title: Amiloride-sensitive sodium absorption is different in vertebrates and invertebrates.
abstract: Amiloride-sensitive Na+ absorption is a well-described feature of numerous transporting epithelia in vertebrates. Yet, very little is known about this important physiological process regarding invertebrates. In the present paper, we compare vertebrate Na+ absorption mediated by the amiloride-sensitive epithelial Na+ channel (ENaC) and its invertebrate counterpart. We used the dorsal skin of the annelid Hirudo medicinalis as a model for the Na+ absorption of invertebrate epithelia. In applying electrophysiological, molecular, and biochemical techniques we found striking functional and structural differences between vertebrate and invertebrate amiloride-sensitive Na+ absorption. Using modified Ussing chambers, we analyzed the influence of different known blockers and effectors of vertebrate ENaC on leech epithelial Na+ absorption. We demonstrate that the serine protease trypsin had no effect on the Na+ transport across leech integument, while it strongly activates vertebrate ENaC. While protons, and the divalent cations Ni2+ and Zn2+ stimulate vertebrate ENaC, amiloride-sensitive Na+ currents in leech integument were substantially reduced. For molecular studies, we constructed a cDNA library of Hirudo medicinalis and screened it with specific ENaC antibodies. We performed numerous PCR approaches using a vast number of different degenerated and specific ENaC primers to identify ENaC-like structures. Yet, both strategies did not reveal any ENaC-like sequence in leech integument. From these data we conclude that amiloride-sensitive Na+ absorption in leech skin is not mediated by an ENaC-like Na+ channel but by a still unknown invertebrate member of the ENaC/DEG family that we termed lENaTP (leech epithelial Na+ transporting protein).
authors.affiliation: Institute of Animal Physiology, Westphalian Wilhelms-University Muenster, Hindenburgplatz 55, D-48143 Muenster, Germany.
authors.forename: Katja
authors.initials: K
authors.lastname: Sobczak
authors.forename: Anne
authors.initials: A
authors.lastname: Willing
authors.forename: Kristina
authors.initials: K
authors.lastname: Kusche
authors.forename: Nadine
authors.initials: N
authors.lastname: Bangel
authors.forename: Wolf-Michael
authors.initials: WM
authors.lastname: Weber
mesh_terms: D000584:Amiloride / Q000008:administration & dosage*; D000818:Animals; D053503:Epithelial Sodium Channels / Q000187:drug effects / Q000502:physiology*; D047709:Hirudo medicinalis / Q000187:drug effects / Q000502:physiology*; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D012869:Skin Absorption / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000493:pharmacokinetics*; D013045:Species Specificity; D014714:Vertebrates / Q000502:physiology*
chemical_list: D053503:Epithelial Sodium Channels; D000584:Amiloride; D012964:Sodium