Relationship between Thermodynamic Driving Force and One-Way Fluxes in Reversible Chemical Reactions

Chemical reaction systems operating in nonequilibrium open-system states arise in a great number of contexts, including the study of living organisms, in which chemical reactions, in general, are far from equilibrium. Here we introduce a theorem that relates forward and re-verse fluxes and free energy for any chemical process operating in a steady state. This rela-tionship, which is a generalization of equilibrium conditions to the case of a chemical process occurring in a nonequilibrium steady state, provides a novel equivalent definition for chemical reaction free energy. In addition, it is shown that previously unrelated theories introduced by Ussing and Hodgkin and Huxley for transport of ions across membranes, Hill for catalytic cycle fluxes, and Crooks for entropy production in microscopically reversible systems, are united in a common framework based on this relationship.Comment: 11 page

Life histories of the copepods Pseudocalanus minutus, P. acuspes (Calanoida) and Oithona similis (Cyclopoida) in the Arctic Kongsfjorden (Svalbard)

The year-round variation in abundance and stage-specific (vertical) distribution of Pseudocalanus minutus and Oithona similis was studied in the Arctic Kongsfjorden, Svalbard. Maxima of vertically integrated abundance were found in November with 111,297 ind m−2 for P. minutus and 704,633 ind m−2 for O. similis. Minimum abundances comprised 1,088 ind m−2 and 4,483 ind m−2 in June for P. minutus and O. similis, respectively. The congener P. acuspes only occurred in low numbers (15–213 ind m−2), and successful reproduction was debatable. Reproduction of P. minutus took place in May/June, and stage distribution revealed a 1-year life cycle with copepodids CIII, CIV, and CV as the overwintering stages. Oithona similis exhibited two main reproductive peaks in June and August/September, respectively. Moreover, it reproduced more or less continuously throughout the whole year with all stages occurring during the entire sampling period, suggesting two generations per year. Both species migrated towards greater depth in November, but O. similis preferred to stay longer in the upper 100 m as compared to Pseudocalanus. The reproduction of the two species in Kongsfjorden seemed to be linked to phytoplankton dynamics

Claudins in renal physiology and disease

The tight junction forms the paracellular permeability barrier in all epithelia, including the renal tubule. Claudins are a family of tight junction membrane proteins with four transmembrane domains that form the paracellular pore and barrier. Their first extracellular domain appears to be important for determining selectivity. A number of claudin isoforms have been found to be important in renal tubule function, both in adults and in neonates. Familial hypomagnesemic hypercalciuria with nephrocalcinosis is an autosomal recessive syndrome characterized by impaired reabsorption of Mg and Ca in the thick ascending limb of Henle's loop. Mutations in claudin-16 and 19 can both cause this syndrome, but the pathophysiological mechanism remains controversial

Prostaglandin- and theophylline-induced Cl secretion in rat distal colon is inhibited by microtubule inhibitors

The aim of the present study was to examine the possible role of microtubules in chloride secretion by distal rat colon stimulated by prostaglandin (PGE 2 ) and theophylline. Distal colonic tissue from male rats was mounted in Ussing chambers, and short-circuit current (I sc ) was measured to assess chloride secretion. Three microtubule inhibitors, colchicine, nocodazole, and taxol, all inhibited the stimulated I sc and reduced the 60-min integrated secretory response to PGE 2 and theophylline (▪I sc dt) by 39–52%, whereas the inactive colchicine analog lumicolchicine did not. Atropine and tetrodotoxin had no effect on stimulated chloride secretion. To confirm the source of I sc , unidirectional 22 Na + and 36 Cl − fluxes were measured in tissues exposed to lumicolchicine (control) or colchicine. Control tissues absorbed both chloride [5.0 (1.1–8.6) (median and 95% confidence interval) μeq/cm 2 /hr] and sodium [2.8 (0.9–7.2) μeq/cm 2 /hr], and this net absorption was reduced by 96% and 79%, respectively, by treatment with PGE 2 and theophylline due to an increase in serosal-to-mucosal chloride and sodium movement. Colchicine-treated tissues exhibited similar net basal chloride and sodium absorption that was reduced by 71% and 75%, respectively, by treatment with PGE 2 and theophylline. Thus the PGE 2 - and theophylline-induced increase in chloride secretion was significantly reduced by colchicine ( P <0.05 by Wilcoxon rank-sum test), whereas colchicine had no effect on PGE 2 - and theophylline-induced changes in sodium fluxes. Furthermore, the colchinine-related changes in stimulated chloride secretion were numerically similar to colchicine-related changes in stimulated I sc . These findings indicate that microtubules are required for normal PGE 2 - and theophylline-induced chloride secretion in distal rat colon and suggest that induced chloride secretion may involve vesicular insertion of ion transporters into the plasma membrane or other microtubule-dependent regulatory processes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44414/1/10620_2005_Article_BF01299864.pd