Alpha 1-adrenergic control of chloride transport in the rat S1 proximal tubule.

To identify in vivo the specific alpha-adrenergic receptor mediating direct neural control of chloride transport in the rat S1 proximal convoluted tubule (PCT), the major effector site of neural regulation, microperfusion was employed in conjunction with the alpha 1- and alpha 2-adrenergic receptor antagonists, prazosin and rauwolscine. Using a glomerular ultrafiltrate-like perfusate, prazosin markedly inhibited chloride transport by -42% (302 +/- 10 to 176 +/- 5 peq.mm-1.min-1, P < 0.0001). Using a sodium chloride perfusate, which measures the active component of chloride absorption (J(Cl)act) (control, 153 peq.mm-1.min-1) plus a constant passive (479 peq.mm-1.min-1) component, both prazosin and acute renal denervation reduced J(Cl)act by -38% and -44% (-58 and -67 peq.mm-1.min-1, each P < 0.05). In contrast, rauwolscine caused no significant change in J(Cl)act using either perfusate. Prazosin regulates chloride transport via protein kinase C (PKC), since preactivation of PKC by phorbol abolished inhibitory impact of prazosin. Inhibition of J(Cl)act by prazosin (-58 peq.mm-1.min-1) was fully additive to either the stimulation or inhibition (losartan) of angiotensin II (55 or -49 peq.mm-1.min-1), which uses the adenosine 3',5'-cyclic monophosphate (cAMP) second messenger system [observed changes, not significantly different from 0 and -99 peq.mm-1.min-1; expected changes, not significantly different from 0 and -107 peq.mm-1.min-1]. In conclusion, neural control of S1 PCT chloride absorption in vivo is mediated by alpha 1-adrenergic receptors, which can selectively regulate J(Cl)act by altering PKC activity, independently of the cAMP second messenger system.

Flow dependence of chloride transport in rat S1 proximal tubules.

These studies examined whether the luminal flow dependency of chloride absorption in the S1 proximal tubule during glomerulotubular balance was due to change in active and/or passive transport of chloride. Using in vivo microperfusion in the Munich-Wistar rat and an essentially pure sodium chloride perfusate (devoid of bicarbonate and organic solutes), we found that an increase in luminal perfusion rate from 30 to 45 nl/min caused stimulation of total chloride absorption (active plus passive) by 87 peq.mm-1.min-1 (632 +/- 17 to 719 +/- 11, P < 0.001). When cyanide was added to this perfusate to eliminate active transport, the flow-induced change in passive transport was 58 peq.mm-1.min-1 (479 +/- 9 to 537 +/- 11, P < 0.001). The cyanide-inhibitable active transport component was therefore 29 peq.mm-1.min-1. With elimination of the transepithelial chloride gradient and, hence, passive transport by isethionate substitution, active transport increased by 63 peq.mm-1.min-1 (121 +/- 4 to 184 +/- 7, P < 0.001) as flow rate rose from 30 to 45nl/min. Removal of organic solutes from a glomerular ultrafiltrate-like perfusate had a minimal effect on flow-induced change in chloride transport (190 vs. 207 peq.mm-1.min-1). In conclusion, flow-dependent active and passive chloride transport in the S1 proximal tubule may both contribute to normal glomerulotubular balance.

Kidney function and age are both predictors of pharmacokinetics of metformin.

The effects of renal impairment and age on the pharmacokinetics of metformin were evaluated. The subjects, including 6 young, 12 elderly, and 3 middle-age healthy adults and 15 adults with various degrees of chronic renal impairment (CRI) each were given a single, 850-mg metformin HCl tablet. Multiple whole blood, plasma, and urine samples were collected and analyzed for metformin levels using a high-performance liquid chromatography (HPLC) method. In healthy elderly individuals, the plasma and whole blood clearance/absolute bioavailability values [CL/F and (CL/F)b], and corresponding renal clearance values (CLR and CLR,b) of metformin were 35-40% lower than the respective values in healthy young individuals. These two groups did not differ significantly with respect to volume of distribution (Vd), time to peak concentration (tmax), and parameters related to metformin's appearance in the urine. In the moderate and severe CRI groups, all clearance values were 74-78% lower than in the healthy young/middle-age group, and all other evaluable pharmacokinetic parameters (with the exception of tmax) differed significantly in this group. In the mild CRI group, clearance values of metformin, which were 23-33% lower than in the young/middle-age group, were the only parameters that differed significantly. Based on a regression analysis of the combined data, both creatinine clearance (CL*cr; corrected for body surface area) and age are predictors of metformin clearance, with the following model best fitting the data: CL/F [or (CL/F)b, CLR, CLR,b] = alpha + beta.CL*cr + gamma.CL*cr.age. Metformin should not be used in patients with moderate and severe CRI, or in patients with mild, but not absolutely stable, renal impairment. The initial and maximum doses in elderly patients and patients with stable mild CRI should be lowered to approximately one third that given to the general (i.e., patients without non-insulin-dependent diabetes) population.

