Osmotic activity of dimethyl sulfoxide in the renal distal tubule

Early targets of lithium in rat kidney inner medullary collecting duct include p38 and ERK1/2

Almost half of patients receiving lithium salts have nephrogenic diabetes insipidus. Chronic lithium exposure induces AQP2 downregulation and changes in the cellular composition of the collecting duct. In order to understand these pathophysiological events, we determined the earliest lithium targets in rat inner medullary collecting duct (IMCD) by examining changes in the IMCD phosphoproteome after acute lithium administration. IMCDs were isolated 9 h after lithium exposure, a time when urinary concentrating impairment was evident. We found 1093 unique phosphopeptides corresponding to 492 phosphoproteins identified and quantified by mass spectrometry. Label-free quantification identified 152 upregulated and 56 downregulated phosphopeptides in response to lithium. Bioinformatic analysis highlighted several signaling proteins including MAP kinases and cell-junction proteins. The majority of the upregulated phosphopeptides contained a proline-directed motif, a known target of MAPK. Four hours after lithium exposure, phosphorylation sites in the activation loops of ERK1/2 and p38 were upregulated. Increased expression of phospho-Ser261-AQP2 (proline-directed motif) was concomitant with the increase in urine output. Pretreatment with MAPK inhibitors reversed the increased Ser261-AQP2 phosphorylation. Thus, in IMCD, ERK1/2 and p38 are early targets of lithium and may play a role in the onset of lithium-induced polyuria.

Small transepithelial osmotic gradients affect apical sodium permeability in frog skin

The aim of the present study was to investigate the effects of small, unilateral changes in solution osmolarity on active sodium transport and cellular electrophysiological parameters in frog skin. The active sodium transport across the skin was measured as the amiloride-sensitive short-circuit current (Isc) and cellular potential was monitored with microelectrodes, while small (+/- 20 mOsm) osmotic gradients were imposed on the skin. Increasing the osmolarity of the apical bathing solution (or decreasing the osmolarity of the basolateral solution) increased ISC, lowered tissue resistance (R), depolarized the cellular potential and decreased the fractional resistance of the apical membrane, which indicates an increased apical sodium permeability. Conversely, a similar increase in basolateral osmolarity (or a decrease in apical osmolarity) lowered the Isc, increased R, hyperpolarized the cells and increased the fractional resistance of the apical membrane, indicating a decrease in apical sodium permeability. The results indicate that the osmotic gradient across the skin, rather than solution osmolarity as such, is responsible for the observed changes in Isc and apical sodium permeability after small osmotic perturbations.

Reduction in sensitivity to Cl- channel blockers by HCO3- -CO2 in rabbit cortical collecting duct

We examined the ability of HCO3- -CO2 to modify the potency of Cl- channel blockers in the renal cortical collecting duct (CCD) for the following two reasons. 1) From a practical point of view, there is, to our knowledge, no information regarding the effect of the HCO3- -CO2 buffer system on the potency of Cl- channel blockers. 2) We showed in the companion manuscript [Am. J. Physiol. 257 (Cell Physiol. 26): C94-C101, 1989] that HCO3- -CO2 stimulates transepithelial anion exchange in the CCD. Based on precedent in the literature, we postulated that HCO3- stimulates the basolateral membrane Cl- conductance. Here, we demonstrate that several Cl- channel blockers can reduce CCD Cl- self exchange when the solutions are buffered in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES). Concentrations of blockers producing 80% inhibition in HEPES, pH 7.4, produced only 20% inhibition in 25 mM HCO3- -CO2, pH 7.4. The ability of HCO3- -CO2 to reduce blocker potency had an IC50 of only 2 mM. We also examined interactions of HCO3- -CO2 and blockers with regard to the principal cell basolateral Cl- conductance. Blockers did not alter the Rb+ flux, a marker of K+ transport, but did reduce transepithelial conductance (GT), i.e., the blockers inhibited the principal cell basolateral Cl- conductance. As was the case with intercalated cell anion exchange, GT measurements indicated that HCO3- -CO2 impaired the ability of Cl- channel blockers to inhibit the principal cell Cl- conductance.(ABSTRACT TRUNCATED AT 250 WORDS)