Response of frog and toad skin to norepinephrine.

Maintenance of a hydrated integument is essential to the normal function of amphibian skin, and amphibians have developed mechanisms to minimize cutaneous dessication. The present work was conducted on skins of amphibians exhibiting a clear preference for either of two such mechanisms to study the influence of such mechanisms on the characteristics of epithelial transport. The response to norepinephrine (NE) was studied in isolated skins of a semiaquatic frog (Leptodactylus chaquensis), known to maintain indispensable skin moisture by secreting a superficial film of mucus via sympathetic stimulation of skin glands, and a terrestrial toad (Bufo arenarum), which replenishes a superficial film of fluid by drawing soil water upward by capillarity. In L. chaquensis skin, NE 5.0 x 10(-7) M, induced slow onset, sustained increases in short-circuit current (SCC) and transepithelial conductance, which were abolished by amiloride, a specific sodium transport inhibitor. At 1.2 x 10(-5) M, the response to NE exhibited a faster onset and a shorter time course. The SCC response also became insensitive to amiloride and could thus be induced by exposing the skin to NE in the presence of the inhibitor. The response was also greatly reduced in the absence of chloride, strongly suggesting a greater dependence on the glandular secretory response. In B. arenarum skin, the response to NE was far more sensitive to amiloride, regardless of the concentration of NE used. Induction of a response in the amiloride-blocked skin required a 10-fold higher concentration of NE, and the resulting effect was still considerably smaller than that observed in the skin of L. chaquensis after the same treatment. The number of mucous glands per unit area in B. arenarum skin was found to be around one-fifth of that observed in L. chaquensis, thus in part explaining the difference in the magnitude of the responses. The response of the skin of L. chaquensis to NE in the presence of sulfate was found to be consistent with the postulated involvement of frog skin glands in sulfate excretion. In contrast, this function was not evident in the skin of B. arenarum. The pattern of response of B. arenarum skin to all concentrations of NE tested closely resembles that seen after exposure to agents known to activate a cyclic AMP-dependent, high-permeability Cl pathway previously described by us in the skin of the toad. Our observations underscore the physiological differences existing in skins from different species, particularly regarding the relative importance of the glandular component of transport.

Effect of theophylline on the electrolyte permeability of the isolated skin of the toad Bufo arenarum.

1. Short-circuit current (SCC, an index of active sodium transport) increased in response to theophylline (Theo) in the isolated skin of Bufo arenarum, regardless of the presence of chloride in the bath. 2. Tissue conductance, G, and the transepithelial potential difference, PD, also increased moderately in skins bathed in chloride-free solutions (main anion: sulfate or iodide); the increases in SCC and G were fully blocked by further addition of amiloride to the epidermal bath. 3. The increase in G following theophylline was greater in the presence of chloride, whereas PD decreased; the increased G was not modified by amiloride at a concentration that completely abolished the SCC. 4. Establishment of a chloride gradient across the theophylline-treated skin whose SCC was completely abolished by amiloride or by removal of sodium from the bath, induced a diffusion current (SCCg) in a direction consistent with the transepithelial flow of chloride following its gradient. The intensity of the SCCg was directly related to the magnitude of the gradient. 5. For a given gradient, both the intensity of the SCCg and the increase in G were greater when the gradient was directed inward (i.e., when the chloride concentration was higher in the epidermal than in the dermal bath), as compared to the opposite case. 6. Exposure of the skin to an epidermal bath made hyperosmotic by addition of urea (epidermal hyperosmolarity, EH, which reversibly increases the permeability of the paracellular pathway by "opening" the tight junctions) induced similar increases in G, whereas SCC and PD decreased irrespective of the anion present in the solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical parameters of the toad skin: effects of forskolin.

Forskolin stimulated short-circuit current (SCC) and transepitelial electrical conductance (G) in the isolated skin of the toad Bufo arenarum in a concentration-dependent manner, between 1.0 x 10(-6) and 2.4 x 10(-5) M. At the latter concentration, glandular secretion appeared to be stimulated also. The increase in G was considerably greater in skins bathed in Ringer solution than in solutions containing no chloride. The increased SCC was abolished by amiloride, a specific blocker of sodium transport in amphibian membranes, irrespective of the anion present in the solution bathing the skin. G was also decreased by amiloride to control values in skins bathed in solutions without chloride, but remained elevated in the presence of Cl-. The increase in SCC following exposure to forskolin, 4.4 x 10(-6) M, was not altered when furosemide, a specific blocker of chloride transport, was present in the Ringer solution bathing the dermal side of the skin. The response to forskolin, 2.4 x 10(-5) M, however, was significantly decreased by dermal furosemide; the inhibitor was ineffective in the absence of chloride. The data indicate that forskolin acts on at least two sites: stratum granulosum cells (the main pathway for sodium transport, and an alternate site, responsible for the increase in permeability to chloride. In addition, at high concentration of the agent, glandular secretion is also stimulated. The data suggest that the adenylate cyclase-cyclic AMP system is involved in the regulation of the permeability of the toad skin to sodium and chloride, probably by separate cell types.

Kinins contribute to the contractile effects of rat glandular kallikrein on the isolated rat uterus.