Chloride transport in the rat S1 proximal tubule.

In vivo microperfusion was used to elucidate the modes and regulation of the powerful chloride transport system resident in the rat early (S1) proximal convoluted tubule (PCT). From a complete, glomerular ultrafiltrate-like perfusate, omission of organic solutes reduced chloride absorption by 93 peq.mm-1.min-1 (302 +/- 10 to 209 +/- 24, P < 0.001). From a high-chloride perfusate (a relatively pure NaCl solution devoid of bicarbonate and organic solutes), luminal addition of the active transport inhibitor cyanide reduced chloride absorption by 153 peq.mm-1.min-1 (632 +/- 17 to 479 +/- 9, P < 0.001). Active transport was also estimated directly as 121 +/- 4 peq.mm-1.min-1 using a solution in which sodium isethionate isosmotically replaced bicarbonate and organic solutes, preventing development of a chloride gradient. Intravenous angiotensin II caused a stimulation of chloride absorption from a high-chloride perfusate by 55 peq.mm-1.min-1 (632 +/- 17 to 687 +/- 14, P < 0.05), which was partially cyanide-sensitive (510 +/- 6 peq.mm-1.min-1). In conclusion, the components of the normal S1 PCT chloride reabsorption (approximately 300 peq.mm-1.min-1) from the glomerular ultrafiltrate consist of the following: active transport (40-50%), which can be regulated by angiotensin II; sodium-coupled organic solute transport (30%); and passive, chloride concentration gradient-driven transport (20-25%).

Pulmonary edema associated with salt water near-drowning: new insights.

In this case report we describe the clinical and laboratory findings of a man who nearly drowned after aspirating a large quantity of seawater. The aspiration of salt water, which is strongly hypertonic with respect to plasma, resulted in severe pulmonary edema, both from the quantity of aspirated seawater and the osmotically driven ultrafiltrate of plasma that accumulated in the air spaces. The initial concentration of protein in the edema fluid sample was very low, 0.7 g/dl, consistent with only a minimal increase in epithelial permeability. Approximately 4 h later, there was a marked increase in the concentration of protein in the residual alveolar fluid associated with improvement in several clinical indices, indicating that the excess alveolar fluid was reabsorbed very rapidly. In addition, the magnesium concentration was markedly elevated because of the aspiration of magnesium-containing seawater, which may have diagnostic importance for near-drowning in salt water. The data from this case provide evidence for well-preserved alveolar epithelial barrier function after aspiration of large quantities of hypertonic salt water.

Comparison of inhibitory potency by nonpeptide angiotensin II receptor antagonists PD123177 and DuP 753 on proximal nephron and renal transport.

The nonpeptide angiotensin II receptor antagonists, DuP 753 and PD123177, have been recently proposed to efficiently discriminate between two receptor subtypes. Because angiotensin II uses two signaling systems to control transport in the proximal tubule, the hypothesis was entertained that angiotensin-regulated proximal reabsorption might be transduced through two receptor subtypes and, therefore, have two components, sensitive to either DuP 753 or PD123177. Using in vivo microperfusion in the rat, a maximally effective dose of PD123177 (60 or 120 mg/kg, i.v.) had a powerful inhibitory effect on transport in the S1 proximal tubule, significantly (P less than .001) reducing bicarbonate absorption by 50% (360 +/- 4 to 179 +/- 8 peq/mm.min), chloride absorption by 38% (211 +/- 1 to 131 +/- 4 peq/mm.min) and sodium and water absorption by 32% (5.7 +/- 0.1 to 3.9 +/- 0.5 nl/mm.min). These results were only slightly different than previously reported effects on proximal solute and water absorption by DuP 753 (10 mg/kg). The two drugs demonstrated virtually no additivity in their transport inhibitory effects. Although downstream reabsorptive elements compensate for the powerful action of PD123177 in the earliest segment of the nephron, we also showed, using free-flow micropuncture and clearance techniques, that PD123177 induces a substantial diuresis, natriuresis and chloruresis, again similar in magnitude to DuP 753. These results suggest PD123177 has diuretic potency roughly equivalent to that of DuP 753, but do not lend support to the simple thesis that there are two separately acting receptor-signal transduction systems in the proximal tubule for angiotensin II.

