An in vivo microperfusion study of distal tubule bicarbonate reabsorption in normal and ammonium chloride rats.

For many years it has been thought that distal nephron hydrogen ion secretion can be importantly modulated by factors such as sodium delivery, sodium avidity, and potassium stores. Free flow micropuncture studies have also indicated that the rate of bicarbonate delivery may also alter the rate of bicarbonate reabsorption. The present studies were undertaken to examine possible luminal influences on total CO2 reabsorption in microperfused distal tubules in the rat in vivo. Tubules from normal and acidotic rats were perfused with five solutions in a manner that induced changes in bicarbonate load, sodium and potassium fluxes (JNa, JK), and luminal sulfate concentration. in each collected perfusate, simultaneous analyses were undertaken to determine water reabsorption, Na, and K concentrations using graphite furnace atomic absorption spectroscopy and total CO2 by microcalorimetry. Using factorial analysis of covariance to account for confounding effects on total CO2 flux (JtCO2) such as water reabsorption, distal tubules of acidotic rats reabsorbed CO2 in the range of 50-112 pmol X min-1 X mm-1 X These JtCO2 values were not significantly correlated with HCO3 load, JNa, or JK despite changes in the latter from net reabsorption to net secretion. Distal tubules of rats with normal acid-base status had JtCO2 values which were neither significantly different from zero nor correlated with changes in JK and JNa. Further, doubling the load from 250-500 pmol/min (by doubling the perfusion rate of 25-mM HCO3 solutions) did not stimulate JtCO2 in these normal animals. Accordingly, these acute in vivo microperfusion studies indicate for the first time that neither load nor potassium or sodium fluxes are important modulators of distal tubule bicarbonate reabsorption.

Bicarbonate secretion in vivo by rat distal tubules during alkalosis induced by dietary chloride restriction and alkali loading.

To examine in vivo the separate effects on distal tubule JtCO2, of dietary chloride restriction, bicarbonate loading, and changes in luminal chloride concentration, we microperfused distal tubules at a physiologic flow rate (8 nl/min) with solutions containing either 45 or 0 mM chloride (after gluconate substitution). Rats were fed a diet containing zero, minimal, or normal amounts of chloride, while drinking either water or a solution of 0.15 M sodium bicarbonate. Neither extracellular fluid volume contraction nor negative chloride balance ensued. Analysis of covariance with repeated measures demonstrated that dietary chloride, drinking sodium bicarbonate, and perfusion with either 45 mM or zero chloride, each have separate and significant modulating effects on distal tubule bicarbonate secretion. During mild alkalemia, there is modest bicarbonate secretion which is significantly different from zero (-9.9 +/- 3.2 pmol.min-1.mm-1, P less than 0.01), and which is suppressed after perfusion with zero chloride. In contrast, during more pronounced metabolic alkalosis after supplemental bicarbonate drinking, the bicarbonate secretory flux is brisk (-26 +/- 3 pmol.min-1.mm-1) and significantly different from zero and persists (-11 +/- 3 pmol.min-1.mm-1) even during perfusion with zero luminal chloride. Accordingly, in this two-day model of alkalosis induced by dietary chloride restriction, there is regulatory secretion of bicarbonate by distal tubules in vivo which is modulated by luminal chloride concentration.

Luminal chloride modulates rat distal tubule bidirectional bicarbonate flux in vivo.

The effects of replacing luminal chloride with gluconate on distal tubule bicarbonate transport were studied in vivo in normally fed rats, overnight-fasted rats, and rats made mildly alkalotic by administration of desoxycorticosterone acetate (DOCA). In paired microperfusions of the same tubule with 0 or 55 mM Cl at 25 nl/min, net secretion of bicarbonate by distal tubules of fed rats was inhibited by chloride replacement. Zero chloride perfusion in DOCA rats also resulted in an inhibition of net bicarbonate secretion at 25 nl/min. In contrast, replacement of 45 mM chloride also perfused at 25 nl/min in fasted rats caused an increase in net bicarbonate reabsorption. To further characterize the effects of changes in luminal chloride, experiments were undertaken in fasted rats with 0, 45, and 100 mM chloride-containing solutions perfused at 8 and 25 nl/min. Perfusion with zero Cl resulted in net bicarbonate reabsorption at 8 nl/min that increased markedly with high flow, whereas bicarbonate reabsorption did not change significantly during perfusion at high flow with a 45-mM Cl perfusate. In marked contrast, perfusion with a 100-mM Cl solution resulted in only minimal bicarbonate reabsorption at 8 nl/min with significant secretion observed at high flow. Thus, chloride-free perfusates inhibit bicarbonate secretion and enhance bicarbonate reabsorption, while high chloride perfusates elicit net bicarbonate secretion in usually reabsorbing distal tubules.

