Atrial natriuretic peptide inhibited the natriferic and hydrosmotic effects of arginine vasotocin on toad skin.

The effect of rat atrial natriuretic peptide (rANP) on hormonal stimulated osmotic water permeability (Jw, hydrosmotic effect) and net ion transport (short-circuit current, SCC, natriferic effect) was studied on toad skin, a tissue with functional similarities to the mammalian distal nephron, in order to assess actions on transport mechanisms. Rat atrial natriuretic peptide, rANP-99-126 (rANP) inhibited stimulated SCC and Jw to submaximal concentrations of arginine vasotocin (AVT) at a site before cyclic AMP generation. The angiotensin-converting enzyme inhibitor (ACEI) MK-422 did not modify the inhibitory effect of ANP in the stimulated Jw.

Effects of angiotensin I of the American bullfrog Rana catesbeiana on amphibian tissues.

1. The effect of bullfrog angiotensin I [Asp1, Val5, Asn9] angiotensin I, (AT I) on short-circuit current (SCC) on isolated toad skin and aorta contractility was examined. 2. AT I increased SCC in toad skin, the effect was partially inhibited by angiotensin-converting enzyme inhibitor (ACEI) teprotide. 3. AT I induced contractile responses in isolated rings of toad aorta. This effect was partially inhibited by captopril and completely blocked by the peptide antagonist [Sar1, Ile8] angiotensin II. 4. Present results indicate that this homologue AT I would act in amphibian tissues by conversion to AT II.

Effect of rat cardionatrin I (rat ANF 99-126) on the response of toad skin to angiotensin II.

The atrial natriuretic peptide cardionatrin I (cardionatrin I is ANF 99-126) was used in studies directed to assess its effects on osmotic water permeability (Posm) and short-circuit current (SCC) in isolated toad skin. Results showed that ANF 99-126 (10(-7) M) added to the dermal side of the skin had no effect on basal Posm or SCC. However, ANF 99-126 (3.3 x 10(-8) M) was able to produce a 50% reversible inhibition of the maximal Posm response to angiotensin II (AII) (3.2 x 10(-8) M). These effects were seen when the skins were preincubated with ANF 99-126 for 10 min or less before the addition of AII. Longer preincubation appeared to inactivate ANF 99-126 through proteolysis. ANF 99-126(10(-7) M) failed to inhibit the SCC response to AII (10(-5) M) in toad skin. These results are compatible with a modulatory function for ANF on several systems including those involved in the regulation of extracellular fluid volume.

Trypsin affects basal and stimulated osmotic water permeability in isolated toad skin.

1. We investigated the effect of trypsin (Tryp) on basal, stimulated and fluphenazine (FPZ)-inhibited net water flow (Jw) through isolated toad skin (Bufo arenarum). 2. Epidermal Tryp (20 min) promoted an increase in basal Jw which was dose-dependent (maximal with 0.5 mg/ml) and was prevented by a Tryp inhibitor (SBTI). 3. Tryp treatment inhibited the subsequent response to substances known to act before (oxytocin, Oxy) or after cyclic AMP (cAMP) generation (theophylline). 4. Tryp-induced Jw was not additive with the maximal response to Oxy or theophylline and did not modify FPZ's inhibitory effect on stimulated Jw. 5. Dermal Tryp (0.5 mg/ml, 20 min) did not modify basal, but inhibited Oxy and isoproterenol-stimulated Jw, without altering the response to theophylline or db-cAMP. 6. Collectively, our results show a differential action for epidermal and dermal Tryp. Tryp's side-selective action enables its use as a pharmacological tool in the functional dissection of Jw across toad skin.

Basal and amiloride-induced short-circuit current across isolated toad skin (Bufo arenarum).

We have previously demonstrated that amiloride (amil) addition to the isolated ventral pelvic (VPel) skin of Bufo arenarum toad induces negative short-circuit current values, which are equivalent to the isotopically measured net chloride transport. In the present work, we found that exposure of various regions of toad skin to amil yielded different values of short-circuit current (aSCC): negative aSCC was found in the VPel and ventral pectoral skin, while those of the dorsal one were not different from zero. The distinct values of aSCC found show a regional difference in the active chloride absorption, probably related to postural adaptations. A possible role of this adaptation would be related to chloride participation in the saline balance of the animals, or the maintenance of epithelial integrity.

Reversed short-circuit current across isolated skin of the toad Bufo arenarum.

Short-circuit current (SCC) across isolated pelvic skin of the toad Bufo arenarum has been shown to be reflected by the algebraic sum of net sodium and chloride transport. After the animals had been maintained in tap water, amiloride--an apical sodium channel blocker--led to a reversal of potential difference (rPD) across this preparation, to which corresponded a reversed short-circuit current (rSCC). Both rSCC and rPD were abolished by dermal treatment of skins with the metabolic inhibitor dinitrophenol, or by omission of chloride ion from the Ringer solution bathing both sides of the skin. There was a significant positive correlation between rSCC and isotopically determined net chloride transport after amiloride. An inhibitory action of amiloride on unidirectional chloride fluxes was detected, but only early after drug addition. rSCC was absent in skins of toads exposed to 110 mmol/l NaCl in tap water during 10 days. Together, our results suggest that amiloride addition--by inhibiting active sodium movement--can in certain conditions reveal the existence of an inward active chloride transport.

Comparative effects of angiotensin II on osmotic water permeability in the toad (Bufo arenarum).

The possible relationship between the renin-angiotensin system and water balance in the toad Bufo arenarum has been indirectly explored. A positive correlation was found between the hydrosmotic response of ventral pelvic toad skin to angiotensin II (A II) and some age indicators (body weight, snout-urostyle length or head width). A different hydrosmotic response for oxytocin and isoproterenol (but not for A II) was found between four cutaneous regions of toad body. We conclude that A II may not be directly involved in the regulation of water balance mediated by water absorption across the skin of Bufo arenarum toads.

Phenothiazines increase active sodium transport across the isolated toad skin.

Fluphenazine (FPZ) and trifluoperazine (TFP) are phenothiazine derivatives commonly used as antipsychotic tranquilizers. Their mechanism of action is incompletely understood. Epidermal addition of each drug promoted biphasic short-circuit current (SCC) changes across isolated pelvic skin of Bufo arenarum toads. By means of radiotracers fluxes, SCC was found to be given by the algebraic sum of net sodium and chloride transport. A readily stimulant effect was detected a low concentrations (from 1 X 10(-6) mol/l up to 1 X 10(-4) mol/l for FPZ, from 1 X 10(-5) mol/l up to 3.2 X 10(-4) mol/l for TFP) above which inhibition prevailed. Dermal FPZ also stimulated SCC. A higher concentration and time threshold were required. Epidermal 1 X 10(-5) mol/l FPZ stimulation was partially reversible, with a diminished membrane resistance and enhancement of sodium influx, without alteration of sodium efflux or net chloride transport. It could be prevented by amiloride pretreatment, or diminished by dermal sodium removal. Variation of epidermal bulk pH from 5.8 to 8.7 demonstrated that ionized and nonionized molecules contribute to FPZ's effect. Our results suggest that SCC stimulation elicited by FPZ and TFP may be a consequence of direct or indirect modifications on apical sodium conductance.