Effects of DuP 753 on proximal nephron and renal transport.

We used the nonpeptide angiotensin II receptor antagonist DuP 753, which lacks the agonist and kinin/prostaglandin-inducing properties of saralasin and captopril, respectively, to examine the role of endogenous angiotensin II in regulating transport in the proximal convoluted tubule (PCT) and whole kidney. During in vivo microperfusion in the Munich-Wistar rat, a maximally effective dose of DuP 753 (10 mg/kg, intravenously) powerfully inhibited absorption of bicarbonate (370 +/- 3 to 200 +/- 9 pEq/mm.min, P less than .001), chloride (214 +/- 3 to 105 +/- 9 pEq/mm.min, P less than .001), and water 5.2 +/- 0.1 to 2.8 +/- 0.2 nL/mm.min, P less than .001) in the S1 subsegment of the PCT. DuP 753 was significantly more effective than captopril (3 mg/kg, intravenously) in inhibiting sodium chloride transport and is the most potent diuretic ever described in this segment. Consistent with the axial decline of angiotensin II receptor density in the PCT, DuP 753 was a less effective transport inhibitor in the S2 subsegment of the PCT, similar to captopril. Using free-flow micropuncture and clearance techniques, though inhibition in the earliest segment of the nephron is partially compensated by downstream reabsorption, DuP 753 induces a substantial diuresis, natriuresis, and chloruresis. In conclusion, DuP 753 markedly decreases S1 PCT fluid and electrolyte absorption, indicating that endogenous angiotensin II exerts significant tonic support of proximal transport in vivo.

Proximal nephron and renal effects of DuP 753, a nonpeptide angiotensin II receptor antagonist.

The purpose of these studies was to quantitatively assess the role of endogenous angiotensin II activity in controlling transport in the proximal convoluted tubule (PCT) and whole nephron. We used the nonpeptide angiotensin II receptor antagonist DuP 753, which lacks the agonist and kinin/prostaglandin-inducing properties of saralasin and captopril, respectively. During in vivo microperfusion in the Munich-Wistar rat, we found that DuP 753 had a powerful inhibitory effect on bicarbonate (370 +/- 3 to 200 +/- 9 pEq/mm.min, P less than 0.001), chloride (214 +/- 3 to 105 +/- 9 pEq/mm.min, P less than 0.001), and water (5.2 +/- 0.1 to 2.8 +/- 0.2 nl/mm.min, P less than 0.001) absorption in the S1 subsegment of the PCT. At maximally effective doses, DuP 753 (10 mg/kg i.v.) was significantly more effective than was captopril (3 mg/kg i.v.) in inhibiting sodium chloride transport in the S1 PCT. DuP 753 is the most potent diuretic ever described in this segment. Consistent with the axial decline of angiotensin II receptor density in the PCT, DuP 753 was a less effective transport inhibitor in the S2 subsegment of the PCT, similar to captopril. Though downstream reabsorptive elements partially compensate for the action in the earliest segment of the nephron, we also showed using free-flow micropuncture and clearance techniques that DuP 753 induces a substantial diuresis, natriuresis and chloruresis. In conclusion, the marked decrease in S1 PCT fluid and electrolyte absorption induced by DuP 753 indicates that endogenous angiotensin II exerts significant tonic support of proximal transport in vivo.

Effects of intracellular calcium on proximal bicarbonate absorption.

Using in vivo microperfusion in the proximal convoluted tubule (PCT) of the Munich-Wistar rat, we assessed the impact of raising intracellular calcium concentration on bicarbonate transport and its regulation by angiotensin II. Luminal perfusion with the calcium ionophore A23187 caused a dose-dependent increase in S1 PCT bicarbonate absorption, with a maximal change of 60 peq.mm-1.min-1. Subsequent addition of phorbol ester (PMA) after A23187 further increased transport but to a level no higher than previously shown with PMA alone, and A23187 was nonstimulatory when added after PMA. These results suggest that intracellular calcium affects the same pathway as protein kinase C in stimulating proximal acidification. Pretreatment with A23187 (+/- PMA) attenuated by approximately one-third the bicarbonate absorptive response in the S1 PCT usually observed after angiotensin II administration. A23187 had similar actions, but of smaller magnitude, in the S2 PCT. In conclusion, a rise in intracellular calcium increases bicarbonate absorption in the S1 and S2 PCT in vivo.

Normal diet NaCl variation can affect the renal set-point for plasma pH-(HCO3-) maintenance.

