Neuroendocrine regulation of salt and water balance in the grapsid crab Metopograpsus messor (Forskal)

Abstract

Metopograpsus messor regulates both hypo- and hyperosmotically in sea water concentrations ranging from 25% to 125%. Within this range of salinities, the blood osmotic concentration varies by only 14%. The urine of animals in all sea water concentrations is maintained slightly hyperosmotic to the blood; however, the urine/blood osmotic concentration ratios do not exceed 1.07. Thus, it is concluded that the antennal glands do not serve in an osmoregulatory function. In 25% sea water, the blood and urine osmotic concentrations of bilaterally eyestalk-ligated animals are lower than that of the normal controls. Eyestalk homogenate injections into ligated animals result in partial replacement of the blood osmotic concentration to the normal value. The lower blood osmotic concentration of the ligated animal is correlated with greater than normal water influx. It is suggested that bilateral eyestalk ligations result in the isolation of an eyestalk component (neural or humoral) which inhibits the ventral ganglionic mass. The ventral ganglionic mass contains a substance which increases the permeability of the animal to '"later. The urine and blood sodium concentrations of both ligated and normal animals reach new "steady states" within 6 hours after exposure to 25% sea water. Blood and urine sodium concentrations are affected by bilateral eyestalk ligations. Ligated animals maintain a sodium "steady state" lower than the normal animals. The effect of eyestalk ligation on active sodium transport is demonstrated by sodium-22 uptake studies. A decreased sodium uptake after eyestalk ligation is restored to normal by the injection of a brain homogenate. Sodium-22 uptake is unaffected by eyestalk or ventral ganglionic homogenate injections. The sodium transport system of the crab's gills is also investigated with reference to adenosine triphosphatase (ATPase) activity. The gill ATPase is Na-K activated, Mg-dependent, and ouabain sensitive. Optimal enzyme activity is obtained with a substrate concentration of 4 mM ATP. The reaction is incubated for 15 minutes at 40 CO in the presence of sodium and potassium in a ratio of 100/20 mM. Magnesium saturation is obtained at a concentration of 4 mM MgCl2. The enzyme activity increases proportionately with increasing osmotic stress conditions. In 25% sea water, the ATPase activity increases within the first 6 hours, but declines to the normal level after 12 hours. Bilateral eyestalk ligations significantly decrease the enzyme activities of animals subjected to 25% sea water for 6 hours. Brain homogenate injections into ligated animals restore the ATPase activity to that found in the gills of normal animals. Thus, it is concluded that a neuroendocrine factor from the brain regulates the active transport of sodium by affecting the enzyme involved in active sodium transport.