Amiloride-sensitive sodium absorption is different in vertebrates and invertebrates

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 divatent 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 IENaTP (leech epithelial Na' transporting protein).