In humans who are ingesting abundant NaCl, blood pH (pHb) and plasma bicarbonate concentration [HCO3-)p) change little or imperceptibly in response to the ingestion of alkali salts. We tested the hypothesis that such tight homeostatic regulation is an artifact of eating a culturally imposed NaCl-enriched diet, not a fundamental physiological trait of humans. In five normal men ingesting a constant acid-producing diet with a low intrinsic NaCl content (0.15 mEq/kg of body weight per day), we measured plasma and urine acid-base composition during four 7-day periods in which the diet was supplemented as follows: no supplements----NaHCO3 only----NaHCO3 plus NaCl----NaCl only. Each sodium supplement was 2.0 mmol/kg body weight per day. With no supplements, pHb was 7.43 +/- 0.005 and (HCO3-)p was 25.0 +/- 0.4 mEq/L. When NaHCO3 only was added, pHb rose 0.02 (to 7.45 +/- 0.004; P less than 0.01) and (HCO3-)p rose nearly 4 mEq/L (to 28.9 +/- 0.6 mEq/L, P less than 0.001). The rise in (HCO3-)p was sustained predominantly by an increased rate of renal bicarbonate reabsorption. When NaCl was added, (HCO3-)p returned to the earlier level, despite continued NaHCO3 supplementation (24.9 +/- 0.6 mEq/L), and remained there when NaHCO3 supplementation was subsequently stopped (24.1 +/- 0.5 mEq/L). Thus, tight homeostatic regulation of plasma acid-base composition in response to a change in dietary base occurred only when dietary NaCl was abundant. To our knowledge, this is the first study in normal humans that demonstrates that diet NaCl variations within the normal range significantly influence plasma acid-base composition.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of angiotensin II in glomerulotubular balance.

Inhibition of angiotensin II activity reduces reabsorption of both bicarbonate and chloride predominantly in the S1 subsegment of the proximal convoluted tubule (PCT). Because the S2 PCT is intrinsically better able to compensate for the increased delivery of bicarbonate compared with chloride under normal conditions, we reasoned that angiotensin II inhibition might selectively raise the amount of sodium chloride emerging from the PCT. Free-flow micropuncture techniques were used in normal and alkalotic Munich-Wistar rats that were euvolemic or plasma volume depleted. In the normal volume-depleted animals, saralasin caused a small rise in single-nephron glomerular filtration rate (29.5 +/- 0.6 to 31.5 +/- 0.6 nl/min, P less than 0.025) and fall in bicarbonate and chloride reabsorption in the 1st mm (S1) PCT (387 +/- 22 to 348 +/- 23 peq.mm-1.min-1, P less than 0.05, and 341 +/- 27 to 217 +/- 57 peq.mm-1.min-1, P less than 0.05, respectively). Reabsorptive compensation by the S2 PCT maintained the end-PCT delivery of bicarbonate unchanged (76 +/- 4 to 78 +/- 3 peq.mm-1.min-1, NS), but end-PCT chloride delivery increased significantly (2,014 +/- 41 to 2,248 +/- 29 peq.mm-1.min-1, P less than 0.01). Early distal convoluted tubule (DCT) and urinary bicarbonate excretion were unchanged, but DCT chloride delivery increased associated with a chloruresis. When metabolic alkalosis was present, however, S2 compensation for increased bicarbonate delivery was attenuated so that end-PCT, DCT, and urinary bicarbonate as well as chloride delivery rates increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II: a powerful controller of sodium transport in the early proximal tubule.

Angiotensin II has recently been shown to exert potent control over sodium and water absorption in the proximal convoluted tubule. This transport stimulation is effected by receptors on both the luminal and basolateral membranes of cells located predominantly in the early, S1 proximal tubule. Angiotensin II increases transport primarily by a Gi protein-mediated reduction in intracellular cyclic adenosine monophosphate, which enhances the affinity of the Na(+)-H+ antiporter. Change in early proximal acidification ultimately causes alteration in the amount of sodium chloride leaving the proximal tubule and entering the urine. These direct tubular transport actions by angiotensin II may participate importantly in various physiological actions of the kidney, including the renal response to change in dietary sodium intake and in extracellular volume, as well as in pathophysiological processes such as hypertension.

Role of protein kinase C in proximal bicarbonate absorption and angiotensin signaling.

