Inhibition of water intake by ouabain administration in sheep

Rat organum vasculosum laminae terminalis in vitro: responses to changes in sodium concentration

Extracellular action potentials were recorded from the organum vasculosum laminae terminalis (OVLT) in rat brain slice preparations; the effect of different concentrations of NaCl on spontaneous firing frequency was studied. From 72 neurons, 67 (93%) were responsive to various perfusion media, while 5 neurons (7%) were not responsive. The change from the standard medium (SM; 124 mM NaCl) to 99 mM, decreased the firing frequency in 24 (65%) and increased it in 13 (35%) out of 37 responsive cells. The change from the SM to 149 mM evoked an increase in the firing rate in 33 (73%) and a decrease in 12 (27%) of 45 responsive neurons; the change to 174 mM, increased the firing rate in 5 (100%) neurons tested. The excitatory effect of increasing [NaCl] in the perfusion medium persisted even in low Ca2+ and high Mg2+ medium. Mannitol (55 mM) added to the SM increased the firing rate of cells; no significant decrease in the firing rate was seen with sodium mannitol 99/55 mM. Ouabain (OUA) (0.1 x 10(-3) mM) added to the SM increased the firing rate in 16 (84%) and decreased it in 3 (16%) out of 19 cells. Diphenylhydantoin (DPH) (1 mM) added to the SM decreased the firing rate in 12 (67%) and increased it in 6 (33%) of 18 cells tested. Hypo- or hypertonic NaCl solutions had no consistent effect on the spontaneous activity of 14 pyramidal cells recorded from the hippocampus (area CA3). The results emphasize the importance of intracellular Na content as a physiological trigger regulating the activity in neurons of the OVLT.

Cerebral sodium sensors in the sodium-deplete sheep

The sodium intake of sodium deplete sheep was studied during local, push-pull perfusion of different solutions within the third cerebral ventricle. Sheep were made sodium deplete by continuous loss of parotid saliva, and were allowed access to 0.6 M NaHCO3 solution for 2 h daily. Local perfusion within the third cerebral ventricle was performed before and during the access to sodium solution. Four perfusion sites were used: anterior dorsal and ventral, and posterior dorsal and ventral. Perfusion of 200 mM Na-csf caused a decrease in sodium intake at each perfusion site. Perfusion of ouabain, 10(-6) M, caused a reduction in sodium intake only during perfusions within the anterior portion of the third ventricle. The results may indicate that specific neuronal elements sensitive to changes in intracellular sodium concentration are located around the anterior portion of the third cerebral ventricle. These neurones, however, are not exclusive sites from where sodium intake of sodium deplete sheep can be influenced.

Central effect of agents which alter sodium transport on water drinking in the rat

The central effect of ouabain (OUA), ethacrynic acid (EA) and diphenylhydantoin (DPH) on water drinking was studied in rats. OUA and EA inhibit cellular Na efflux, increasing intracellular (IC) Na content, while DPH inhibits cellular Na influx lowering IC sodium content. The animals were injected into the third ventricle (3V), fourth ventricle (4V), lateral ventricle (LV), or lateral hypothalamus (LH). Ouabain (50 pg) and EA (50 ng) when injected into the 3V or LV significantly decreased the water intake induced by 24 hr water deprivation. No effect was observed after injections in other loci, thus supporting the hypothesis that sensors could be located in the circumventricular organs, in an area without the blood brain barrier. DPH (270 ng) injected into the 3V or LV significantly enhanced water intake in rats deprived of water for 14 hr, but it did not increase water intake in water-repleted rats. DPH also enhanced water intake induced by cellular-dehydration (2 M NaCl IP). These results suggest that changes in the Na content of sensor cells located on or near the walls of the 3V could be responsible for the movement of water between the cells and their medium, in agreement with the osmoreceptor hypothesis for thirst.

