Membrane pathways for water and solutes in the toad bladder: II. Reflection coefficients of the water and solute channels

Effects of glutaraldehyde fixation on renal tubular function. I. Preservation of vasopressin-stimulated water and urea pathways in rat papillary collecting duct

Using the in vitro microperfusion technique on isolated rat papillary collecting duct (PCD), we examined whether the glutaraldehyde-fixation method can be also applied to the mammalian collecting duct for preservation of the vasopressin-stimulated water and urea transport. Arginine vasopressin (AVP) at 10(-9) mol/l increased diffusional water permeability (Pdw) from 101.9 +/- 10.76 to 283.3 +/- 16.67 X 10(-7) cm2 s-1 (n = 8, P less than 0.01) and urea permeability (Purea) from 30.3 +/- 2.24 to 83.5 +/- 7.80 X 10(-7) cm2 s-1 (n = 8, P less than 0.01). Both parameters remained elevated after fixation with 0.1 mol/l glutaraldehyde even in the absence of AVP, with the values being 265.0 +/- 14.47 and 74.5 +/- 7.15 X 10(-7) cm2 s-1, respectively. Glutaraldehyde fixation did not affect the basal levels of Pdw or Purea. Phloretin at 2.5 X 10(-4) mol/l decreased glutaraldehyde-fixed AVP-stimulated Purea from 79.0 +/- 7.96 to 29.7 +/- 3.66 X 10(-7) cm2 s-1 (n = 4, P less than 0.01) and from 73.2 +/- 7.05 to 38.7 +/- 3.53 X 10(-7) cm2 s-1 (n = 4, P less than 0.01) when the drug was added to the lumen or to the bath, respectively. Phloretin also decreased glutaraldehyde-fixed non-stimulated Purea by 25-40%. However, this drug did not affect glutaraldehyde-fixed Pdw. These findings indicate that the glutaraldehyde fixation method can be applied to mammalian collecting tubules for studying vasopressin stimulated Pdw and Purea. Purea fixed by glutaraldehyde is functionally flexible and may be distinct from the water pathway.

Effect of salt depletion on sodium concentration in serum and urine of Bufo chilensis. Evidences for increased levels of neurohypophysial principles in their plasma

Salt-depleted toads Bufo chilensis were compared with animals maintained in NaCl solution and a control group with respect to Na+ content in serum and urine. Plasma hydro-osmotic activity of the animals was measured by increased water transfer across the isolated urinary bladder of the frog (Caudiverbera caudiverbera). Sodium in serum is not affected by pre-adaptation in distilled water. Urine Na+ is markedly reduced. Plasma from depleted animals increases water transfer across the isolated urinary bladder. Immersion in NaCl solution did not have this effect. An increase in neurohypophysial hormones in the blood of the animals is postulated.

Inhibition of vasopressin-stimulated water flow in toad bladder by phorbol myristate acetate, dioctanoylglycerol, and RHC-80267. Evidence for modulation of action of vasopressin by protein kinase C

The action of vasopressin (AVP) in transporting epithelia is mediated by cyclic AMP(cAMP), whereas its effects in hepatocytes are mediated by calcium and phosphoinositides. Based on our recent observation that AVP stimulates phosphoinositide turnover in toad bladder, we examined the role of calcium-phospholipid-dependent kinase (protein kinase C) as a modulator of AVP's hydroosmotic effect. Phorbol myristate acetate (PMA), which can substitute for diglyceride as an activator of protein kinase C, the diglyceride dioctanoylglycerol, and RHC-80267, a glyceride lipase inhibitor that should increase diglyceride levels, inhibited AVP-stimulated water flow, but not water flow stimulated by cAMP, suggesting inhibition of cyclic AMP production. Both the dioctanoylglycerol and RHC-80267, but not PMA, also decreased water flow in response to 8-bromo cAMP indicating a potential inhibition at post-cAMP events as well. PMA increased prostaglandin synthesis; however, inhibition of water flow persisted even when prostaglandin synthesis was completely blocked by incubation with naproxen. Furthermore, water flow was not inhibited by incubation with the inactive diglyceride substitute phorbol didecanoate, supporting the specificity of the PMA inhibition. Consistent with the site of action at adenylate cyclase suggested by the transport experiments, PMA and RHC-80237 decreased both cell cAMP content and the cyclic AMP-dependent kinase ratio (-cAMP/+cAMP), an index of intracellular cyclic AMP effect. Assay for protein kinase C activity in toad bladder epithelial cell supernatant demonstrated that the toad bladder indeed contains a kinase stimulable by phospholipid, calcium, and PMA. As an apparently independent effect, we found that addition of PMA, but not dioctanoylglycerol or RHC-80267, to the mucosal bath increased both water permeability and the frequency of granular cell luminal membrane aggregates in the absence of vasopressin, consistent with stimulation of fusion events at the luminal membrane. Our data suggest that protein kinase C can modulate AVP-stimulated water flow in toad bladder by inhibiting cAMP generation, and perhaps post-cAMP steps as well, and support the hypothesis that AVP-stimulated turnover of membrane phosphoinositides antagonize the effects of AVP via changes in diglyceride, calcium, and protein kinase C.

