Ringer Bicarbonate Solution Research Articles

Effects of osmotic stress on rabbit corneal endothelium

The effects of osmotic stress on corneal endothelium were investigated by exposing rabbit corneas to anisosmotic conditions, and then perfusing the corneas with isosmotic glutathione bicarbonate Ringer solution for 4 hr at 35 °C. During the perfusion, endothelial function was assessed by measuring corneal thickness with a specular microscope. After perfusion, the corneas were prepared for scanning and transmission electron microscopy. Endothelial ultrastructure and function were well maintained after exposure to a wide range of osmolality (0.12–2.7 osmol/kg), but this tolerance of osmotic stress was dependent both on the duration and the temperature of exposure to the anisosmotic conditions. Exposure to an osmolality of 2.7 osmol/kg for 15 min at 23 or 37 °C resulted in gross damage to the endothelium when the hyperosmotic agent was sodium chloride. This damage was not the result of increased osmolality per se nor cellular shrinkage because the endothelium tolerated exposure to a sucrose solution of the same osmolality for 15 min at 37 °C. The detrimental effect of sodium chloride, however, was mitigated by reducing the temperature of exposure to 0 °C or reducing the duration of exposure to 5 min. These results suggest that endothelial cells become more tolerant of high electrolyte concentrations with reducing temperature, and this could be an important factor in the survival of the endothelium in corneal cryopreservation. The results also help to define the limits of osmotic shrinkage and swelling tolerated by endothelial cells. This will be of value in overcoming the detrimental osmotic effects associated with the addition and, in particular, the removal of cryoprotectants.

Influence of prostaglandins on unidirectional zinc fluxes across the small intestine of the rat.

1. The regulatory role of prostaglandins (PGs) E2 and F2 alpha on the zinc transport rate across the jejunal segments of rats was examined by employing the Ussing chamber technique. The Zn flux rate from mucosa to serosa across jejunal segments (Jms) was 5.24 (SE 1.54) nmol/h per cm2 (n 48) and that from serosa to mucosa (Jsm) was 15.16 (SE 2.38) nmol/h per cm2 (n 48) when both sides of the segment were bathed with Ringer's bicarbonate solution containing 0.5 mM-zinc chloride and 3 mM-L-histidine. 2. When 5.0 or 50 microM of either PGE2 or PGF2 alpha were added to the serosal side of the tissue, Jsm generally decreased and Jms generally increased, compared with controls. On the other hand, when PGE2 or PGF2 alpha was added to the mucosal side of the tissue, Jms either did not change or increased while Jsm had a tendency to decrease. 3. The Zn uptake capacity of tissue increased significantly when PG was added to the serosal side of the tissue-bathing medium, but not when PG was added to the mucosal side. The uptake capacity of mucosal Zn by jejunal segments was approximately twice that of serosal Zn. 4. When PG was included in the tissue-bathing medium, the short-circuit current, potential difference and conductance between the mucosa- and serosa-bathing media generally decreased. 5. These results suggest that (a) PGs influence Zn flux rate not by chelating Zn and carrying it across the mucosal cell membrane but by interacting with the cytosolic components, (b) it is the serosal PGs which control the Zn flux rate and (c) PGs play a part in triggering a transduction mechanism in the intestinal Zn transport process.

Silver ion (Ag+)-Induced increases in cell membrane K+ and Na+ permeability in the renal proximal tubule: Reversal by thiol reagents

The initial mechanisms of injury to the proximal tubule following exposure to nephrotoxic heavy metals are not well established. We studied the immediate effects of silver (Ag+) on K+ transport and respiration with extracellular K+ and O2 electrodes in suspensions of renal cortical tubules. Addition of silver nitrate (AgNO3) to tubules suspended in bicarbonate Ringer's solution caused a rapid, dose-dependent net K+ efflux (Km = 10(-4) M, Vmax = 379 nmol K+/min/mg protein) which was not inhibited by furosemide, barium chloride, quinine, tetraethylammonium, or tolbutamide. An increase in the ouabain-sensitive oxygen consumption rate (QO2) (13.9 +/- 1.1 to 25.7 +/- 4.4 nmol O2/min/mg, P less than 0.001), was observed 19 sec after the K+ efflux induced by AgNO3 (10(-4) M), suggesting a delayed increase in Na+ entry into the cell. Ouabain-insensitive QO2, nystatin-stimulated QO2, and CCCP-uncoupled QO2 were not significantly affected, indicating preserved function of the Na+,K+-ATPase and mitochondria. External addition of the thiol reagents dithiothreitol (1 mM) and reduced glutathione (1 mM) prevented and/or immediately reversed the effects on K+ transport and QO2. We conclude that Ag+ causes early changes in the permeability of the cell membrane to K+ and then to Na+ at concentrations that do not limit Na+,K+-ATPase activity or mitochondrial function. These alterations are likely the result of a reversible interaction of Ag+ with sulfhydryl groups of cell membrane proteins and may represent initial cytotoxic effects common to other sulfhydryl-reactive heavy metals on the proximal tubule.

