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Bradykinin (BK) is one of the most important peptide regulating vascular tone, water, and ionic balance in the body, playing a key role in controlling blood pressure. Interestingly, patients with essential hypertension excrete less BK than do normotensive subjects. To elucidate the mechanism by which BK regulates renal water transport, AQP2-transfected collecting duct CD8 cells, expressing the BK receptor (BK2), were used as experimental model. In CD8 cells, BK pretreatment impaired forskolin-induced AQP2 translocation to the apical plasma membrane. To clarify the signal transduction cascade associated with this effect, we first investigated whether BK induced increase in citosolic calcium, via the G protein Gq known to be coupled to BK2 receptor. Spectrofluorometry employing fura-2-AM revealed that 100 nM BK elicited a significant increase in Cai (from 72.8 ± 7.4, to 310.7 ± 43.4 nM, n=6, P which was abolished by the receptor antagonist HOE-140. In renal cells, Gq coupled receptors may activate Rho and its downstream effectors. In CD8 cells it has been shown that forskolin-induced AQP2 translocation is associated with a decrease in Rho activity and depolymerization of F-actin which facilitates the translocation of AQP2 to the plasma membrane. Interestingly, BK treatment in CD8 cells resulted in a significant increase in Rho activity, as assessed by selective pull down experiments. In agreement with these data, BK induced a significant increase of F-actin content as assessed by actin polymerization assay and by immunofluorescence experiments. BK effects on actin assembly were abolished by the BK2 agonist HOE-140. We conclude that the diuretic effect of bradykinin may in part be explained by impairement of AQP2 translocation via activation of Rho and F-actin formation. [ABSTRACT FROM AUTHOR]

The effects of noradrenaline on cGMP levels of epithelial cells isolated from frog skin are reported. Noradrenaline does not change cGMP levels either in a medium containing Ca++ and Mg++, or without Ca++ or without Mg++.

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.

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.

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.