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M Svelto

Publications and source records attributed to M Svelto.

At least 55 records · Page 3Linked to original sources
PubMed1987

Evidence for a two-site model of forskolin action in frog urinary bladder.

In the present study several aspects of the osmotic water-flow-regulation mechanism in frog urinary bladder were examined, utilizing forskolin either as a direct hydrosmotic agent or in association with vasopressin. It was found that forskolin induces a hydrosmotic effect similar to the one induced by vasopressin. This effect is rapid, reversible and dose-dependent. The half-maximally effective concentration (Ec50FSK) is 1.37 microM forskolin. No additional effect on the osmotic water flow was observed when maximal concentrations of forskolin and vasopressin were given simultaneously. Moreover, forskolin can also markedly potentiate vasopressin-induced hydrosmotic response. This potentiation was maximal with submaximal doses of vasopressin, whereas it disappeared when the hormonal concentration was increased to very high levels. Therefore, forskolin increases vasopressin potency without affecting vasopressin efficacy. The Ec50FSK for the forskolin-induced increase in vasopressin potency was 11 nmol, about two orders of magnitude lower than the Ec50FSK for the direct effect of forskolin on the osmotic water transport. On the whole, our results are compatible with a two-site model of forskolin action in frog urinary bladder: a low affinity site (Ec50FSK = 1.37 microM) that mediates the direct action of forskolin on the osmotic water flow and a high affinity site (Ec50FSK = 11 nmol), which mediates the synergic effect of forskolin with the antidiuretic hormone.

Animals↗
PubMed1987

Phorbol ester effect on the hydrosmotic response to vasopressin in frog skin.

Serosal preincubation of frog skin with tetradecanoyl phorbol acetate, TPA, an activator of protein kinase C, inhibits the hydrosmotic response elicited by vasopressin (AVP) but not that induced by 8br-cAMP. This proves that serosal TPA primarily influences a pre-cAMP step. The TPA-induced inhibition of AVP response appears to be related to TPA-induced prostaglandin synthesis. The pretreatment with naproxen, in fact, prevents the inhibition induced by serosal TPA on the AVP response. On the contrary, mucosal TPA produces a more marked inhibition of the response to AVP and significantly diminishes the water flow induced by 8br-cAMP; this suggests that mucosal TPA interferes mainly with a post-cAMP step. Furthermore, naproxen is unable to completely prevent the inhibition induced by mucosal TPA on AVP response thus indicating that mucosal TPA may also activate a prostaglandin-independent mechanism able to inhibit one of the last steps of the hydrosmotic response to AVP.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1985

Calcium role in base-line and ADH-stimulated sodium transport in frog skin.

Treatment of ventral frog skin with serosal A23187 calcium ionophore caused an initially transient increase in transepithelial sodium transport. After 60 min of treatment with A23187, a steady-state transport value was reached which was significantly lower than the initial one. Furthermore, it was found that ionophore treatment greatly inhibited the natriferic response to ADH and to 8br-cAMP. A further analysis on the possible ionophore action mechanism was carried out through pretreatment of the skin with indomethacin, very powerful prostaglandin synthesis inhibitor. In the experimental conditions reported, A23187 seems no longer capable of inducing a transient increase in sodium transport, although it does inhibit the natriferic response to ADH.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1985

Hydrosmotic response to vasopressin in frog skin. Control by endogenous factors.

The A23187 calcium ionophore strongly inhibits the hydrosmotic response to vasopressin. On the contrary neither exogenous cAMP nor theophylline-induced hydrosmotic response are affected by Ca++-ionophore. The effects obtained with A23187 suggest that calcium ions modulate vasopressin-induced osmotic water flow by interfering with a pre-cAMP step. The increase in intracellular calcium concentration induced by A23187, in fact, may inhibit the rate of cAMP formation by interfering with the vasopressin-stimulated adenylate cyclase system. This inhibition seems to be restricted to a locus proximal to the catalytic subunit of adenylate cyclase, whereas the hydrosmotic response to forskolin, a non-hormonal activator, is not affected by calcium ionophore addition. In order to clarify whether calcium ions act directly on the adenylate cyclase system or activate a more complex pathway, we studied the interaction between calcium and prostaglandins in modulating the hydrosmotic response to vasopressin. Direct measurements by radioimmunoassay of PGE2 release in the serosal medium show that A23187 notably increases PGE2 release. Furthermore, the inhibition of prostaglandin biosynthesis by hydrocortisone (a phospholipase inhibitor) or by indomethacin and naproxen (agents that inhibit arachidonic acid oxygenase) results in augmented vasopressin-stimulated water flow and prevents the inhibitory effect of the ionophore. Collectively these results strongly suggest that the effects obtained with A23187 on hydrosmotic response to ADH, are closely linked to calcium-stimulated PGE2 biosynthesis.

Animals↗
PubMed1984

Evidence for the role of calcium in the hydrosmotic response to antidiuretic hormone in frog skin.

Treatment with the calcium ionophore A23187 on either the serosal or mucosal sides of frog skin, strongly inhibits the hydrosmotic response to vasopressin. On the contrary, the hydrosmotic response to 8-br-cAMP is not affected by treatment with the A23187. Trifluoperazine, a drug which inhibits the Ca2+-calmodulin complex, selectively inhibits vasopressin-induced water transport. Collectively, our results suggest that an increase in the intracellular concentration of Ca2+, obtained by treatment with the ionophore A23187, interferes with a pre-cAMP step of the hydrosmotic response to the antidiuretic hormone. Calcium ions could regulate adenyl-cyclase activity and consequently intracellular levels of cAMP. This effect may probably involve calmodulin.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1983

[Effect of ionophore A23187 on the response to ADH in the ventral skin of Rana esculenta].