Secretion of bicarbonate by rat distal tubules in vivo. Modulation by overnight fasting.

We have performed microperfusion studies on distal tubule bicarbonate reabsorption (JtCO2) of fed and fasted rats to extend our previous observations of in vivo bicarbonate secretion and to resolve certain discrepancies between free-flow and microperfusion data. When rats are fasted overnight, as in previous free-flow studies, distal tubule microperfusion with a 28-mM tCO2 solution results in significant JtCO2 (53 +/- 6 pmol.min-1.mm-1) at normal flow and increases briskly (91 +/- 16 pmol.min-1.mm-1) with bicarbonate load. This response is not influenced by the addition of other normal tubular fluid constituents. However, when normally fed rats are used, as in our previous microperfusion studies, distal tubule JtCO2 is not different from zero when a 28-mM tCO2 solution is perfused at normal flow rates but becomes negative (-54 +/- 13 pmol.min-1.mm-1) at high flow rates, which indicates the existence of bicarbonate secretion against a concentration gradient. Alkali loading of fasted rats also elicits bicarbonate secretion at high flow. These results demonstrate for the first time that normal feeding or alkali loading can induce bicarbonate secretion in a mammalian nephron segment in vivo, and resolves previous discrepancies between free-flow and microperfusion data.

The effects of chronic hypoxemia on electrolyte and acid-base equilibrium: an examination of normocapneic hypoxemia and of the influence of hypoxemia on the adaptation to chronic hypercapnia.

We have carried out balance studies in normal dogs in order to appraise the effects of chronic hypoxemia on acid-base and electrolyte equilibrium. During the first phase of observation we produced a state of "pure" hypoxemia by reducing the oxygen concentration (utilizing nitrogen as a diluent) and by adding carbon dioxide to the environment in a concentration sufficient to keep arterial CO(2) tension (PCO(2)) within normal limits. The data demonstrate that such a 9-day period of normocapneic hypoxemia has no effect on electrolyte excretion and is virtually without effect on plasma composition. During the second phase of observation we subjected the hypoxemic dogs to stepwise increments in arterial carbon dioxide tension in order to evaluate the effects of the low oxygen tension on the acid-base adjustments to a chronic state of hypercapnia. At least 6 days was allowed for extracellular composition to reach a new steady state at each level of inspired carbon dioxide. The data demonstrate a rise in both plasma bicarbonate concentration and renal acid excretion that was not significantly different from that which has been described previously for hypercapnia without hypoxemia. Just as in these earlier studies, plasma hydrogen ion concentration rose with each increment in carbon dioxide tension, each millimeter Hg increment in PCO(2) leading to an increase in hydrogen ion concentration of 0.32 nmole per L. It thus appears that the chronic"carbon dioxide response curve" is not significantly influenced by moderately severe hypoxemia.

In vivo evidence of impaired solute transport by the thick ascending limb in potassium-depleted rats.

The objective of this investigation was to determine if thick ascending limb (TAL) solute removal is impaired in potassium-depleted rats, in vivo. We estimated TAL NaCl concentration by measuring in situ conductivity of tubular fluid presented to the early distal site after stop-flow periods of 10-60 s, during which a proximal equilibrium solution remained in contact with the reabsorbing epithelium. This allowed us to calculate the rate constant of the decrease in tubular fluid NaCl concentration and to determine equilibrium values for control, potassium-depleted, and potassium-repleted rats. After 60 s of stop-flow, NaCl concentration of TAL fluid decreased to 18.3 +/- 2.73 mM in control rats, while potassium-depleted rats had values almost twice as high (36.5 +/- 2.97 mM, P less than 0.01). The amount of NaCl remaining after 60 s of stop-flow in K-depleted rats was highly correlated with the plasma K concentration. Calculated rates of NaCl efflux from the TAL appeared to be normal in K-depleted rats while the concentration of NaCl achieved at equilibrium was nearly twice that measured in control rats. Acute systemic administration of KCl by gavage or infusion in K-depleted rats was associated with a decrease in TAL NaCl concentration to normal values. Addition of K to the perfusate, however, did not repair the defect. Our results can best be explained by assigning a special role to the peritubular K concentration. We suggest that the defect in TAL solute removal in K-depletion can be rapidly reversed, because decreases in peritubular K concentration limit Na efflux across the peritubular membrane by decreasing the activity of the Na-K-ATPase pump. We recognize that factors such as regional renal blood flow, local angiotensin II levels, and products of the cyclo-oxygenase enzyme system may play a role.

Role of angiotensin II in dietary modulation of rat late distal tubule bicarbonate flux in vivo.

