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M A Knepper

Publications and source records attributed to M A Knepper.

At least 181 records · Page 10Linked to original sources
PubMed1989

Renal actions of atrial natriuretic factor: a mathematical modeling study.

Atrial natriuretic factor (ANF) is a peptide hormone that increases renal NaCl and water excretion. Several renal sites of ANF action have been identified, but general agreement has not been reached concerning the quantitative contribution of each action to the natriuresis and diuresis. Using a five-nephron central core model of NaCl, urea, KCl, and water transport in the rat kidney, we have quantitatively evaluated the hypothetical effects on whole kidney function of three experimentally observed ANF actions: 1) inhibition of active NaCl absorption in the collecting duct, 2) inhibition of osmotic water permeability in the collecting duct, and 3) increased NaCl and water delivery out of the proximal convoluted tubule simulating an increase in glomerular filtration rate. The simulations show that inhibition of collecting duct active NaCl absorption by greater than or equal to 50% can increase NaCl and water excretion to levels that match experimental values. In addition, the model predicted that the urinary sodium concentration will increase to greater than plasma levels as observed experimentally. Simulated decreases in collecting duct water permeability predicted an increase in water excretion with little change in NaCl excretion. Simulated 2.5-5% increases in glomerular filtration rate also increased simulated NaCl and water excretion rates to experimentally observed levels in response to ANF. However, this action was less effective than inhibition of collecting duct active NaCl absorption in increasing the urinary NaCl concentration. We conclude that a combination of several actions are likely to account for the overall renal effect of ANF.

Animals↗
PubMed1988

Active NH4+ absorption by the thick ascending limb.

Isolated perfused medullary thick ascending limbs from rabbits were studied to determine the mechanism of ammonium ion absorption. Under control conditions, thick ascending limbs spontaneously absorbed NH4+ and generated a lumen-positive potential. When these tubules were chemically voltage clamped to lumen-negative potentials by lowering the bath NaCl concentration, NH4+ absorption persisted. Thus NH4+ was absorbed against an electrochemical gradient. The active flux accounts for most of the net flux under control conditions, the remainder being due to passive paracellular NH4+ diffusion. The NH4+ permeability, measured in separate experiments, was high (1.50 +/- 0.25 x 10(-4) cm/s) compared with values in other segments. The NH3 permeability was relatively low (3.1 +/- 0.5 x 10(-3) cm/s). Luminal furosemide (10(-4) M) eliminated most of the active NH4+ flux, indicating that a major fraction of the active flux is dependent on apical entry of NH4+ via the Na+ -K+ -2Cl- cotransporter (presumably by substitution for K+). The remaining active flux was completely inhibited by 10(-4) M ouabain in the bath. Active chloride absorption was maintained when NH4+ entirely replaced K+ in bath and perfusate, indicating that NH4+ substitutes for K+ on the apical cotransporter and the basolateral Na+ -K+ -ATPase. Ammonium absorption provides an active "single effect" for countercurrent multiplication of NH4+ in the renal medulla.

Absorption↗
PubMed1988

Hormone effects on NaCl permeability of rat inner medullary collecting duct.

It has been proposed that regulation of NaCl excretion occurs in part by hormonal effects on NaCl permeability in the inner medullary collecting duct (IMCD). We carried out experiments in isolated perfused terminal IMCDs to determine whether atrial natriuretic factor (ANF), vasopressin, or deoxycorticosterone (DOC) affects NaCl permeability. Apparent Cl- or Na+ permeabilities (PCl and PNa) were determined by measuring ion fluxes resulting from imposed electrochemical gradients. Transepithelial resistance (RT) was calculated from voltage deflections at the perfusion and collection ends of the tubule, which resulted from perfusion end current injection (cable analysis). ANF [rat ANF-(1-28), 100 nM in the peritubular bath] significantly decreased PCl from 2.20 to 1.84 x 10(-5) cm/s and did not alter PNa (1.11 to 1.18 x 10(-5) cm/s). ANF also decreased PCl in IMCDs from DOC-treated rats (1.14 to 0.98 x 10(-5) cm/s). Vasopressin (10 nM in the peritubular bath) did not affect PCl. RT averaged 39.3 omega.cm2 in IMCDs from control rats and was significantly increased to 62.3 omega.cm2 in tubules from DOC-treated rats. Neither ANF nor vasopressin significantly affected RT in either group. We conclude the following: 1) the results do not support the hypothesis that ANF causes natriuresis by increasing the NaCl permeability of the terminal IMCD. Instead, ANF significantly decreases the chloride permeability. 2) Vasopressin does not affect NaCl permeability. 3) Mineralocorticoid-induced antinatriuresis may be due in part to reduced NaCl permeability in the terminal IMCD.

Animals↗
PubMed1988

Calcium and cyclic adenosine monophosphate as second messengers for vasopressin in the rat inner medullary collecting duct.