Autoradiographic studies of solute transport across the toad bladder

The autoradiography of diffusible, hydrophilic solutes presents special problems in localization of the labeled solute under study. We present studies of the movement of 14C-labeled urea, and 14C- and 3H-labeled sucrose across the isolated urinary bladder of the toad, a vasopressin-sensitive epithelium, using a technique that avoids exposure to water throughout all processing steps and minimizes error caused by isotope scatter. We have shown a significant increase in 14C urea entry into epithelial cells following vasopressin, and a significant decrease following phloretin, an agent that selectively blocks vasopressin-stimulated urea transport. The autoradiographic technique confirms the luminal site of action of phloretin. Studies of 14C and 3H sucrose labeling show that this molecule is virtually excluded from the cell. The current method of grain counting is capable of yielding reliable information in studies of epithelial transport.

Independent action of prostaglandins and kinins on vasopressin-stimulated water flow

The kallikrein-kinin and the prostaglandin systems are both important modifiers of vasopressin action. This study examines whether the systems are dependent on one another for their action. Four groups of toad hemibladders were examined. In groups 1 and 2 animals the endogenous prostaglandin system was inhibited. Inhibition of kallikrein by aprotinin caused vasopressin-stimulated water flow to increase further (24.8 +/- 4.9 to 34.5 +/- 4.8 microliters/min) while potentiation of kinins by captropril caused vasopressin-stimulated water flow to decrease (45 +/- 6.3 to 30.5 +/- 5.4 microliters/min). In groups 3 and 4 endogenous kallikrein was inhibited by aprotinin. The addition of prostaglandin E2 caused vasopressin-stimulated water flow to decrease (17.5 +/- 2.7 to 5.71 +/- 1.0 microliter/min) while the inhibition of endogenous prostaglandins caused vasopressin-stimulated water flow to increase (26.7 +/- 3.4 to 39.2 +/- 3.5 microliters/min). Thus, the inhibitory effects of prostaglandins and kinins on vasopressin-stimulated water flow are independent of one another.

Does water drag solutes through kidney proximal tubule?

Coupling of salt and water movements across kidney proximal tubules was studied in the presence of an induced transepithelial osmotic water flux. Convoluted proximal tubules from rabbit kidney were perfused in vitro with a control solution, with or without 50 mM/l of mannitol or raffinose in the both. Osmolalities of the perfused and collected fluids as well as the net water flux Jv were measured in each experiment. The net solute flux Js was calculated from the difference between the amount of total solutes delivered and collected at each end of the tubule. No apparent net transepithelial solute movements were detected in the presence of an osmotic water flux when active solute transport was inhibited either by an external to of 26 degrees C or by ouabain in the bath. The water flux observed was similar to that calculated assuming that only water crossed the epithelium, and no streaming potential was measured, whether or not active transport was blocked. It is concluded that the osmotic water flux through kidney proximal tubule does not drag a significant amount of solutes, probably because of the absence of convective solvent flux. This suggests the existence of different pathways for water and salt movement.

Role of the endogenous kallikrein-kinin system in modulating vasopressin-stimulated water flow and urea permeability in the toad urinary bladder

This study investigates the endogenous kallikrein-kinin system's role as a modulator of vasopressin action in the toad urinary bladder. Kalli-krein inhibition by aprotinin, which results in decreased kinin production, significantly increased both vasopressin and 8-Br-cyclic (c) AMP-stimulated water flow. Kinin potentiation by the kininase II inhibitor captopril (SQ 14225) significantly decreased vasopressin and 8-Br-cAMP-stimulated water flow. In contrast to water flow, vasopressin-stimulated urea permeability was decreased by aprotinin and increased by captopril. We conclude that the endogenous kallikrein-kinin system represents a significant modulator of vasopressin action and it permits separate control of vasopressin-stimulated water flow and solute transport.