Regulation of Na(+) transport across leech skin by peptide hormones and neurotransmitters

An increase in intracellular cyclic AMP concentration stimulates transepithelial Na(+) transport across the skin of the leech Hirudo medicinalis, but it is unclear how cytosolic cyclic AMP levels are elevated in vivo. In search of this external stimulus, we performed Ussing chamber experiments to test several peptide hormones and neurotransmitters for their effect on Na(+) transport across leech dorsal integument. Although all the peptide hormones under investigation significantly affected ion transport across leech integument, none of them mimicked the effect of an experimental rise in intracellular cyclic AMP level. The invertebrate peptides conopressin and angiotensin II amide inhibited short-circuit-current- (I(sc)) and amiloride-sensitive Na(+) transport (I(amil)), although to slightly different degrees. The vertebrate peptide hormones 8-arginine-vasopressin and 8-lysine-vasopressin both produced an inhibition of I(amil) comparable with that caused by angiotensin II amide. However, 8-lysine-vasopressin reduced I(sc), whereas 8-arginine-vasopressin induced a moderate increase in I(sc). The neurotransmitter dopamine, which occurs in the leech central nervous system in relatively large amounts, and its precursor l-dopamine both induced large decreases in I(sc) and I(amil). However, the reactions evoked by the catecholamines showed no pronounced similarity to the effects of intracellular cyclic AMP. Two other neurotransmitters known to occur in leeches, serotonin (5-hydroxytryptamine) and gamma-n-aminobutyric acid (GABA), had no influence on transepithelial ion transport in leech skin.