Evaluation of the functional properties of invertebrate hemoglobins

Physiological responses to temperature and haeme synthesis modifiers in earthworm Lumbricus terrestris (Annelida: Oligochaeta)

Earthworms (Lumbricus terrestris) acclimated at 2° and 6°C above their average habitat temperature (10°C) had respectively 15 and 40% higher rate of respiration than those at habitat temperature. At 14°C, the rate of respiration and blood hemoglobin (Hb) concentration both increased by ∼60 and 50%, respectively, of the values at habitat temperature. At higher temperatures the rate of respiration and Hb synthesis started decreasing. At 20-23°C, the respiration and Hb concentration decreased respectively by about 85% and 35% of that at 14°C. Decrease in blood Hb concentration at higher temperatures appeared to be due to the lowering of the activity of blood enzyme δ-aminolaevulinic acid dehydratase (ALAD). Exposure of 20-23°C-acclimated pale worms to ALAD inhibitor (lead), lowered the already compromised rate of respiration and blood Hb concentration; while exposure to hexachlorobenzene (HCB, inducer of haeme synthesis) and ferric chloride (enhancer of haeme synthesis) did not overcome the inhibitory effect of high temperature on Hb synthesis. At 20-23°C the affinity of Hb for oxygen also decreased as indicated by the lowering of oxy-Hb (HbO) concentration in blood. The lowering of concentration of blood Hb and its affinity for oxygen may lower the amount of oxygen delivered to cells, which may limit the level of aerobic metabolism (glycolysis, oxidative phosphorylation), as indicated by an increase in blood glucose concentration and a decrease in in vitro activities of mitochondrial electron transport system components (ETS) namely NADH-cytochrome c reductase, succinate dehydrogenase, cytochrome c oxidase, and ATPases. Although the oxygen concentration in air, at sea level, does not decrease significantly from 6° to 20-23°C (lack of hypoxia), lowering of both Hb and HbO concentrations by high temperature may cause significant hypoxemia. The latter may lead to inhibition of the activity of muscle mitochondrial respiratory enzymes (ETS). The resulting inhibition of ATP synthesis and hydrolysis may cause deficit of energy needed for peristalsis/fictive locomotion of body and heart muscles (as indicated by a decrease in heart rate) to facilitate diffusion and transport of gases. The upper critical temperature (20-23°C) also slows down the heart rate and causes hyperosmotic stress (hypovolemia). Thus, a rise in soil temperature above 18°C, which inhibits Hb synthesis, Hb oxygenation, and mitochondrial ETS activity, and slows down the heart rate and causes hyperosmotic stress, can make this and higher temperatures lethal to populations of these earthworms, especially in the presence of metabolic inhibitors and respiratory poisons.

Is monitoring of respiratory properties of the haemoglobin of the Lugworm Arenicola marina meaningfull?

Haemoglobin in Arenicola has a high oxygen affinity, with a half saturation oxygen pressure of about 180 Pa at pH 7.4 and 15°C. The oxygen affinity decreases with increasing copper concentration in the blood to about 300 Pa at pH 7.4, 15°C. at a copper concentration of 35 μg ml(-1). Copper exerts only a slight influence on the subunit cooperativity in oxygen binding. The oxygen affinity, subunit cooperativity and the Bohr effect are pH dependent. Copper affects the functioning of the blood by inhibiting oxygen binding to the haemoglobin in the respiratory area and oxygen delivery in the tissues.

Mechanims of extracellular acid-base regulation as temperature changes in decapod crustaceans

In the crab Carcinus maenas, as in other decapod crustaceans, the extracellular pH varies with temperature so that the relative alkalinity remains approximately constant. However, as temperature changes, the blood acid-base behavior is not quite the same in an in vitro closed system and the living animal. In fully acclimated crabs, the higher the temperature, the lower the blood total CO2 concentration. After an abrupt temperature increase in vivo a new acid-base state is established instantaneously, comparable to that observed in vitro; but during the following hours, both PCO2 and bicarbonate concentration decrease concomitantly at constant pH. The relative gill ventilation, evaluated as the ratio of the ventilatory flow rate to the oxygen uptake rate, does not vary significantly as a function of temperature. No significant change in the base output can be detected during the hours following a temperature shift. These observations suggest that the mechanisms involved in the temperature-dependent acid-base regulation act mainly to control the blood bicarbonate level.

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.

The function of hemocyanin in respiration of the blue crab Callinectes sapidus

Blood PO2 in the blue crab Callinectes sapidus, a very active species of tropical origin, is lower at 22 degrees C than that of larger crabs in colder waters. These low oxygen levels permit its hemocyanin to be highly oxygenated at the gill, and to deliver almost half of its oxygen to the tissues in resting animals. Sustained muscular activity results in conspicuous decreases in blood PO2, pH and hemocyanin oxygenation. Although the venous reserve is fully utilized, hemocyanin oxygenation at the gill decreases so much that there is no change in its total quantitative function. The large Bohr shift becomes functional during activity, but its quantitative importance is not clear.