UNLABELLED: Vasopressin increases both the urea permeability and osmotic water permeability in the terminal part of the renal inner medullary collecting duct (terminal IMCD). To identify the second messengers that mediate these responses, we measured urea permeability, osmotic water permeability, intracellular calcium concentration, and cyclic AMP accumulation in isolated terminal IMCDs. After addition of vasopressin, a transient rise in intracellular calcium occurred that was coincident with increases in cyclic AMP accumulation and urea permeability. Half-maximal increases in urea permeability and osmotic water permeability occurred with 0.01 nM vasopressin. The threshold concentration for a measurable increase in cyclic AMP accumulation was approximately 0.01 nM, while measurable increases in intracellular calcium required much higher vasopressin concentrations (greater than 0.1 nM). Exogenous cyclic AMP (1 mM 8-Br-cAMP) mimicked the effect of vasopressin on urea permeability but did not produce a measurable change in intracellular calcium concentration. CONCLUSIONS: (a) Cyclic AMP is the second messenger that mediates the urea permeability response to vasopressin in the rat terminal IMCD. (b) Vasopressin increases the intracellular calcium concentration in the rat terminal IMCD, but the physiological role of this response is not yet known.

Animals↗
PubMed1988

Atrial natriuretic factor inhibits vasopressin-stimulated osmotic water permeability in rat inner medullary collecting duct.

The inner medullary collecting duct (IMCD) has been proposed to be a site of atrial natriuretic factor (ANF) action. We carried out experiments in isolated perfused terminal IMCDs to determine whether ANF (rat ANF 1-28) affects either osmotic water permeability (Pf) or urea permeability. In the presence of a submaximally stimulating concentration of vasopressin (10(-11) M), ANF (100 nM) significantly reduced Pf by an average of 46%. Lower concentrations of ANF also significantly inhibited vasopressin-stimulated Pf by the following percentages: 0.01 nM ANF, 18%; 0.1 nM, 46%; 1 nM, 48%. Addition of exogenous cyclic GMP (0.1 mM) mimicked the effect of ANF, decreasing Pf by an average of 48%. ANF also inhibited cyclic AMP-stimulated Pf by an average of 31%. ANF did not affect urea permeability, nor did it alter vasopressin-stimulated cyclic AMP accumulation. We conclude that ANF at physiological concentrations causes a large inhibition of vasopressin-stimulated Pf in the rat terminal IMCD, and that cyclic GMP is the second messenger mediating the effect. ANF appears to act at a site distal to cyclic AMP generation in the chain of events linking vasopressin receptor binding to an increase in osmotic water permeability.

Animals↗
PubMed1987

NH3 and NH4+ transport by rabbit renal proximal straight tubules.

UNLABELLED: Isolated perfused S2 proximal straight tubules from rabbits spontaneously secreted ammonia (-1.34 pmol X mm-1 X min-1) and absorbed bicarbonate (49.3 pmol X mm-1 X min-1) when perfusate and bath solutions contained 1 mM NH4Cl and 25 mM bicarbonate (pH 7.4). The NH3 concentration in the collected fluid was on average 40% lower than that of the bath as a consequence of a lower pH in the lumen. To test whether diffusion of NH3 down the bath-to-lumen NH3 concentration gradient could account for the measured ammonia secretion, we measured the permeabilities to NH3 (1.6 X 10(-2) cm/s) and NH4+ (4.5 X 10(-5) cm/s). From these values, we calculated predicted rates of passive NH3 secretion (-3.6 pmol X mm-1 X min-1) and passive NH4+ absorption (0.9 pmol X mm-1 X min-1). The predicted rate of net ammonia secretion exceeded the measured rate, indicating that passive NH3 secretion can fully account for the measured flux. In additional experiments, 10(-4) M acetazolamide in bath and perfusate inhibited net bicarbonate absorption by approximately 80%, but the rate of ammonia secretion was unaffected. CONCLUSIONS: S2 proximal straight tubules spontaneously secrete ammonia as required for generation of a corticomedullary ammonia concentration gradient by counter-current multiplication. Diffusion of NH3 down a concentration gradient created by luminal acidification can account for the ammonia secretion. A substantial passive lumen-to-bath backflux of NH4+ occurs. Acetazolamide does not inhibit ammonia secretion.

Acetazolamide↗
PubMed1987

Luminal disequilibrium pH and ammonia transport in outer medullary collecting duct [corrected and issued with original paging in Am J Physiol 1987 Aug;253(2 Pt 2)].

