engleski

Characterization of ion transport in the isolated epipodite of the lobster Homarus americanus

Unfolded epipodite isolated from American lobsters (Homarus americanus) acclimated to dilute seawater was mounted in an Ussing-type chamber for ion transport studies. The split epipodite is an electrically polarized, one-cell-layer epithelium supported with cuticle. Under open-circuit conditions, the transepithelial potential was -4.2 +/- 1.0 mV (N=38). In the short-circuited epithelium, the current averaged over all of the preparations was -185.4 +/- 20.2 A cm(-2) (N=38) with a high conductance of 55.2 +/- 11.4 mS cm(-2) (N=38), typical for a leaky epithelium. The Na:Cl absorptive flux ratio was 1:1.6 ; ion substitution experiments indicated that the transport of Na(+) and Cl(-) is coupled. Basolateral application of the Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino) benzoate (NPPB) and niflumic acid (NFA) dose-dependently inhibited short-circuit current (I(SC)). Secretory K(+) (Rb(+)) fluxes exceeded influxes and were inhibited by the Na(+)/K(+)-ATPase inhibitor ouabain and the K(+) channel blocker cesium. Western blot analysis showed that Na(+)/K(+)-ATPase alpha-subunit protein was more highly expressed in the epipodite of lobsters acclimated to 20 p.p.t. compared with animals acclimated to seawater (34. p.p.t.). 3-Isobutyl-1-methyl-xanthine (IBMX) stimulated a negative I(SC) and enhanced apical secretory K(+) flux. Basolateral application of NPPB inhibited J(B -> A)(Rb) fluxes, suggesting the interaction of K(+) channels with NPPB-sensitive Cl(-) channels. The results are summarized in a transport model, suggesting apical Na(+)/K(+)/2Cl(-) co-transport, a dominant apical K(+)-secreting channel and basolaterally located Cl(-) and K(+) channels. This study represents the first comprehensive characterization of ion transport processes across the lobster epipodite epithelium and indeed in any tissue within the branchial cavity of the American lobster.

crustacean ; epithelial transport ; chloride channel ; potassium channel ; Na ; K-ATPase

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