Effect of oxytocin on transepithelial transport of water and Na+ in distinct ventral regions of frog skin (Rana catesbeiana)

Thoracic, abdominal, and pelvic fragments of ventral skin of Rana catesbeiana were analysed regarding the effect of oxytocin on: (1) transepithelial water transport; (2) short-circuit current; (3) skin conductance and electrical potential difference; (4) Na+ conductance, the electromotive force of the Na+ transport mechanism, and shunt conductance; (5) short-circuit current responses to fast Na+ by K+ replacement in the outer compartment, and (6) epithelial microstructure. Unstimulated water and Na+ permeabilities were low along the ventral skin. Hydrosmotic and natriferic responses to oxytocin increased from thorax to pelvis. Unstimulated Na+ conductance was greater in pelvis than in abdomen, the other electrical parameters being essentially similar in both skin fragments. Contribution of shunt conductance to total skin conductance was higher in abdominal than in pelvic skin. Oxytocininduced increases of total skin conductance, Na+ conductance, and shunt conductance in pelvis were significantly larger than in abdomen. An oscillatory behaviour of the short-circuit current was observed only in oxytocin-treated pelvic skins. Decrease of epithelial thickness and increase of mitochondria-rich cell number were observed from thorax to pelvis. Oxytocin-induced increases of interspaces were more conspicuous in pelvis and abdomen than in thorax.

Effect of crude extract of roots of Bredemeyera floribunda Willd. I. Effect on arterial blood pressure and renal excretion in the rat

The infusion of the dried roots of Bredemeyera floribunda Willd. is used in Brazilian popular medicine as a potent diuretic, especially in the treatment of hypertension and nephrolithiasis (renal calculi). Intravenous administration of crude root-extract (20-80 mg/kg) to anesthetized rats induces clear dose-dependent and reversible hypotensive responses. At higher doses the extract leads to bradycardia and death. In doses that do not alter the arterial blood pressure, the extract elicited immediate and dose-dependent reversible increase of water, sodium, and potassium renal excretion. The results, apart from indicating that the renal effect of the extract is not due to its systemic hypotensive action, support the folk therapeutic use of the infusion of the root-extract as a diuretic.

Effect of crude extract of roots of Bredemeyera floribunda Willd. II. Effect on glomerular filtration rate and renal tubular function of rats

The effect of crude extract of roots of Bredemeyera floribunda Willd., Polygalaceae used by Brazilian popular medicine as a potent diuretic, on renal function in antidiuresis or water diuresis in rats, was studied. During intravenous infusion of the extract (0.05 mg/min/100 g), mean arterial pressure did not change significantly but urine flow, glomerular filtration rate, fractional water and sodium excretion and solute clearance increased significantly, in both groups of animals. In antidiuresis rats the extract significantly increased reabsorption of water by the collecting duct and in water diuresis animals the extract significantly increased free water clearance. Our findings indicate a direct effect of extract on glomerular filtration rate, possibly by detergent like interactions with structural components of glomerular membranes and/or by decreasing renal perfusion pressure. The study on the concentrating and diluting mechanisms suggests preferential action of extract in the proximal tubular cells, possibly on the (Na(+)-K+)-ATPase countertransport system and/or on other proteins components of tubular cell membranes involved with salt transport mechanisms.

Effect of crude extract of roots of Bredemeyera floribunda Willd. III. Effect on hormone-stimulated water transport in isolated frog skin

The diuretic activity of the crude extract of roots of Bredemeyera floribunda Willd. (Polygalaceae) was investigated on basal and hormone-stimulated absorptive water transport in the isolated frog skin. The extract, added to the mucosal skin surface, at the peak of oxytocin (Oxy) action induces steady and dose-dependent reduction of water transport. Pre-treatment of the skins during 2 h with the extract, added to mucosal skin surface, renders the epithelium non-responsive to oxytocin. These effects are not observed when the extract is added to the serosal skin surface. Basal water transport is not altered by the extract. The results, apart from indicating inhibition of the oxytocin-stimulated water transport in frog skin epithelium, support the folk therapeutic use of infusion of the root extract as a diuretic.

Control of sodium permeability of the outer barrier in toad skin

The 24Na efflux (JNaeff) (i.e., the rate of appearance of 24Na in the outer compartment) in the isolated short-circuited toad skin bathed by NaCl-Ringer's solution on both sides is composed of para- and transcellular components of almost equal magnitudes. This relies on the assumption that amiloride acts on the transcellular component only and could block it completely. Ouabain induces a large transient increase of the transcellular component. This increase, which starts within a few minutes after the addition of ouabain, is due to electrical depolarization of the outer barrier, rather than a consequence of blocking Na recirculation across the inner barrier. The subsequent decline of JNaeff, which takes place after the ouabain-induced JNaeff peak, is due to a progressive block of outer barrier Na channels with time, which can eventually be complete, depending on the duration of action of ouabain. As the external Na concentration was always kept high and constant in these experiments, the results indicate that a rise in cell Na concentration, and not in the outer bathing solution, is the signal that triggers the reduction of outer barrier Na permeability (PNao). Ouabain has no effect upon JNaeff with Na-free solution bathing the outer and NaCl-Ringer's solution the inner skin surface, showing the importance of Na penetration across the outer barrier, and not across the inner barrier due to its low Na permeability, in the process of closing the Na channels of this structure. Step changes from Na 115 mM to Na-free external solution, or vice-versa, may affect both the outer barrier electrical potential difference (PDo) and cell Na concentration (Na)c. Therefore, the behavior of JNaeff depends on which variable (if PDo or (Na)c regulated outer barrier Na permeability) is most affected by step changes in outer bathing solution Na concentration. Amiloride in the control condition blocks the transcellular component of JNaeff. However, in the condition of approximate short-circuiting of the outer barrier and high cellular Na concentration induced by long term effects of ouabain, when the Na channels of the outer barrier are already blocked by elevated cell Na concentration, amiloride may induce the opposite effect, increasing Na permeability of the outer barrier. With outer barrier Na channels completely blocked by high cell Na concentration, PCMB in the outer bathing medium induces a large increase of JNaeff, rendering these channels again amiloride sensitive. The results are consistent with the notion that Na efflux from cell compartment to the outer bathing solution goes through the amiloride-sensitive Na channels of the apical border of the superficial cell layer of toad skin, with an apparent Na permeability modulated by cell ionic environment, most probably the cell Na concentration.