Glandular kallikrein is known to promote contractions of the isolated, estrogenized rat uterus, perhaps independently of kinin formation. The recent availability of kinin receptor antagonists led us to study whether they might affect the oxytocic activity of kallikrein. DArg0-Hyp3-Thi5,8-DPhe7-bradykinin (8.5 x 10(-7) M) displaced the dose-response curves to both bradykinin (from 1.0 x 10(-9) to 4.0 x 10(-6) M) and kallikrein (from 4.7 x 10(-11) to 8.0 x 10(-9) M) approximately one order of magnitude to the right. This inhibition could not be due to a nonspecific effect on the uterine muscle, as the contractile response to oxytocin was not altered. In addition, carboxypeptidase B (a potent kininase) and kinin antibodies reduced the contractile response to kallikrein by 70 and 60%, respectively. Removal of the intervening agent restored the normal response. The effect of kallikrein depended on its enzymatic activity, inasmuch as kallikrein inactivated with D-Phe-Arg-Arg-CH2Cl was not oxytocic. Prolonged or multiple exposures to kallikrein completely abolished uterine response, whereas the effect of bradykinin was unaltered. Uterine horns rendered insensitive to kallikrein by prolonged exposure still contracted in response to trypsin. Kininogen was present in the uterine tissue in a concentration of 1.5 +/- 0.3 ng of bradykinin equivalents per mg wet wt. No more than 15.9 +/- 1.2% of this total was due to plasma contamination. Only 21.5 +/- 2.9% of total kininogen could be cleaved by kallikrein. We conclude that part of the oxytocic activity of kallikrein is related to generation of kinins from a kallikrein-sensitive kininogen present in the isolated rat uterus.(ABSTRACT TRUNCATED AT 250 WORDS)

A role for the kallikrein-kinin system in the regulation of osmotic water permeability in the toad bladder.

Captopril (CA), a specific inhibitor of kininase II, did not alter osmotic water permeability (Posm) when present in the mucosal bath of the urinary bladder isolated from the toad Bufo arenarum at a concentration of 2.3 X 10(-3) M. This treatment, however, caused a 65% enhancement in the increase in Posm following serosal exposure to vasopressin, oxytocin or theophylline, agents that increase intracellular cyclic AMP levels. The effect of captopril was prevented by procedures that reduce the kallikrein (KK)-like alkaline esterase activity present in the bladder (such as simultaneous exposure to 2.3 X 10(-5) M aprotinin, or pretreatment of the toads with 0.1 N NaCl for several days before the experiment) or by replacing the mucosal bath with fresh solution of identical composition after exposure to captopril. In contrast, changing the serosal bath did not alter the effect of the drug. These results are consistent with an effect of CA at a step beyond cAMP generation, and suggest it is mediated by release of a soluble factor, probably a kinin, into the mucosal bath. These observations, together with data previously published, suggest that the KK-kinin system may participate in the control of epithelial water and electrolyte permeability in the toad bladder. In particular, under environmental stress, it may become important in the regulation of the animal's extracellular fluid volume, thus exhibiting an adaptive value.

Captopril inhibits sodium and water transport in the toad skin, a model of the distal nephron.

Captopril (SQ 14225), an orally active angiotensin I-converting enzyme inhibitor (CEI), increases natriuresis and diuresis in man and experimental animals in vivo, as well as in the isolated perfused rat kidney, raising the possibility of a direct renal action of the drug. We tested this hypothesis by studying its effects in the isolated toad skin, a model of the distal nephron devoid of vascular and nervous influences. When added to the dermal bath, captopril caused a reversible, concentration-related decrease in short-circuit current (SCC), a measure of active transepithelial Na transport. Keeping the toads in 0.1 M NaCl for 4 or more days increased sensitivity to the drug, which then inhibited SCC maximally (49 +/- 12% at 3.4 X 10(-3) M, P less than 0.01, n = 10), suggesting its effect might be modulated by endogenous mineralocorticoid activity. Captopril also inhibited the increase in SCC and in osmotic water permeability caused by neurohypophyseal peptides (NHP). The increases in SCC by non-peptidic agents (nystatin, a polyene antibiotic, or norepinephrine, an adrenergic agonist) were not altered, ruling out a generalized toxic effect, or any significant inhibition of the Na pump by captopril. The apparently specific effect of the drug on the permeability responses to NHP seems to be exerted proximally to the apical border, since the response of the latter to other agents was preserved. The present data suggest SH groups may be involved, since other CEI lacking such groups (teprotide and MK-422) do not produce such effects. These observations support the notion that a direct tubular effect may be involved in the increased diuresis and natriuresis observed after administration of captopril.

Inhibition of short-circuit current in toad urinary bladder by inhibitors of glandular kallikrein.

Aprotinin, a reversible inhibitor, and D-Phe-Phe-Arg-chloromethyl ketone (DPPA), an irreversible inhibitor of mammalian glandular kallikreins, decreased short-circuit current (SCC) in the isolated toad urinary bladder. Both were more potent and rapidly acting on the mucosal than serosal surface. The maximal inhibition in basal SCC was 29% for aprotinin and 41% for DPPA at concentrations of 7.0 X 10(-6) and 1.0 X 10(-5) M, respectively. SCC inhibition with mucosal aprotinin was reversed by rinsing, whereas inhibition with mucosal DPPA was not reversible. The presence of either agent in the mucosal bath inhibited the SCC increase to serosal vasopressin, but neither modified this response when present in the serosal bath. Neither agent affected basal or vasopressin-stimulated osmotic water permeability. Aprotinin did not prevent aldosterone-induced increases in SCC. Soybean trypsin inhibitor, an inhibitor of plasma but not glandular kallikrein, did not affect SCC. We postulate that these inhibitors of mammalian glandular kallikreins act upon some accessible serine proteinase(s) to reduce short-circuit current. This protein(s) might be an amphibian homologue of mammalian renal kallikrein.