OSMOTIC AND IONIC REGULATION IN LUMBRICUS TERRESTRIS L

¹ Department of Zoology, Oregon State University, Corvallis, Oregon 97331

1. Earthworms maintain a hyperionic steady state while living in a dilute solution containing Na⁺, K⁺, Ca⁺², Cl^- and HCO₃^-(PW).

2. The inside (coelomic fluid) of worms is electronegative by 12 to 30 mV relative to the bath (PW).

3. Sodium and chloride are transported across the skin against an electrochemical gradient. Each ion may be transported independently of the other presumably in exchange for an endogenous ion of like charge.

4. For animals in PW, Cl^- is exchanged at a rate of 0.9 µeq/10g-hr and Na⁺ at a rate of 0.3 µeq/10g-hr. The influx of Na⁺ is dependent on Na⁺ concentration in the bath and displays saturation kinetics (V_max= 1 µeq/10 g-hr; K_s = 1.3 mMoles/l). Chloride kinetics were not analyzed.

5. Salt depletion increases the influx of Na⁺ and C1^- causing a net uptake of each ion.

6. Inulin and dextran are cleared from the coelomic fluid of PW-adapted worms at a rate of 75 µl/10 g-hr. Excretion of these compounds is probably through nephridia so that this is a maximum estimate of the rate of urine production. Water is also lost as a dilute rectal fluid which is formed at a rate of about 20 µl/10 g-hr.

7. Drinking of the bath occurs at a slow rate (4 µl/10g-hr). The gut does not play a significant role in ion uptake from the bath but may be involved in the elimination of water. Possible mechanisms are discussed.

8. When worms are transferred from moist soil to PW they experience a net uptake and retention of water which dilutes their body fluids. Subsequently Na⁺ and Cl^-influxes are elevated resulting in the net accumulation of Na⁺ and Cl^-and, within seven days, the restoration of the original (soil) concentrations in the coelomic fluid (75 mM Na⁺/l, 47 mM Cl^-l).

9. Several lines of evidence indicate that hydromineral metabolism in worms is delicately controlled and involves the integrated activity of several organs. These control systems are discussed.