The inhibition by chlorate of the sulphation of polyethyleneglycol in the isolated perfused guinea pig liver.

1. The sulphation of polyethyleneglycol 200 by the isolated perfused guinea pig liver is inhibited to about 60% by 10 mM ClO3- in the plasma of the perfusate when the concentration of SO4(2-) therein is 1.18 mM. 2. The inhibition is almost complete when the concentration of SO4(2-) is about 0.1 mM, a level which can be achieved by using a modified Ringer-bicarbonate solution, devoid of sulphate, to prepare the perfusate. 3. Chlorate, presumably through its action on ATP-sulphurylase, may therefore be a useful inhibitor of sulphation in the isolated perfused liver when the activity of the sulphurylase is rate-limiting. 4. The rate of bile production in the presence of chlorate is no different from that in its absence showing that, in the time scale of the perfusion, chlorate is not a general liver poison. 5. When the synthesis of PAPS is not rate-limiting, as in the sulphation of oestrone metabolites by rat liver, chlorate has no effect on the rate of sulphation.

Sided effects of bicarbonate on rabbit corneal endothelium in vitro

Stroma-endothelium preparations from albino rabbits were studied either in a specular microscope (and measurements made of corneal thinning under silicone oil) or mounted between two half-chambers (to permit direct measurement of endothelial fluid pump activity). Preparations were first equilibrated with bicarbonate-Ringer solutions (supplemented with glucose, adenosine and glutathione and equilibrated with 5% CO2-air). If the preparations were equilibrated with a low (2 mM) level of bicarbonate on the stromal side and variable levels (2 to 50 mM) of bicarbonate on the endothelial side, corneal thinning increased with bicarbonate concentration while fluid pump was found to be constant from 2 to 35 mM bicarbonate. In contrast, if the preparations were equilibrated with a low (2 mM) bicarbonate level on the endothelial side and 2 to 50 mM bicarbonate on the stromal aspect, corneal thinning did not occur but fluid pump declined gradually as a function of stromal bicarbonate levels. These results contrast sharply with the significant corneal thinning and fluid pump that can be measured if both sides of the preparations are equilibrated with 35 mM bicarbonate. These results further emphasise that bicarbonate ions do not simply serve to drive an endothelium-sited fluid pump in the mammalian cornea.

Effect of carbachol on rabbit corneal endothelium.

We investigated the current reformulation of the commercially available intraocular preparation of carbachol for its effect on corneal thickness and endothelial cell ultrastructure. A 15-minute perfusion in the specular microscope with 0.01% carbachol caused a significant increase in the corneal swelling rate, which returned to control values after 75 minutes of reperfusion with Krebs Ringer bicarbonate solution. Continued reperfusion with Krebs Ringer bicarbonate solution led to the final average corneal swelling rate being statistically similar to that of controls over the entire 180-minute perfusion period. Transmission and scanning electron microscopy demonstrated no changes in endothelial cell ultrastructure.

Mechanisms of transport of Na + and Cl − in the lizard colon

1. 1. Ionic fluxes of sodium and chloride across lizard colon mucosa were measured and compared with the electrical characteristics of the tissue under voltage-clamped conditions. 2. 2. In a Ringer-bicarbonate solution there was both a net sodium flux ( J net Na) and a net chloride flux ( J net cl) from mucosa to serosa. The net flux residual ( J R) was near zero, indicating that net sodium and chloride transport is the result of an electrically neutral transport mechanism. 3. 3. In the presence of sodium, the net chloride flux was abolished and the short-circuit current ( I sc ) and the electrical potential difference (PD) were unchanged. In the absence of chloride the net sodium flux was abolished and the short-circuit current and electrical potential difference were not modified. 4. 4. From an analysis of the effects of the inhibitors, furosemide, amiloride and disulfonic stilbene (DIDS), a plausible model was developed to explain the characteristics of sodium and chloride absorption.

