Inhibitory effect of sodium ursodeoxycholate on basal and stimulated short-circuit current across the isolated toad skin.

The effect of sodium ursodeoxycholate (U) on short-circuit current (SCC), an index of basal and stimulated net ion transport across isolated skins of Bufo arenarum toads, was tested. U inhibited basal SCC when added to the epidermal side of the skins. The inhibitory effect was reversible after rinsing the preparation during 60 min. U also inhibited the natriferic response to oxytocin, db-cAMP and theophylline by 82%, 49% and 47%, respectively. Inhibition of SCC by exposure to U was reversed by the polyene antibiotic nystatin. In turn, SCC induced by nystatin in the amiloride-treated skin was insensitive to U and blocked by ouabain, a Na+, K(+)-ATPase inhibitor. These results strongly suggest that the effect of U is exerted at the apical membrane of sodium transporting cells, and rule out the existence of an additional site of inhibitory action of U.

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.

Effect of alpha-2 adrenergic receptor stimulation on short-circuit current across isolated skin of the toad Bufo arenarum.

1. The effect of the alpha-2 adrenergic agonist clonidine on short-circuit current (SCC) across isolated skins of Bufo arenarum toads was investigated. 2. Clonidine inhibited basal SCC in a dose-dependent manner. 3. Blockade of the effect of clonidine on basal SCC by the selective alpha-2 antagonist yohimbine supports the hypothesis that the inhibitory effect is mediated by the stimulation of alpha-2 adrenergic receptors. 4. The fact that the inhibitory effect of clonidine is higher in skins with spontaneous positive SCC than in the negative ones, and that the alpha-2 agonist was unable to alter amiloride-induced negative SCC suggests that the inhibitory effect of clonidine may probably be mediated by inhibition of sodium transport.

Alpha-2 adrenergic agonists inhibit basal and stimulated osmotic water permeability in toad skin.

The effect of selective alpha adrenergic agonists and antagonists on osmotic water permeability (Posm) across isolated skins of Bufo arenarum toads was investigated. Clonidine, an alpha-2 agonist, inhibited basal Posm and oxytocin, isoproterenol and theophylline stimulated Posm, but did not alter the hydrosmotic effect of exogenous cAMP. Blockade of the effect of clonidine on basal and stimulated Posm by the selective alpha-2 antagonist yohimbine supports the hypothesis that the inhibitory effect is mediated by the stimulation of alpha-2 adrenergic receptors.

Vascular responses to angiotensins in the toad. Role of the adrenergic system.

The isolated hindlimbs (vasoconstrictor) and the whole toad pressor assays were used to investigate the effects of some angiotensin II (AT II) analogues and the influence of the adrenergic system on the vascular response to AT II in the toad Bufo arenarum. Three of the analogues assayed (Leu8AT II, Sa1, Ala8AT II, Sar1, I1e8AT II) were unable to block the response to AT II, and a fourth, Ala1, Ile8AT II, potentiated it. In the case of Sar1, Ala8AT II (saralasin), this confirms previous negative findings from this laboratory on water and electrolyte transport in toad skin, probably reflecting differences between AT II receptors in mammals and amphibians. This notion is further supported by the fact that desamino AT II, and AT II analogue reported not to produce tachyphylaxis in the guinea-pig ileum, behaves like AT II when injected repeatedly in the isolated hindlimbs. Tyramine was devoid of vascular effects in this preparation. Phentolamine (which blocked the response to norepinephrine) and Ro 4-1284 (an in vitro catecholamine depleting agent) failed to alter the vasoconstrictor action of AT II. An unexpected finding was the inhibiting effect of Ro 4-1284 on the vasoconstrictor response to exogenous norepinephrine.