Abstract
We present a new invertebrate model for the study of epithelial sodium transport in tight epithelia, the earthworm integument. Dissected segments of earthworm integument were mounted in modified Ussing chambers and perfused with either pond water (PW) or earthworm ringer solution (ERS) on the apical side. In order to investigate ion transport under near-in vivo physiological conditions, measurements were performed under current-clamp conditions by monitoring the transepithelial potential (V-T), as well as the transepithelial resistance (RT). These were recorded continuously and the virtual short circuit current (ISC) was calculated. The integument has a high transepithelial resistance (R-T=9,037 +/-502 Omega cm(2) for PW, n=24, and 11,055 +/-1,320 Omega cm(2) for ERS, n=32). V-T was -3.7 +/-2.2 mV for PW (n=24) and -1.5 +/-1.0 mV for ERS (n= 32), and I-SC was -0.57 +/-0.30 mu A/cm(2) for PW (n=24) and - 0.44 +/-0.24 mu A/cm(2) for ERS (n=32). Only under PW, but not under ERS conditions, was there a pronounced inhibition of ISC by low doses of amiloride or its analogues phenamil and benzamil. The resistance of the paracellular pathway was found to be very high. The terrestrial oligochaete Lumbricus seems especially adapted to the environmental conditions because it has an ultra-tight integument and a very fast up- and down-regulation of apical Na+ channels.