BSS Plus: a potential irrigating solution for neurosurgery

BSS Plus is a pH-stable balanced salt solution similar to glutathione bicarbonate Ringer's solution. Extensively used in ophthalmology, it is of potential value in neurosurgery. In comparative tests of its effectiveness, 28 cats underwent bilateral irrigation of the surface of the cerebral cortex with normal saline on one side and BSS Plus on the other. After 2 hours, a marked decrease was seen in the surface pH of the hemisphere irrigated with normal saline but not of the hemisphere treated with BSS Plus. Blood-brain barrier changes (measured with Evans blue dye techniques) were more evident following saline irrigation. Somatosensory evoked potentials and cerebral blood flow were not significantly altered. Conventional light microscopy using three standard stains did not reveal a significant difference. Transmission electron microscopy studies were performed in 14 animals and scanning electron microscopy in six. In five animals both transmission and scanning electron microscopy studies were conducted after irrigation with both agents without a cottonoid cover and with immediate harvest of superficial layers from the living brain and immersion-fixation in glutaraldehyde. Tissue preservation was superior on the BSS Plus side in all studies. This agent may represent an improved irrigation solution for neurosurgery, but further studies are required.

Myo-Inositol metabolism and (Na+ + K+)-ATPase activity in relation to peripheral nerve function in rats with streptozotocin-induced diabetes

Experimental diabetes in the rat is associated with a well-characterized decrease in motor nerve conduction velocity (Gillon et al., 1983). In rat sciatic nerve, induction of diabetes results in marked elevation of the concentrations of glucose, sorbitol and fructose, whereas the concentration of myo-inositol is reduced (Palmano et al., 1977). Diabetic nerve malfunction may occur as a result of a relationship between the increased polyol pathway activity, the decreased free myo-inositol concentration and an alteration in the activity of the enzyme (Na' + K+)ATPase. The present study was designed to examine the possibility of such a correlation and to investigate a report by Greene & Lattimer (1983) that (Na+ + K+)ATPase activity is lower in sciatic nerve homogenates from diabetic rats compared with that in homogenates from control rats. The steady-state rate of energy utilization is known to be decreased in diabetic nerves (Greene & Winegrad, 1981). This study compares the rates of ouabain-sensitive 86 Rb+ uptake by endoneurial preparations from diabetic and control rats. Diabetes was induced in male Wistar rats (253-314 g) by a single intraperitoneal injection of streptozotocin (60 mg/kg body wt.) in 0.2 ml of 50 mM-sodium citrate buffer. Age- and weight-matched controls received a similar volume of buffer alone. Animals whose blood glucose concentration exceeded 16 mM 2 days after injection were included as diabetics in the study. Diabetic and control animals were divided into two groups and used at either 4 or 6 weeks after injection. Sciatic nerves were removed from the rats under pentobarbitone anaesthesia (60 mg/kg body wt., by intraperitoneal injection). Two ligatures (with 5/0 grade silk) were applied to each nerve before excision, one at the sciatic notch and the other approx. 4cm distal to the first. The epineurium and the perineurial membrane were removed from the ligated nerves by the method of Gillon & Hawthorne (1 983) and two more ligatures were applied at the centre of the resulting endoneurial preparation. The nerve was bisected between these ligatures to yield two endoneurial preparations from each sciatic nerve. Endoneurial preparations were incubated at 37°C for 20 min in Krebs-Henseleit bicarbonate Ringer solution (Krebs & Henseleit, 1932) containing 4% (w/v) bovine serum ium ion-dependent adenosine triphosphatase. Abbreviations used: (Na+ + K+ )-ATPase, sodium ion-plus-potass

PH sensitivity of the basolateral membrane of the rabbit proximal tubule

Conventional microelectrodes were used to study the effects of bath pH and bicarbonate concentrations on the basolateral membrane potential (Vbl) of cells from the superficial proximal convoluted (PCT) and proximal straight (PST) tubules of the rabbit kidney perfused in vitro. Bathing solution pH was varied over the range of 5.9-7.4 using either control (22-25 mM) or low bicarbonate (5.0-6.6 mM) Ringer solutions and the appropriate CO2 tensions. The results show a strong pH dependence of the steady-state values of Vbl in both the convoluted and straight tubule segments. The pH-dependent depolarization was approximately 35 mV/pH unit change of the bathing solution in the acid direction and could be demonstrated in CO2-free HEPES-buffered solutions. A depolarizing response to increased bath potassium concentration (HK) was observed that was linearly related to the absolute value of the Vbl under control conditions. Under acidotic conditions, reduced HK depolarizations indicate that a decrease in the relative potassium permeability of the basolateral membrane is the principle mechanism underlying the effects of bath pH on Vbl.