Effect of agents which alter the Na transport on the sodium appetite in rats

The effect of ouabain (OUA), ethacrinic acid (EA) and diphenylhydantoin (DPH) on sodium appetite was evaluated in sodium-depleted rats. These animals were injected with either OUA (50 pg) or EA (150 ng) into the third ventricle (3V), fourth ventricle (4V), lateral ventricle (LV), or lateral hypothalamus (LH). Decreased Na appetite was observed only after injections either into the 3V or LV. DPH injection (270 ng) enhanced sodium appetite in mildly depleted rats. These effects seem to be specific for Na appetite since EA and OUA injections into the 3V did not alter food intake in food-deprived rats. The drugs did not impair gustatory inputs since the water-glucose preference was not altered after OUA, EA or DPH administration. The decrease in Na appetite induced by a 3-hour infusion of OUA (1 microliter/hr) was reversed by a 3-hour infusion of DPH (1 microliter/hr), while a pretreatment with DPH prevented the inhibitory effect of OUA. The data show that agents which inhibit cellular Na efflux such as OUA and EA decreased sodium appetite while DPH, which inhibits cellular Na influx, induced the opposite effect, i.e., enhanced Na appetite. The results are consistent with the hypothesis that the Na content of sensor cells located on or near the walls of the 3V could be the signal for the central control of Na appetite.

Centrally mediated effect of vanadate on diuresis and natriuresis in normal rats. Interaction with ouabain

Prolonged infusion of vanadate (VO-3) (51 ng/microliter/hr during 15 hr), an in vitro inhibitor of Na,K-ATPase activity, into the 3rd brain ventricle (3BV) increased diuresis, sodium and potassium excretion in normal rats (p less than 0.001). Since the animals did not receive food or water during the infusion period, the effect of VO-3 was not related to hydration or volume expansion. A single injection of ouabain (a specific inhibitor of the sodium pump) 1 microliter, 50 pg/microliter, followed by a single injection of vanadate (1 microliter/51 ng/microliter) 1, 5 or 10 minutes after ouabain, prevented the diuretic and natriuretic effect of VO-3. When vanadate injections were delayed 20 or 30 minutes, diuresis reappeared. Ouabain (50 pg) decreased sodium intake induced by sodium depletion, an effect that lasted for 10 to 15 minutes after injection, a time coincident with the inhibition of the natriuretic effect of VO-3 by OUA. Then the life-span of ouabain effect on prevention of VO-3-induced diuresis as well as on inhibition of sodium intake, lasted between 10 to 15 minutes. These results provide further evidence for the powerful diuretic, natriuretic and kaliuretic effect of centrally injected vanadate. However, the finding that ouabain suppresses this effect, does not support the suggestion that the active transport system might be involved.

Effect of intracerebroventricular vanadate administration on salt and water intake and excretion in the rat

The effect of vanadate (VO-3), an "in vitro" inhibitor of Na,K-ATPase activity, on sodium and water intake and excretion of Na-depleted and water deprived rats, was investigated. Injection of sodium orthovanadate Na3V04, H20 14) 1 microliter, 1.0 mM solution, 51 ng/microliter free base vanadium (V) into the 3rd brain ventricle (3BV) inhibited by 34% the sodium intake induced by peritoneal dialysis (PD). Urinary water and sodium excretion increased and potassium excretion decreased. The same concentration of vanadate administered by continuous infusion into the 3BV (1 microliter/hr, 24 hr, 51 ng/microliter, 1.2 micrograms/24 hr) during 24 hours after PD, decreased sodium intake by 69%. The same rate of infusion through the jugular vein failed to inhibit sodium intake or to increase urinary water and sodium excretion. Injections into lateral hypothalamus were also ineffective. Vanadyl (VO+2), the reduced form of vanadate, did not affect sodium intake. Similar or larger doses of vanadate injected into the 3BV of water deprived rats, did not modify water intake significantly. The present results suggest that the Na-K, active transport system is involved in salt and water balance regulation at the central nervous system level.