REGULATION OF ELECTROGENIC NA+ TRANSPORT ACROSS LEECH SKIN

The dorsal integument of the medical leech Hirudo medicinalis exhibits a marked amiloride-sensitive Na+ absorption. With 20 mM Na+ in the apical solution, the transepithelial short-circuit current (I-sc) was similar to 40% higher than with 115 mM Na+, whereas the transepithelial potential (VT) with 20 mM Na+ was -35.7 +/- 4.5 and -20.6 +/- 2.6 mV with 115 mM Na+. Amiloride (100 mu M) inhibition at 20 mM apical Na+ was also significantly larger than with 115 mM Na+ in the solution. Benzamil (100 mu M) showed additional inhibition after amiloride. Large transient overshooting currents occurred only when 115 mM Na+ was added after some minutes of Na+-free apical solution. Addition of adenosine 3',5'-cyclic monophosphate (cAMP) to the serosal side in the presence of 115 mM apical Na+ nearly doubled I-sc. This cAMP effect was reduced to only 20% in the presence of 20 mM Na+. Guanosine 3',5'-cyclic monophosphate (cGMP) slightly increased I-sc, whereas ATP showed biphasic potency. Removal of calcium from the apical side resulted in a large stimulation of amiloride-sensitive I-sc only in the presence of 115 mM Na+. When currents were activated with cAMP, a deprivation of Ca2+ modestly reduced the amiloride-sensitive I-sc. The Na+ channel of leech integument was found highly selective for Na+ over other monovalent cations. The permeability ratio for Na+ over K+ was similar to 30:1; the selectivity relationship for the investigated cations was Na+ > Li+ > NH4+ > K+ approximate to Cs+ approximate to Rb+.