New observations on bicarbonate-pH effects on thickness changes of rabbit corneas under silicone oil in vitro.

Specular microscopy was used to measure thickness changes of stroma-endothelium preparations from rabbit corneas in vitro. The preparations were first bathed on both sides for 90 min with different bicarbonate-Ringer solutions (2 to 50 mM bicarbonate) maintained in equilibrium with 5% CO2-air (pH 6.2 to 7.9). During this equilibration, the stroma attained a stable thickness that was inversely related to the hydrostatic pressure (20 to 100 cm H2O) applied to the endothelial surface. After equilibration and at 20 cm H2O pressure, covering the anterior stromal surface with silicone oil (Dow Corning 200 dielectric fluid, 20 cs viscosity) resulted in stromal thinning. The rate of this deturgescence increased (from 5 to 75 micron/h) as the equilibration solution bicarbonate level increased from 2 to 30 mM. The net size of the thickness change was also related to the equilibration bicarbonate level. However, indirect studies on the cornea using phenol red indicator and pH electrode measurements of solutions revealed that the stromal bathing solutions became more alkaline under the silicone oil layer. CO2 is soluble in silicone oil. Implications of these CO2 and pH effects on mechanisms of corneal deturgescence are discussed.

Effects of adenosine on renin release from isolated rat glomeruli and kidney slices.

Adenosine produced by the macula densa cells in response to changes in the tubular NaCl-concentration has been suggested to inhibit renin release in vivo. In order to test this suggestion we studied: incubated kidney cortical slices (KS) which contain both the macula densa and the entire afferent arteriole; superfused single microdissected glomeruli (LAG) without macula densa but with the afferent arteriole preserved; and superfused batches of selected glomeruli (SAG) containing only the juxtaglomerular cells closest to the glomerulus. For superfusion and incubation a bicarbonate Ringer solution was used. The specificity of the renin release process was validated by measuring adenylate kinase as a marker for cytoplasmatic leak. Adenosine (10 micrograms/ml) halved basal renin release from incubated KS as compared to controls (P less than 0.001, n = 8, 8). Renin release from LAG stimulated by calcium depletion was also inhibited (P less than 0.05, n = 8, 9) whereas basal release was not affected (n = 6, 12). No effect was detected neither on basal nor on calcium stimulated renin release from SAG. We conclude that adenosine inhibits renin release in vitro by a mechanism independent of a functioning nephron, and which involves only the JG-cells located in the afferent arteriole at some distance from the glomerulus.

Effects of bilirubin on transepithelial transport of sodium, water, and urea

Urinary concentrating defects and renal salt wasting have been described in the hyperbilirubinemic Gunn strain strain of rat. Homozygous animals demonstrate significant reductions in renal medullary urea and sodium ion concentrations. These observations are consistent with possible bilirubin associated disorders in the transepithelial transport of water and solute. To test this hypothesis, measurements of active sodium transport and passive water and urea fluxes were made in hemibladders isolated from the Dominican toad, Bufo marinus. Tissues were exposed to amphibian bicarbonate Ringer's solution containing 0.1 mM bilirubin with 0.05% bovine serum albumin (BSA) or BSA alone. Vasopressin-stimulated sodium transport, as reflected by short circuit current (SCC), was inhibited by 18 +/- 6% in the presence of bilirubin (N = 10; P less than 0.02). Cyclic AMP (p-Cl-phenylthio cAMP 10(-5) M) stimulated SCC was inhibited to a similar degree in the presence of bilirubin. The inhibition was noted only when bilirubin was in the serosal bath, and it could be abolished with BSA 0.5%. Bilirubin had no effect on the increase in SCC induced by higher concentrations of cyclic AMP (10(-4) M), aldosterone, or amphotericin B. Furthermore, bilirubin had no effect on the hydro-osmotic response to vasopressin and vasopressin-induced changes in urea permeability. These findings show that short-term exposure to bilirubin exerts a tissue-specific effect on the vasopressin-stimulated active transport of sodium but has no effect on the vasopressin-induced fluxes of water and urea.

