Prostaglandin E2 enhances the sodium conductance of exocrine glands in isolated frog skin (Rana esculenta)

Effects of lipids on ENaC activity in cultured mouse cortical collecting duct cells

Direct effects on epithelial Na+ channels (ENaC) activity by lipids, e.g., arachidonic acid (AA), eicosatetraynoic acid (ETYA), linoleic acid (LA), stearic acid (SA), hydroxyeicosatetraenoic acid (HETE), 11,12-epoxyeicosatrienoic acid (EET), (PGF2), and (PGE2), in cultured mouse cortical collecting duct (M1) cells were clarified by using single-channel recordings in this study. In a cell-attached recording, a bath application of 10 microM AA significantly reduced the ENaC open probability (NPo), whereas 10 microM ETYA or 5 microM LA only induced a slight inhibition. The inside-out recording as a standard protocol was thereafter performed to examine effects of these lipids on ENaC activity. Within 10 min after the formation of the inside-out configuration, the NPo of ENaC in cultured mouse cortical collecting duct (M1) cells remained relatively constant. Application of ETYA or LA or SA exhibited a similar inhibition on the channel NPo when applied to the extracellular side, suggesting that fatty acids could exert a nonspecific inhibition on ENaC activity. 11,12-EET, a metabolite of AA via the cytochrome P450 epoxygenase pathway, significantly inhibited the ENaC NPo, whereas 20-HETE, a metabolite of AA via the hydroxylase pathway, only caused a small inhibition of the ENaC NPo, to a similar degree as that seen with ETYA and LA. However, both PGE2 and PGF2alpha significantly enhanced the ENaC NPo. These results suggest that fatty acids exert a nonspecific effect on ENaC activity due to the interaction between the channel proximity and the lipid. The opposite effects of 11,12-EET and prostaglandin (PG) implicate different mechanisms in regulation of ENaC activity by activation of epoxygenase and cyclooxygenase.

The adrenergic receptor subtypes present in frog (Rana esculenta) skin

Frog skin transports ions and water under hormonal control. In spite of the fundamental role played by adrenergic stimulation in maintaining the water balance of the organism, the receptor subtype(s) present in the skin have not been identified yet. We measured the increase in short-circuit current (ISC, an estimate of ion transport) induced by cirazoline, clonidine, xamoterol, formoterol, or BRL 37344, in order to verify the presence of alpha1, alpha2, beta1, beta2, or beta3 receptor subtypes, respectively. Only after treatment with formoterol, BRL 37344 and, to a lesser extent, cirazoline was measured a significant increase in ISC (57%, 33.2%, and 4.7%, respectively). The formoterol and BRL 37344 concentrations producing half-maximal effect (EC50) were 1.12 and 70.1 nM, respectively. Moreover, the formoterol effect was inhibited by treatment with ICI 118551 (antagonist of beta2 receptors) while SR 59230A (antagonist of beta3 receptors) had no effect; opposite findings were obtained when the BRL 37344 stimulation was investigated. Finally, by measuring the transepithelial fluxes of 22Na+ and 36Cl-, we demonstrated that Na+ absorption is increased by activation of beta2 and beta3 and is cAMP-sensitive, whereas the Cl- secretion is only increased by activation of beta2 receptors and is cAMP- and calmodulin-sensitive.

Correlation between chloride flux via the mitochondria-rich cells and transepithelial water movement in isolated frog skin (Rana esculenta)

The coupling between net transepithelial Cl- influx and net water flow was investigated. Experiments were performed on isolated frog skin bathed in isotonic Cl- Ringer's solution in the presence of the Na+ channel blocking agent amiloride in the mucosal solution. The skins were voltage-clamped at -80 or -100 mV (with the serosal solution as reference). Under these conditions the current across the skin is carried by an influx of Cl-. In the absence of antidiuretic hormone the correlation between current and net water flow was low, but in the presence of the antidiuretic hormone, arginine vasotocin, there was a highly significant correlation between current and net water flow. The data presented here indicate that under steady state conditions about 70 molecules of water follow each Cl- ion across the skin. If the water influx is driven by electroosmosis one would expect that a change in current should result in an immediate change in the water flow. There was, however, a considerable time delay between the change in current and water flow. This indicates that the observed coupling between Cl- flux and water flow is caused by current-induced local osmosis and not electroosmosis.