How to overcome osmotic stress? Marine crabs conquer freshwater. New insights from modern electrophysiology

Abstract

In the present article we review our findings on split lamella preparations of crab gills mounted in modified Ussing-chambers with respect to mechanistic and ecophysiological aspects. The leaky gill epithelium of shore crabs adapted to brackish water absorbs Na + and C1-in a coupled mode, and shows similarities to other salt-absorbing epithelia exposed to moderately diluted media. The results so far obtained for NaCI uptake across the gills of the shore crab are compatible with a transport model where two cell types operate in parallel, one displaying cotransport-like NaC1 absorption, similar to that in the thick ascending limb of Henle's loop of the mammalian mephron, and the other one with characteristics of amiloride-sensitive, channel-mediated Na + uptake by frog skin. Although there is no clear evidence for the apical mechanisms in this model, it may serve as a good basis for more detailed studies in the future. The moderately tight gill epithelium of freshwater adapted Chinese crabs absorbs Na + and C1-independently from each other, and shows similarities to other salt-absorbing epithelia exposed to freshwater. The characteristics of a positive, Na +-dependent short-circuit current with externally C1--free saline indicate that active Na + uptake proceeds in a frog-skin-like mode via apical Na+-channels and the basolateral Na+/K+opump. The nature of a negative short-circuit current with external C1--saline indicates that active and Na +-independent CI-uptake is driven by an apical V-type H~--pump and proceeds via apical C1-/ HCO3--exchange and basolateral C1--channels.

Electrophysiology of posterior, NaCl-absorbing gills ofChasmagnathus granulatus: rapid responses to osmotic variations

“…On the other hand, the Gte increase at increased Isc after also reducing the osmolarity (but at constant ionic strength) indicates a change in the transcellular conductance. In many epithelia, including crab gills (Onken et al, 1991(Onken et al, , 1995, the apical membrane is, by far, the barrier of highest resistance along the transcellular pathway, and significant conductance changes are most likely due to modulations in this membrane. Thus, it seems likely that the Gte increase in the presence of Hyposmotic saline is related to the modulation of an apical, electrogenic transporter.…”

A Malaria Parasite-encoded Vacuolar H+-ATPase Is Targeted to the Host Erythrocyte

“…This channel has a high permeability to chloride ions, but it is not known whether it is encoded by the parasite genome (10). Interestingly, vacuolar H ϩ -ATPases (V-H ϩ -ATPases) 3 are associated with chloride channels in the plasma membrane of numerous cells (11)(12)(13)(14)(15), and some stages of malaria parasites selectively invade V-H ϩ -ATPase-containing cells (16). Normal erythrocytes, however, do not possess a V-H ϩ -ATPase.…”

The carbonic anhydrase of the Chinese crabEriocheir sinensis: Effects of adaption from tap to salt water

“…Due to the necessity of inward C1-transport in tap water, the increase of CA activity seems to be a response to the external low C1-concentration. But here some contradictions arise: In E. sinensis, the results derived from electrophysiological and biochemical investigations indicated that electrogenic C1-transport depends on the cooperation of a proton pump and the CA (Onken et al, 1995). In contrast, experiments on C. maenas concerning potential difference, fluxes of Na + and C1- (B6ttcher et al, 1991) and short circuit current experiments (Onken & Siebers, 1992) do not reveal any participation of CA activity in Na+/C1 -transport.…”