The addition of the Ca++ ionophore A23187 (10 microM) to the inside solution of the frog skin induced a transient increase in the active Na+ transport in frog skin (Rana esculenta) which decayed to the control values 60 minutes after the addition. At the same time the skin resistance failed significantly; antidiuretic hormone addition resulted in no-more increase of the Na+ active transport; the skin resistance remained unchanged. To further investigate the role of intracellular calcium on the skin transepithelial permeability, the effect of A23187 ionophore on thiourea permeability has been tested. Increase in intracellular Ca++ concentration brought about by calcium ionophores have been shown to modify both basal and ADH-stimulated thiourea transport.

Animals↗
PubMed1983

[Effect of niflumic acid on the non-acid transport of chlorides in the gastric mucosa of amphibians].

The in vivo gastric mucosa actively transport Cl- (serosa to mucosa) and it has been shown that the e.m.f. generated by the epithelium, as well as the short circuit current are both manifestations of the same phenomenon: the Cl- movement. Also, it has been postulated the presence of a Cl- - HCO-3 exchange in the gastric epithelium probably located on different cell types. The addition of niflumic acid (10(-4) M in the serosal solution) resulted in a decline towards zero of both transepithelial p.d. and Isc. The mechanism of the niflumic acid action is postulated to be a blockage of the Cl- - HCO-3 exchange, similarly to the SITS action mechanism.

Animals↗
PubMed1983

cAMP levels and alpha and beta receptors stimulation by noradrenaline in frog skin.

The effect of noradrenaline on intracellular levels of cAMP in epithelial cells isolated from frog skin, has been studied both in the presence and in the absence of alpha (phentolamine) or beta (propranolol) adrenergic blocking agents. Such levels are increased by hormonal treatment and are linked to the stimulation of beta-adrenergic receptors, while alpha-stimulation has an inhibitory effect. Higher levels of cAMP are reached in the presence of both Ca2+ and Mg2+, confirming the existence of a Ca2+ and Mg2+-dependent adenyl-cyclase, stimulated by noradrenaline. The experimental conditions necessary to induce the highest intracellular cAMP levels, are those in which the permeabilizing effect reaches its maximum.

Animals↗
PubMed1982

[The nature of urea transport across the skin of of Rana esculenta].

In several epithelial tissues such as toad bladder, gallbladder and human red cells, it has been established that urea movement implies a phloretin sensitive mediated transport. In the skin of the toad Bufo viridis also it has been described an active transport of urea. Our data, obtained on the frog skin seem to demonstrate the existence of some specific mechanism for urea transport towards the inside solution. In fact, two molecules having the some molecular diameter, such as urea and thiourea, show a large difference in permeability at low concentration. In addition 0.1 mM urea influxes and outfluxes, measured on paired skin halves in the absence of concentration gradient, exhibit an evident asymmetry. Further approaches with phloretin experiments were made in order to characterize the urea transport system. Phloretin (5.10(-4)M) added to the external solution significantly inhibits the urea influx. Little can be said at this time about the composition or kinetics of the carrier involved in the transport.

Animals↗
PubMed1982

Phloretin sensitive active urea absorption in frog skin.

This report presents evidence for urea active absorption by isolated skin of Rana esculenta. One of the supporting factors of such evidence is that at a low concentration the urea influx is five times greater than the outflux, in the absence of a chemical gradient. The transport shows a saturation kinetics with an apparent Km = 1.33 mM and is inhibited by un uncoupling agent (FCCP). 5 x 10(-4) M Phloretin, added to the external side, markedly inhibits inward urea transport, whereas it is ineffective when added to the serosal fluid. This provides evidence for a phloretin-sensitive mechanism located at the external side of the epithelium. Phloretin stimulates the sodium active transport; the possible coupling of urea and sodium movement is analysed.

Animals↗
PubMed1981

Protective effect of hydrocortisone on vasopressin response in frog skin.

The effect of microtubular-poisons, such as colchicine and vincristine, on frog skin permeability has been investigated. Three-hour treatment with the drugs has no effect on nonelectrolyte basal transepithelial permeability, but completely suppresses the effect of ADH. Colchicine and vincristine, in addition, affect both basal sodium transport and the rise in short circuit current induced by vasopressin. The inhibition produced by microtubular-poisons disappears, however, when hydrocortisone, a glucocorticoid known to preserve junctional communications is used. Together with the results previously obtained with isolated epithelial cells (Svelto et al. 1979), these findings provide further support for our hypothesis that the microtubular-microfilament-system, is involved in cell-to-cell exchange.

Animals↗
PubMed1979

Colchicine effect on the permeability of the whole epithelium and of isolated cells of frog skin.

The effect of 2 X 10(-5) M colchicine on epithelial cells isolated from frog skins was investigated. Three hours of treatment with colchicine did not change either Na+ and K+ content of isolated cells or nonelectrolyte permeability. When ADH (50 mU/ml) was added, thiourea uptake values became greater than without the hormone; the same values were found in the cells previously treated with colchicine. Na+ transepithelial transport, measured by means of short-circuit current, was inhibited by the antimitotic agent both under control conditions and after ADH stimulation. These results support the view that colchicine does not directly affect ADH action on membrane permeability, but influences some mechanism that controls ADH action on transepithelial transport. Intercellular junctions appear to be the location of such a mechanism.

Animals↗
PubMed1978

Colchicine inhibition of ADH effect on frog skin permeability.

ADH and AMPc enhance both thiourea unidirectional fluxes in frog skin. This effect is completely abolished by colchicine pretreatment. The ADH increase of thiourea discharge with or without colchicine led us to suppose that colchicine does not directly affect ADH action on outer membrane permeability, but exerts its effects on a site which is limiting for the ADH action on transepithelial permeability.

Animals↗