Using in vivo microperfusion in the proximal convoluted tubule (PCT) of the Munich-Wistar rat, we investigated the impact of varying protein kinase C (PKC) activity on the rate of bicarbonate reabsorption and on its regulation by angiotensin II. Activation of PKC with luminal perfusion of phorbol 12-myristate 13-acetate (PMA, 5 x 10(-7) M) caused bicarbonate absorption in the S1 PCT to increase by 25%, from 346 +/- 7 to 432 +/- 4 peq.mm-1.min-1 (P less than 0.001), without affecting intracellular cAMP level. Another PKC stimulator, dioctanoylglycerol, had the same effect. Inhibition of PKC activity with luminal perfusion of 5 x 10(-6) M sphingosine had the opposite effect, decreasing bicarbonate absorption by 45% to 190 +/- 2 peq.mm-1.min-1 (P less than 0.001). Pretreatment with PMA or with sphingosine each attenuated by approximately one-third the bicarbonate absorptive response usually observed following angiotensin II administration. Similar results for the action of PKC, but of smaller magnitude, were found in the S2 PCT. In conclusion, activation of PKC increases bicarbonate and water absorption in the S1 and S2 PCT in vivo, and PKC may participate in as much as one-third of the transport stimulation induced by angiotensin II.

Cine-computed tomographic assessment of regional renal blood flow.

The new modality of cine-computed tomography was used to assess regional renal blood flow following hemorrhagic hypotension and administration of atrial natriuretic factor (ANF). As has also been established with other techniques, hypotension induced by hemorrhage caused cortical blood flow to fall from 5.3 (SD 0.8) to 2.1 (SD 1.1) ml.min-1g-1 (p less than 0.005; n = 7) proportionally more than inner medullary blood flow which fell from 0.45 (SD 0.24), to 0.26 (SD 0.08) ml.min-1g-1 (p less than 0.05) so that the ratio of the cortical to inner medullary blood flow decreased from 11.8 (SD 4.8) to 8.2 (SD 5.1) (p less than 0.01). In contrast, during natriuresis induced by ANF, cortical blood flow was unaltered (5.8, SD 1.1, vs 5.4, SD 0.9 ml.min-1g-1; NS; n = 5) while inner medullary blood flow declined from 0.66 (SD 0.13) to 0.51 (SD 0.11) (p less than 0.005) so the ratio of cortical to inner medullary blood flow increased from 8.8 (SD 1.3) to 10.9 (SD 1.7) (p less than 0.005). These results indicate that cine-CT appears to be an appropriate technique for demonstrating changes in distribution of intrarenal blood flow. Cine-CT may, therefore, be an attractive alternative for measuring regional renal blood flow.

Angiotensin II stimulates early proximal bicarbonate absorption in the rat by decreasing cyclic adenosine monophosphate.

These studies explored the hypothesis that angiotensin II increases bicarbonate absorption in the proximal convoluted tubule (PCT) by decreasing intracellular cAMP. In vivo microperfusion was performed in rat PCT with measurements of bicarbonate absorption and of tubular fluid cAMP delivery, as a reflection of intracellular cAMP. Intravenous angiotensin II potently increased S1 PCT bicarbonate absorption (348 +/- 11 to 588 +/- 8 peq/min.min, P less than 0.001) and decreased tubular fluid cAMP (18 +/- 2 to 12 +/- 2 fmol/mm.min, P less than 0.05). Parathyroid hormone had the expected opposite effects, which were additive to those of angiotensin II. Over a wide range of hormonal activities, there was an excellent inverse relationship between hormonally modulated bicarbonate absorption and cAMP delivery. Pertussis toxin pretreatment significantly attenuated (by 35-45%) the angiotensin-induced increase in bicarbonate absorption and decrease in cAMP delivery, indicating Gi-protein intermediation. Luminal dibutyryl cAMP abolished the transport response to angiotensin II. In conclusion, these in vivo results suggest angiotensin II stimulates bicarbonate absorption in the S1 PCT by a G1-mediated depression in intracellular cAMP.

Urinary cGMP as biological marker of the renal activity of atrial natriuretic factor.

Current evidence suggests guanosine 3',5'-cyclic monophosphate (cGMP) serves as the second messenger for atrial natriuretic factor (ANF) in the kidney in vivo. We examined whether extracellular cGMP accumulation quantitatively reflected the concentration of cGMP within renal cells and whether urinary excretion of cGMP correlated with the physiological action of ANF. cGMP egression was examined in renal epithelial LLC-PK1 cells. ANF augmented intracellular cGMP concentration and extracellular cGMP appearance. Extracellular cGMP was an excellent function of the time-integrated intracellular cGMP concentration. In clearance studies in awake rats, urinary cGMP was primarily of renal cellular origin and correlated with the natriuresis induced by ANF in a time-dependent and concentration-dependent fashion. Urinary cGMP excretion may be useful as a biological marker for the renal activity of ANF in vivo.