Managing High Salt Loads: From Neuron to Urine in the Leech

Abstract

In a single meal, the medicinal leech, Hirudo medicinalis, ingests up to 10 times its own body weight. Extra salt and water are eliminated rapidly by 17pairs of nephridia and bladders. The nephridia of jawed leeches are secretory organs. The primary urine-forming cells resemble a Cl⁻-secreting epithelium and secrete hypertonic K⁺-enriched fluid into their interconnected lumina. In the course of final urine formation, 85% of Na⁺ and 97% of K⁺ is taken out. After a blood meal, the combination of more primary urine of a higher Na⁺/K⁺ ratio and a decrease of NaCl resorption leads to an 80-fold increase of NaCl output. As discussed here, the ability to control primary and final urine formation separately makes secretory organs preferable to filtration kidneys when dealing with high salt loads. In Hirudo, salt excretion is under nervous control. Sensory and neurosecretory innervation of the nephridia is accomplished by a single set of neurons, the peripheral nephridial nerve cells (NNCs). Their electrical activity depends on the extracellular Cl⁻ concentration, which changes under different physiological conditions. At normal, low extracellular Cl⁻ concentration, the NNC is spontaneously active and releases its endogenouspeptide, FMRF-NH2,, in the nephridium. FMRF-NH₂ enhances NaCl resorption. An increase of extracellular Cl⁻ concentration, as after a blood meal or after the invasion of brackish water, inhibits the NNC's discharge and, hence, peptide release. Less NaCl is resorbed, which results in a strong natriuresis. This local feedback regulation is compared to the afferent and efferent control mechanisms of maintaining salt homeostasis in other species.