We have reported that overnight fasting stimulates bicarbonate reabsorption (JtCo2) in rat distal tubules. The present in vivo microperfusion studies evaluated the hypothesis that endogenous angiotensin II (AII) mediates this response. Rat late distal (LD) tubules were perfused at 8 nl/min in vivo with a hypotonic solution containing 28 mM bicarbonate. In overnight-fasted rats, LD JtCO2 was significantly higher than in normally fed rats (50 +/- 4 vs. 16 +/- 6 pmol/min.mm, P < 0.05). When overnight-fasted rats were salt-loaded, JtCO2 fell significantly (38 +/- 3 pmol/min.mm, P < 0.05). Conversely, in fed rats ingesting a zero-salt diet, JtCO2 increased three-fold (45 +/- 5 pmol/min.mm, P < 0.05). Enalaprilat infusion (0.25 micrograms/kg body wt, intravenously), in these zero-salt and overnight-fasted rats, reduced LD JtCO2 values to normal. Further, infusion of losartan (5 mg/kg body wt, intravenously), the specific AII AT1 receptor blocker, reduced JtCO2 in overnight-fasted rats by two-thirds (16 +/- 4 pmol/min.mm, P < 0.05). Finally, we perfused 10(-11) M AII intraluminally with and without 10(-6) M losartan: AII increased JtCO2 to 45 +/- 6 pmol/min.mm, equal to the zero-salt flux. This was completely abrogated by simultaneous losartan perfusion. Therefore, these results suggest that AII is an in vivo stimulator of late distal tubule bicarbonate reabsorption.

Distal tubule bicarbonate accumulation in vivo. Effect of flow and transtubular bicarbonate gradients.

We have performed microperfusion studies on distal tubules of normal and alkalotic rats in an attempt to demonstrate in vivo bicarbonate secretion. All perfusion solutions were free of phosphate and other nonbicarbonate buffers. In both normal and alkalotic rats, distal perfusions elicited significant tCO2 entry only at high flow (24 nl/min). Even when perfusate tCO2 concentration closely matched plasma tCO2 concentration (30 mM tCO2), significant tCO2 entry again occurred at high flow. This was associated with a rise of the perfusate tCO2 concentration, which indicated net entry of tCO2 against a concentration gradient. In this "symmetrical" perfusion situation, acetazolamide blockade prevented tCO2 entry. Accordingly: distal tubule tCO2 entry is demonstrable in both alkalotic and normal rats at high flow rates; increasing perfusate tCO2 concentration can suppress tCO2 entry; and entry can occur in the absence of a gradient and this effect can be blocked by acetazolamide.

Proximal bicarbonate reabsorption during Ringer and albumin infusions in the rat.

Several studies have clearly shown that extracellular volume expansion is associated with suppression of whole kidney bicarbonate reabsorption, although little is known concerning the single nephron correlates of this response. More recently, attention has also been focussed on bicarbonate transport in attempts to identify a possible role for this ion in enhancing the rate of net fluid efflux by proximal tubules. To further explore proximal tubular bicarbonate handling in the rat, we carried out recollection micropuncture studies to assess the effects of infusions of modified Ringer or salt-poor hyperoncotic human albumin. With stable levels of arterial PCO2, plasma [HCO3-] or plasma [K+], marked suppression of fractional HCO3- reabsorption occurred: during Ringer infusion fractional reabsorption fell by 31% (P less than 0.001) while during albumin infusion a decrease of 20% (P less than 0.001) was observed. Despite this, absolute net HCO3- reabsorptive rates did not change significantly. Simple and partial correlation analysis of single tubular responses revealed strong linkage effects between changes in absolute net reabsorptive rates for HCO3- and H2O in both types of infusion; the partial r was 0.91 (P less than 0.001) and 0.94 (P less than 0.001) during Ringer and albumin infusions, respectively. We conclude that under these free-flow conditions, Ringer and albumin infusions do not suppress absolute net HCO3- reabsorption by proximal tubules, and that strongly linked changes in absolute HCO3- and H2O fluxes are characteristic of both protocols.

Loop of Henle bicarbonate accumulation in vivo in the rat.

We have carried out perfusion studies on hydropenic and bicarbonate-loaded rats to provide direct in vivo observations on bicarbonate accumulation in the short loops of Henle. Analysis of early distal tubular fluid was made during bicarbonate-free saline perfusion from the end proximal to the early distal site, documenting accumulation of "new" bicarbonate. During perfusion in hydropenic rats, steady-state bicarbonate concentrations were suggested by early distal values of approximately equal to mM, which were independent of perfusion rate and virtually indistinguishable from bicarbonate concentration measured during free flow when filtered bicarbonate was allowed to enter the loop. Thus, loop bicarconate accumulation was apparently sufficient to allow new bicarbonate to enter at a rate comparable to that delivered to the early distal site during free flow, recognizing of course that free-flow delivery rates are the result of complex components of filtration and bidirectional fluxes. In bicarbonate-loaded rats, however, bicarbonate accumulation rates although higher than in hydropenia, were much lower than free-flow delivery rates. Furthermore, early distal bicarbonate concentrations during bicarbonate loading fell as perfusion rate increased, presumably because of a limitation to increasing ionic bicarbonate entry.

