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D Manahan

Publications and source records attributed to D Manahan.

5 recordsLinked to original sources
PubMed1997

Metabolic importance of Na+/K+-ATPase activity during sea urchin development.

Early stages of animal development have high mass-specific rates of metabolism. The biochemical processes that establish metabolic rate and how these processes change during development are not understood. In this study, changes in Na+/K+-ATPase activity (the sodium pump) and rate of oxygen consumption were measured during embryonic and early larval development for two species of sea urchin, Strongylocentrotus purpuratus and Lytechinus pictus. Total (in vitro) Na+/K+-ATPase activity increased during development and could potentially account for up to 77 % of larval oxygen consumption in Strongylocentrotus purpuratus (pluteus stage) and 80 % in Lytechinus pictus (prism stage). The critical issue was addressed of what percentage of total enzyme activity is physiologically active in living embryos and larvae and thus what percentage of metabolism is established by the activity of the sodium pump during development. Early developmental stages of sea urchins are ideal for understanding the in vivo metabolic importance of Na+/K+-ATPase because of their small size and high permeability to radioactive tracers (86Rb+) added to sea water. A comparison of total and in vivo Na+/K+-ATPase activities revealed that approximately half of the total activity was utilized in vivo. The remainder represented a functionally active reserve that was subject to regulation, as verified by stimulation of in vivo Na+/K+-ATPase activity in the presence of the ionophore monensin. In the presence of monensin, in vivo Na+/K+-ATPase activities in embryos of S. purpuratus increased to 94 % of the maximum enzyme activity measured in vitro. Stimulation of in vivo Na+/K+-ATPase activity was also observed in the presence of dissolved alanine, presumably due to the requirement to remove the additional intracellular Na+ that was cotransported with alanine from sea water. The metabolic cost of maintaining the ionic balance was found to be high, with this process alone accounting for 40 % of the metabolic rate of sea urchin larvae (based on the measured fraction of total Na+/K+-ATPase that is physiologically active in larvae of S. purpuratus). Ontogenetic changes in pump activity and environmentally induced regulation of reserve Na+/K+-ATPase activity are important factors that determine a major proportion of the metabolic costs of sea urchin development.

Journal Article↗
PubMed1995

Coulometric measurement of oxygen consumption during development of marine invertebrate embryos and larvae

Determining the metabolic rate of larval invertebrates from aquatic habitats is complicated by the problems of small size and the scarcity of suitable measurement techniques. In this study, coulometric respirometry (a new technique for the study of marine embryos and larvae) was used to explore several issues associated with the rate of energy use during embryonic and larval development of marine invertebrates from three phyla. Coulometric respirometry measures rates of oxygen consumption under normoxic conditions by electrochemically replacing the oxygen consumed by organisms during an experiment. This technique is based on the assumption that all electrons consumed by the anodic reactions result in the production of oxygen. We verify this assumption using direct measurements of oxygen production and show that the technique is sensitive enough (1 nmol O2 h-1) to quantify the oxygen consumption of a single individual swimming freely in a relatively large volume (2 ml). Continuous measurements can span days, and embryos in the coulometric respiration chambers develop to the larval stage at normal rates of differentiation. Measurements of metabolic rates were made with the coulometric respirometer during the complete life-span of larvae of three species (asteroid, Asterina miniata; bivalve, Crassostrea gigas; echinoid, Dendraster excentricus). For these species, metabolic power equations had mass exponents near unity (0.9­1.1), showing that metabolic rate scales isometrically with mass during larval growth. Metabolic rates were independent of the concentration of larvae used in the respirometer chambers for a range of larval concentrations from 4 to 400 larvae ml-1 (coulometric respirometer) and from 241 to 809 larvae ml-1 (polarographic oxygen sensor). Metabolic rates were measured using coulometric respirometry and two other commonly used techniques, polarographic oxygen sensors and Winkler's titration. Polarographic oxygen sensors in small, sealed chambers (100 µl) consistently gave the lowest values (by as much as 80 %) for the asteroid, echinoid and molluscan larvae. By comparison, rates of oxygen consumption measured using coulometric respirometry and Winkler's titration (to measure the change in oxygen concentration over time) were similar and consistently higher. Although the polarographic oxygen sensor is the most widely used method for measuring the metabolism of small animals in sealed 100­1000 µl chambers, it appears that the metabolism of some larvae is adversely affected by the conditions within these respirometers.

Journal Article↗
PubMed1995

Taurine metabolism in larvae of marine invertebrate molluscs (Bilvalvia, Gastropoda)

Nonfeeding larvae of the gastropod Haliotis rufescens maintained a constant amount of taurine during embryonic and larval development and, since no de novo synthesis of taurine was observed in these larvae, the maternal endowment of taurine to the egg was sufficient for larval development to metamorphosis. In contrast, feeding larvae of the bivalve Crassostrea gigas increased their taurine content by a factor of 43 during growth to metamorphosis (from 86 to 311 µm, valve length). Taurine was not present in algae used to feed the larvae, suggesting that de novo synthesis of taurine by the larvae met their requirements. In unfed larvae, cysteic acid, cysteine sulfinic acid and hypotaurine were labeled from a [35S]cysteine precursor, but taurine was not. Hyperosmotic treatment (from 33 to 44 salinity for up to 3 h) did not induce taurine synthesis in unfed larvae. However, larvae fed the alga Isochrysis galbana up-regulated their taurine synthesis from [35S]cysteine by a factor of 11 (fed, 11.7±2.2 fmol taurine larva-1 h-1; unfed controls, 1.08±0.33 fmol taurine larva-1 h-1; means ± s.e.m.). Fed larvae also synthesized taurine from [35S]methionine (18.4 fmol larva-1 h-1). I. galbana contained 5 fmol cell-1 of cysteine and methionine (combined) and, based on known feeding rates, we calculated that there were sufficient taurine precursors in the algae to supply the taurine requirements of growing larvae. The lack of significant de novo taurine synthesis reported for adult bivalve molluscs has led to the conclusion that taurine is a dietary requirement. Our findings for larval forms differ in that there is significant de novo synthesis of taurine during development.

Journal Article↗
PubMedDate not supplied

A Kenyan experience.

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Acquired Immunodeficiency Syndrome↗