A new preparation for microcirculatory studies of the hamster cheek pouch.

Existing methods of preparing the hamster cheek pouch for observation under an intravital microscope have several disadvantages. The everted method, described by Duling (Microvasc. Res. 5: 423-429, 1973), appears to restrict blood flow by placing unnatural tension on the retractor muscle and by requiring an incision in the tip of the pouch. The method of Yamaki et al. (Microvasc. Res. 21: 299-301, 1981) requires an incision in the tip of the pouch and complete disconnection of the retractor muscle. The chamber method of Greenblatt et al. (Microvasc. Res. 1: 420-432, 1969) has a limited optical resolution because the tissue cannot easily be transilluminated with properly condensed light. We have devised a less traumatic method of preparing the pouch, which eliminates these disadvantages. The hamster is anesthetized, and a thin, glass support plate is inserted into the left cheek pouch. The plate is constructed and positioned so that it does not restrict flow to any part of the pouch. The free end of the plate is secured to the animal stage. An incision is made in the skin to expose the cheek pouch, and the loose, avascular connective tissue investing the pouch and the retractor muscle is removed. The pouch is positioned at an angle of 20 degrees from the hamster's body in a temperature-controlled chamber over a standard microscope condensor system. Throughout both surgical and experimental procedures, the pouch is superfused with Ringer-bicarbonate solution at 37.5 degrees C. This preparation minimizes surgical trauma and allows the entire vascular supply to the cheek pouch to be studied.

Cell pH of rat renal proximal tubule in vivo and the conductive nature of peritubular HCO3- (OH-) exit.

Intracellular pH (pHc) was measured on surface loops of rat kidney proximal tubules under free-flow conditions in vivo using fine tip double-barrelled pH microelectrodes based on a neutral H+ ligand. The microelectrodes had Nernstian slopes and a resistance of the order of 10(12) omega. By using a driven shield feed back circuit the response time to pH jumps was lowered to around 1 s. At a peritubular pH of 7.42 and a luminal pH of 6.68 +/- 0.13 (n = 27), pHc was 7.17 +/- 0.08 (n = 19). Perfusing the peritubular capillaries suddenly with bicarbonate Ringer solutions of plasma-like composition which were equilibrated with high or low CO2 pressures, acidified or respectively alkalinized the cells rapidly as expected from the high CO2 permeability of the cell membranes. Such data allowed us to calculate the cytoplasmic buffering power of the tubular cells. Sudden peritubular perfusion with Ringer solution containing only 3 mmol/l of HCO3- at constant physiological CO2 pressure led to a similar fast cell acidification which indicated that the peritubular cell membrane is also highly permeable for bicarbonate or OH- (H+). The latter response was completely blocked by the stilbene derivative SITS at the concentration of 10(-3) mol/l. The observations indicate first that pHc of rat proximal tubule is more acidic than was previously thought on the basis of distribution studies of weak acids, second that intracellular bicarbonate concentration is around 13 mmol/l and third that bicarbonate exit across the peritubular cell membrane is a passive rheogenic process via a conductive pathway which can be inhibited by SITS.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactive Effects of Ethanol and Silver on Sodium Transport across Toad Skin

Both ethanol and silver ions have been shown to affect ion transport across various epithelia. This investigation was principally undertaken to further define mechanisms of silver ions and ethanol, and their possible interactions, on sodium transport across toad skin. Isolated toad skin, mounted between identical oxygenated amphibian bicarbonate Ringer solutions, maintained stable transepithelial potential differences (serosa positive) and short-circuit currents for several hours at 25 degrees C. It was observed that (1) ethanol inhibited the active transcellular component of sodium absorption and this effect was reversible; (2) inhibition of sodium transport by ethanol was directly proportional to the applied concentration; (3) pretreatment with silver ions prevented any ethanol effects; and (4) pretreatment with ethanol prevented any silver ion effects. It was concluded from these results that ethanol induced its inhibitory effects on membrane phospholipids thereby perturbing the function of a sulfhydryl ligand, while silver ion or silver chloride complex binding to this ligand would maintain its function in sodium transport despite the presence of ethanol.

Mechanisms of adverse effect of air-oxidized soy bean oil-feeding in rats.

