular proteins, remain practically constant whatever the temperature. From examples in the literature together with these results, it is concluded that poikilothermic air-breathers and poikilothermic water-breathers regulate their blood pH in the face of temperature changes by contrasting mechanisms. In the first, regulation is almost instantaneous and takes place at the pulmonary level through adjustment of CO2 exchanges. In the second this regulation is slow and mainly extraventilatory, occurring through ionic exchanges. This contrast must be considered in relation with differences in blood PCO2 values, caused by the much higher O2 capacitance of air compared to water.

Temperature-induced variations of blood acid-base status in the lugworm, Arenicola marina (L.): I. In vitro study

Blood of the lugworm Arenicola marina studied in vitro behaved like a Rosenthal system: when temperature rose, pH decreased and PCO2 increased, whereas [HCO3] remained practically constant. pH values were low whatever the CCO2 and SO2. The temperature coefficient dpH/dt was always significantly different from the mean temperature-induced variations of the neutral pH of pure water between 0 and 30 degrees C. Consequently, the relative alkalinity of the blood, [OH-]/[H+], was very low (range, 1.53-5.06) and increased appreciably with temperature. Calculated changes in the fractional dissociation of protein imidazole groups, alphaIm, were smaller. The very variable buffer power of Arenicola hemoglobin was maximum (beta max) for a strictly defined, temperature-dependent value of pH (pH beta max), suggesting that as yet unidentified ionizable group on the hemoglobin molecule, RH, could be responsible for the pH-dependent changes of blood buffer power in Arenicola. Assuming pK'RH = PH beta max, the calculated fractional dissociation of RH, alpha RH, was constant between 0 and 30 degrees C. The nature of RH is discussed in relation with Reeves's hypothesis concerning the preeminence of protein imidazole groups in the regulation of extra- and intracellular pH.