Differentiation of renal intercalated cells in fetal and postnatal rats.

An ultrastructural study was conducted on the kidneys from rat fetuses and pups from ages ranging from birth to 8 weeks to identify the time of appearance of each of the two intercalated cell types. With transmission electron microscopy. A-intercalated cells were recognized by their large apical microvilli and microplicae as well as by the numerous subapical vesicles. Their identification was confirmed by the presence of typical studs at the cytoplasmic face of the apical plasma membrane. By scanning electron microscopy the cells were recognized by their typical microplicae at the apical surface. In 19-day-old fetuses and newborns. A-intercalated cells were numerous in the epithelium lining the renal pelvis and inner medullary intercalated ducts. Two weeks after birth they disappeared from these regions but became numerous at the outer medullary collecting ducts and also at the cortical collecting ducts although to a lesser degree. B-intercalated cells were recognized by the scarcity of microvilli, the absence of microplicae, and the large number of basal infoldings. Their identification was confirmed by the presence of studs at the cytoplasmic face of the basolateral membrane. B-cells started to appear 3 weeks after birth and increased thereafter. We speculate that the particular stages at which the two cell types differentiate might be related to changes in acid-base status.

Poor ultrafiltration during nighttime dialysis in CAPD patients and its effects on fluid balance.

To evaluate fluid retention during the long nighttime peritoneal dwell in continuous ambulatory peritoneal dialysis (CAPD) patients, we measured remaining volumes in 70 patients. In only 50% of these patients were more than 2 L of fluid recovered; in 30% between 1.5 and 2 L were recovered; and in 17% of patients we retrieved less than 1.5 L of peritoneal fluid. In 3 of these patients, who were edematous and had marked pitting edema, we shortened the nighttime dwell by having the patients awaken after 4 hours and drain the dwell. This resulted in 3-5 kg of weight loss in each patient, when compared with each patient's previous use of long nighttime dwells. Finally, we propose in this report two automated methods whereby the period of nighttime dialysis can be controlled, while patients sleep, using a system of timer clamps.

Ciguatera: current concepts.

Ciguatera poisoning develops after ingestion of certain coral reef-associated fish. With travel to and from the tropics and importation of tropical food fish increasing, ciguatera has begun to appear in temperate countries with more frequency. The causative agents are certain varieties of the protozoan dinoflagellate Gambierdiscus toxicus, but bacteria associated with these protozoa may have a role in toxin elaboration. A specific "ciguatoxin" seems to cause the symptoms, but toxicosis may also be a result of a family of toxins. Toxicosis develops from 10 minutes to 30 hours after ingestion of poisoned fish, and the syndrome can include gastrointestinal and neurologic symptoms, as well as chills, sweating, pruritus, bradycardia, tachycardia, and long-lasting weakness and fatigue. More severe features are rare. In this review, the pathophysiologic features and symptoms of ciguatera are reviewed and compared with those of other seafood-related syndromes. Although no definitive therapy is known, the most promising treatment for ciguatera is intravenous administration of mannitol. Physicians should warn patients who are traveling to endemic areas about this toxicosis.

Burning pain in an extremity. Breaking the destructive cycle of reflex sympathetic dystrophy.

The pathogenesis of reflex sympathetic dystrophy is controversial, but the condition can result from a major or seemingly minor injury to a limb, or even an insult to an organ, such as stroke or myocardial infarction. Onset can be sudden or insidious. The syndrome is characterized primarily by localized, deep, burning pain in a limb--pain that may not follow any logical distribution. Nonpitting edema, skin hyperesthesia, and guarding of the limb usually accompany the pain. If treatment is not instituted, deformity, contracture, and wasting of the limb can eventually occur. With appropriate therapy, the process can be stopped and often reversed. The keys are a high index of suspicion, early diagnosis, and aggressive treatment.

Interstitial cystitis. An overlooked cause of pelvic pain.

Interstitial cystitis is a disease primarily of young and middle-aged women that is characterized by pelvic pain, urinary frequency, and dyspareunia. Its cause is unknown, but defects in the protective glycosaminoglycan layer of the bladder mucosa may be responsible. The diagnosis is mainly one of exclusion. Cystoscopy reveals characteristic glomerulations in the bladder mucosa. Of the available treatments, the most common are intermittent hydrodilation of the bladder and intermittent intravesical instillation of dimethyl sulfoxide. Other methods and medications are currently under investigation. Although interstitial cystitis is uncommon, its potentially devastating effects may be modified or even averted if primary care physicians are familiar with its presentation and maintain a high index of suspicion.