Effect of hypertonic urea and mannitol on distal nephron permeability

Calcein labelling and electrophysiology: insights on coral tissue permeability and calcification

The mechanisms behind the transfer of molecules from the surrounding sea water to the site of coral calcification are not well understood, but are critical for understanding how coral reefs are formed. We conducted experiments with the fluorescent dye calcein, which binds to calcium and is incorporated into growing calcium carbonate crystals, to determine the permeability properties of coral cells and tissues to this molecule, and to determine how it is incorporated into the coral skeleton. We also compared rates of calcein incorporation with rates of calcification measured by the alkalinity anomaly technique. Finally, by an electrophysiological approach, we investigated the electrical resistance of coral tissues in order to better understand the role of tissues in ionic permeability. Our results show that (i) calcein passes through coral tissues by a paracellular pathway, (ii) intercellular junctions control and restrict the diffusion of molecules, (iii) intercellular junctions should have pores of a size higher than 13 Å and lower than 20 nm, and (iv) the resistance of the tissues owing to paracellular junctions has a value of 477 ± 21 Ohm cm(2). We discuss the implication of our results for the transport of calcium involved in the calcification process.

Pathways for water absorption and physiological role of the lateral interspaces in the kidney tubule

Possible routes for water and salt flow and the most likely theories that describe coupling between water and salt flow across leaky epithelia are presented. The osmotic theories seem the most likely ones. However, several of the theories have weaknesses that render them unsatisfactory, in particular because of the possibility of paracellular water flow in these epithelia. Puzzling are the findings that measurements of the cellular water osmotic permeability give figures that are too low for some of the exclusively transcellular theories to work. If these observations hold in the future, it may be shown that part of the water moves through paracellular pathways in these leaky epithelia. This view is supported by the observation that large extracellular markers are dragged by volume flow. Finally, experimental evidence is reviewed indicating that changes in the luminal area concentration may modulate the functional state of the nephron junctional complexes.

The effects of extracellular potassium, ouabain, and prostaglandins on intracellular potassium activity in sheep cardica Purkinje fibers

(1) Intracellular K activity (aKi) of sheep heart Purkinje fibers was measured using K-selective microelectrodes (liquid ion exchanger). aKi in the resting state with an extracellular K of 4 mmol . l-1 EK was 112.9 +/- 6.1 mmol . l-1 (n = 47) for a membrane potential (VM) of -73.3 +/- 0.9 mV. VM deviated from the calculated potassium equilibrium potential (EK = -93mV). (2) When extracellular K was decreased to 2 mmol . l-1 or increased to 6 and 10 mmol . l-1 EK changed from -114 to -84 and -73 mV, with little change in aKi. (3) aKi and VM significantly decreased after administration of 10(-6) mol . l-1 ouabain. (4) Prostaglandins (PGI2 10-100 micrograms . l-1 and PGE2 0.01-1 microgram . l-1) decreased aKi without greatly changing VM. The differences between VM and EK became smaller. These effects indicate an increase in K permeability and may explain the antiarrhythmic action of prostaglandins.

Membrane permeability during low potassium depolarization in sheep cardiac Purkinje fibers

Exposure of sheep Purkinje fibers to low [K]o leads to marked depolarization to a stable potential of about -40 mV. This level is equivalent to the plateau of the Purkinje fiber action potential. The low [K]o depolarization could be prevented by removal of [Na]o and was modified by tetrodotoxin. The membrane potential in the depolarized state was unresponsive to changes in [Cl]o or [Ca]o and it was poorly responsive to changes in [K]o between 0 and 2 mM. Repolarization was induced by decrease in [Na]o with a slope response of 30 mV/10-fold change in [Na]o. Average internal K activity (aK) in the resting state with a [K]o of 5 mM was 121.4 mM for a membrane potential of -80 mV. During low K depolarization aK was 119.7 mM with a membrane potential of -34 mV. The depolarization was therefore due to a change in membrane permeability, with little change in aK. Upon restoration of [K]o the fiber repolarized to values transiently more negative than the prior resting potential. These transient potentials were more negative than the K equilibrium potential (VK), if it is calculated assuming a uniform [K]o. The hyperpolarization was reduced by ouabain [10(-6)] or by low [Ca]o.