In order to clarify the mechanism of the adverse effects of air-oxidized oil feeding, the effects of dietary soy bean oil on body weight gain, food consumption, fecal consistency and gastrointestinal functions were investigated in rats. Feeding of oxidized soy bean oil with a peroxide value above 350 produced significant reductions in body weight gain, food consumption and intestinal sucrase activity, and severe diarrhea. These adverse effects were prevented with the concurrent feeding of Gobo dietary fiber and were rapidly eliminated within a day after switching to the diet containing the unoxidized soy bean oil. Furthermore, the remarkable release of sucrase from the small intestine was observed on perfusion of the jejunum with Ringer's bicarbonate solution containing the oxidized soy bean oil at the 1% level. These findings suggest that the adverse effects occurring after air-oxidized oil feeding are due to a disturbance of gastrointestinal functions, and raise the possibility that the primary cause might be solubilization or exfoliation of the brush border membrane.

Differential Vulnerability of Large and Small Mammalian Myelinated Fibers to Glucose Lack

The excitability of A fibers of peripheral nerves in vitro is more susceptible to depression by energy lack than is excitability of C fibers. Whether there is also a differential vulnerability between small myelinated A delta fibers that conduct fast pain and large myelinated A beta fibers is unclear. We evaluated the relative abilities of A beta and A delta fibers to withstand energy lack in rabbit vagus nerve by measuring the amplitudes and latencies of the compound action potentials during incubation in glucose-free Ringer's-bicarbonate solution. We also evaluated the reversibility of the effects after returning the nerves to glucose containing solution. We found that A delta fibers are more susceptible to such energy lack and recover from it less completely than A beta fibers.

Effect of volume expansion and plasma chloride on function of the loop segment.

To determine the effect of acute volume expansion and changes in plasma chloride on fluid and chloride uptake in superficial loop segments of rats, this segment was microperfused in vivo at 22 nl/min with a fluid containing Na 145, Cl and 36Cl 130, and HCO3 15 meq/liter during hydropenia and after acute volume expansion with 0.15 M NaCl, 0.15 M NaHCO3, or an isotonic bicarbonate Ringer (Cl 106 meq/liter) solution. Fractional fluid, chloride, and 36Cl reabsorption and early distal chloride concentration did not change during maintained hydropenia (time control) or during volume expansion with NaCl (plasma chloride 120 meq/liter) or bicarbonate Ringer solution (plasma chloride 104 meq/liter). Absolute and fractional reabsorption of chloride and 36Cl increased, without change in fluid reabsorption, and early distal chloride diminished after infusion of NaHCO3 (plasma chloride 90 meq/liter). It is concluded that acute volume expansion, per se, has no effect on either net fluid or net chloride absorption in the superficial loop segment at the load studied. Hypochloremia is associated with increased net reabsorption of chloride and an increased unidirectional efflux of chloride from the loop segment during acute volume expansion, most likely due to a gradient effect on the thick ascending limb of the loop of Henle.

Active electrogenic mechanisms for alkali and acid transport in turtle bladders.

Immediately after mounting in the Ussing chamber between choline bicarbonate Ringer solutions devoid of exogenous Na and Cl, the serosal fluid is electronegative to the luminal fluid in bladders from postabsorptive and acidotic turtles; and electropositive in bladders from alkalotic turtles. In bladders from postprandial turtles, the electrical orientation, initially serosal positive, reverses to serosal negative. Serosal additions of 3-isobutyl-1-methylxanthine (IBMX) and adenosine 3',5'-cyclic monophosphate (cAMP) produce no changes in the negative short-circuiting current (Isc) of acidotic turtles but induce large positively-directed increases of Isc in bladders from other turtle groups. With IBMX and cAMP in the (HCO3 + CO2)-rich serosal fluid at pH 7.2 and with luminal pH maintained at 4.0-5.0, the rate at which titratable alkali enters the luminal fluid is electrochemically equal to the positive Isc; and this increased positive Isc is the same as that in the absence of transepithelial gradients. The effects of acetazolamide and 4-acetamido-4-isothiocyanostilbene-2,2'-disulfonic acid on positive and negative Isc are presented. It is concluded that isolated bladders from alkalotic, postprandial or postabsorptive turtles, but not those from acidotic turtles, possess an active electrogenic mechanism for a Na-independent Cl-independent secretion of bicarbonate. This transport process is accelerated by phosphodiesterase inhibitors (IBMX) and cAMP or its eight substituted derivatives.