Comparative Biochemistry and Physiology Part A: Physiology最新文献

The temperatures in the polar oceans are not only low, but also relatively stable. Blood becomes more viscous at cold temperatures and it is assumed that this increase of viscosity is responsible for a number of adaptations of the cardiovascular system. The Antarctic Nototheniids show large changes in haematocrit compared to other fishes, and this phenomenon may be related to the high viscosity of their blood at low temperatures. Reduction of the haematocrit will reduce the viscosity, and thus diminish cardiac work. Indeed, one group (the “icefish,” Channichthydae) has disposed of erythrocytes altogether.

The cholinergic tonus on the heart is remarkably high under “resting” conditions—up to 80% in the bottom-dwelling Trematomus bemacchii—and changes in cardiac performance appear to depend chiefly on modulation of this tonus, rather than activity in excitatory (adrenergic) fibres. Sequestering of erythrocytes by the spleen is a major factor in the reduction of haematocrit, and cholinergic autonomic nerves control release of these cells to increase haematocrit during periods of demand. Thus, the studies of the autonomic control of the heart and spleen of the Antarctic fish show that these are unusual among fishes in that both organs appear more or less solely cholinergically controlled.

Traditional karyotyping and molecular cytogenetic techniques were used to study the chromosomes of Champsocephalus gunnari from the Indian Sector of the Antarctic Ocean. C. gunnari has 24 pairs of chromosomes, giving a diploid number of 48. Most are acrocentric, six are metacentric and submetacentric and the level of divergence from the supposed ancestral notothenioid karyotype remains low. The chromosomal features of this icefish do not differ very much from those of the other channichthyids, the karyotypic macrostructure being kept stable in these fishes. Although providing relevant basic biological information, traditional cytogenetics do not allow a significant comparative analysis between species in this notothenioid family. Molecular cytogenetic techniques such as in situ hybridization have been successfully used on C. gunnari, opening new opportunities for a deeper characterization of Antarctic fish chromosomes. The in situ location of specific DNA sequences could provide valuable new information for phylogenetic reconstruction, and shed light on the relationship between chromosome change and the evolutionary process.

The effects of hypoxia, sodium azide and pentachlorophenol (PCP) exposure on high-energy phosphorylated compounds, intracellular pH (pH_i) and intracellular free Mg²⁺ (Mg_f) in intact red abalone (Haliotis rufescens) were determined using ³¹P-NMR. Abalone made hypoxic by bubbling sea water with N₂ showed modest changes in phosphoarginine (PA) and inorganic phosphate (P_i) concentrations, no significant changes in pH_i and a moderate decrease in Mg_f that was not statistically significant. Azide (50 mg/l) exposed animals displayed severe declines in PA dropping to 0.53 of reference values, coupled with large increases in P_i to 10.66 times resting concentrations that occurred just after the 2-hr exposure period. pH_i also showed significant declines from a resting value of 7.17-7.06 (P < 0.05) but fully recovered by the end of the 6-hr clean seawater recovery period, whereas Mg_f concentrations declined slightly during the exposure period but increased by 18% at the end of the recovery period relative to reference Mg_f. PCP (1.2 mg/l) exposed animals displayed similar increases and declines in P_i and PA, respectively, as did azide-exposed animals by the end of the exposure period, but recovery was much slower and occurred in a bimodal fashion with some animals completely recovering at the end of 6 hr and others essentially stabilized at the end of the exposure period and did not show any significant changes during the recovery period.

The kinetic properties of two metabolic enzymes, glucose-6-phosphate dehydrogenase and hexokinase, were studied in four strains of entomopathogenic nematodes that had been recycled for two years at various temperatures: Steinernema feltiae NF strain, Steinernema feltiae Umeå strain, Steinernema carpocapsae All strain, Steinernema riobravis TX strain. The recycling temperatures influenced the activities of glucose-6-phosphate dehydrogenase and hexokinase in an adaptive fashion in all the strains. At each assay temperature (5–35°C), the maximum specific activity of both the enzymes was greater in the nematodes that had been recycled at lower temperatures than in those reared at higher temperatures. In three enzyme-nematode strain combinations, the lowest K_m values measured at each assay temperature occurred in nematodes that had been recycled at the lower temperatures. However, the assay temperatures at which the minimum K_m values occurred were ≥15°C. The capacities of these nematodes to adjust to different recycling temperatures is discussed in relation to the physiological mechanisms involved.

The regulation of body water balance was examined in the reedfrogs Hyperolius marmoratus taeniatus and Hyperolius viridiflavus ommatostictus. Temperature and stage of post-metamorphic development significantly affected the rate of water uptake. Hydrated reedfrogs prevented hyperhydration by voiding diluted urine when obtaining water. Within 48 hr after rehydration, body fluid osmolality remained at low levels, which may be supportive to counter excessive cutaneous water influx in hydrated frogs. Once evaporative water loss exceeded 10–12% total body mass, reedfrogs became anuric. The rate of water uptake strongly increased with increasing body water deficit. Both the anuric response and the increased rate of water uptake are assumed to strongly enhance the efficacy of using very briefly available water sources during dry-period conditions. Dry-adapted and estivating reedfrogs survived evaporative water losses between 40 and 55% total body mass. Bladder fluid stores contributed substantially to this desiccation tolerance. During a 16-day period of desiccation, H. v. ommatostictus could replace approximately 25% of evaporative water loss from the bladder fluid store. During desiccation, the level of free amino acids selectively increased in the gastrocnemius muscle tissue, which may support cell volume regulation and/or protect cellular structures from osmotic stresses. Even strongly dehydrated reedfrogs rehydrated quickly with no obvious osmoregulatory problem. Rehydration was associated with a higher than expected decrease of free amino acids in the gastrocnemius muscle tissue, a response that may help to protect cells from bursting during fast rehydration.