ELECTROGENIC CATION-TRANSPORT ACROSS LEECH CECAL EPITHELIUM

Electrogenic cation transport across the caecal epithelium of the leec h Hirudo medicinalis was investigated using modified Ussing chambers. Transepithelial resistance (R(T)) and potential difference (V-T) were 61.0 +/- 3.5 Omega . cm(2) and -1.1 +/- 0.2 mV (n = 149), respectively , indicating that leech caecal epithelium is a ''leaky'' epithelium Un der control conditions short circuit current `(I-SC) and transepitheli al Na+ transport rate (I-Na) averaged at 22.1 +/- 1.5 mu A . cm(-2) an d 49.7 +/- 2.6 mu A . cm(-2), respectively. Mucosal application of ami loride (100 mu mol . l(-1)) or benzamil (50 mu mol . l(-1)) influenced neither I-SC nor I-Na. The transport system in the apical membrane sh owed no pronounced cation selectivity and a linear dependence on mucos al Na+ concentration. Removal of mucosal Ca2+ increased I-SC by about 50% due to an increase of transepithelial Na+ transport. Trivalent cat ions (La3+ and Tb3+, 1 mmol . l(-1) both) added to the mucosal Ringer solution reduced I-Na by more than 40%. Serosal ouabain (1 mmol . l(-1 )) almost halved I-SC and I-Na while 0.1% (= 5.4 mmol . l(-1)) DNP dec reased I-Na to 11.8 +/- 5.1% of initial values. Serosal addition of cA MP increased both I-SC and I-Na whereas the neurotransmitters FMRFamid e, acetylcholine, GABA, L-dopa, serotonin and dopamine failed to show any effects; octopamine, glycine and L-glutamate reduced I-Na markedly . On the basis of these results we conclude that in leech caecal epith elium apical uptake of monovalent cations is mediated by non-selective cation conductances which are sensitive to extracellular Ca2+ but ins ensitive to amiloride. Basolaterally Na+ is extruded via ouabain-sensi tive and -insensitive ATPases. cAMP activates Na+ transport across lee ch caecal epithelium, although the physiological stimulus for cAMP-pro duction remains unknown.