We measured bicarbonate, ammonia, and luminal pH in segments of the rabbit outer medullary collecting duct (OMCD) to determine the relationship between luminal pH and ammonia transport. Both the inner-stripe and outer-stripe portions of the OMCD absorbed bicarbonate at high rates. The outer stripe OMCD generated an acidic pH disequilibrium that was reversibly dissipated by exogenous luminal carbonic anhydrase. In contrast, the inner stripe OMCD did not generate a spontaneous pH disequilibrium unless perfused with the carbonic anhydrase inhibitor acetazolamide. Ammonia secretion was three times more rapid in the outer stripe OMCD than in the inner stripe OMCD. We conclude the following. 1) Both the inner-stripe and outer-stripe portions of the rabbit OMCD secrete protons at substantial rates. 2) Functional luminal carbonic anhydrase is present in the inner stripe OMCD but absent from the outer stripe OMCD. 3) Ammonia secretion occurs predominantly by NH3 diffusion in both portions. 4) The luminal pH disequilibrium, which is normally present in the outer stripe OMCD, enhances ammonia secretion.

Acetazolamide↗
PubMed1987

Bicarbonate and ammonia transport in isolated perfused rat proximal straight tubules.

Bicarbonate, ammonia, and fluid transport were studied in isolated perfused proximal straight tubules from rats. The mean rate of fluid absorption (0.77 nl X min-1 X mm-1) and the mean rate of total CO2 absorption (42 pmol X min-1 X mm-1) exceeded corresponding rates measured previously in rabbit proximal straight tubules. The limiting total CO2 concentration when the tubules were perfused at slow flow rates was 5 mM, a value similar to those reported previously for rat proximal convoluted tubules and thick ascending limbs. When rat proximal straight tubules were perfused and bathed with solutions containing 1 mM total ammonia at slow perfusion rates, the measured total ammonia concentration in collected fluid rose to a level predicted by the diffusion trapping model of ammonia secretion in the absence of a luminal disequilibrium pH. We conclude the proximal straight tubule of the rat can absorb bicarbonate at a rate that can account for a large portion of the bicarbonate absorption measured in vivo between the late proximal convoluted tubule and the early distal tubule, the rat proximal straight tubule is capable of transepithelial ammonia secretion, most likely by NH3 diffusion down a concentration gradient generated by luminal acidification, and the rat proximal straight tubule apparently does not generate a luminal disequilibrium pH despite the occurrence of proton secretion, implying the presence of endogenous luminal carbonic anhydrase.

Ammonia↗
PubMed1987

Vasopressin effects on urea and H2O transport in inner medullary collecting duct subsegments.

The inner medullary collecting duct (IMCD) is widely viewed as a single renal tubule segment with homogeneous properties. However, recent morphological and functional studies have raised the possibility that the initial and terminal parts of the IMCD may differ. To test this possibility further and to localize sites of action of arginine vasopressin (AVP) along the IMCD, we measured osmotic water permeability (Pf) and urea permeability (Purea) in isolated perfused rat IMCDs. In the initial third of the IMCD, 10 nM AVP increased Pf from 16 +/- 8 to 148 +/- 50 micron/s. The terminal two-thirds of the IMCD had a significantly higher basal Pf (70 +/- 12 micron/s), which increased to 186 +/- 25 micron/s with AVP. The initial IMCD had a relatively low basal Purea (3 +/- 1 X 10(-5) cm/s), which did not change with AVP. The terminal IMCD had a significantly higher basal Purea (17 +/- 4 X 10(-5) cm/s), which increased to a very high value (69 +/- 15 X 10(-5) cm/s) with AVP. The results support the premise that (from the point of view of vasopressin effects on water and urea transport) there are two functionally distinct parts of the inner medullary collecting duct: an initial part that resembles the cortical or outer medullary portions of the mammalian collecting duct and a terminal part that resembles the toad urinary bladder. The significance of these findings for the urinary concentrating mechanism is discussed.

Animals↗
PubMed1987

Disequilibrium pH and ammonia transport in isolated perfused cortical collecting ducts.

The present study was carried out to test directly whether isolated perfused rabbit cortical collecting ducts (CCDs) spontaneously generate a luminal disequilibrium pH. We determined disequilibrium pH as the difference between 1) the actual luminal pH measured by perfusing the lumen with a membrane-impermeant pH-sensitive dye [1,4-dihydroxyphthalonitrile (1,4-DHPN)] and 2) equilibrium pH calculated from the measured total CO2 concentration in fluid collected at the end of the tubule. When the peritubular bath and perfusate had the same composition, a statistically significant acidic disequilibrium pH was found (mean -0.14 units). To determine whether the disequilibrium pH is due to an absolute lack of luminal carbonic anhydrase, we measured the effective rate constant for carbonic acid dehydration in the lumen (k-1). To do this, a lumen-to-bath NH3 concentration gradient was imposed, and the luminal pH was measured along the tubule with 1,4-DHPN. NH3 absorption caused a luminal disequilibrium pH (due to dissociation of NH+4 to NH3 and H+), whose profile along the lumen is dependent on k-1 and NH3 permeability (PNH3). PNH3 and k-1 were estimated from the luminal pH profiles using a mathematical model of proton and buffer transport. The measured k-1 (37 s-1) is within the reported range of values for uncatalyzed H2CO3 dehydration. Calculations demonstrate that the measured PNH3 (2 X 10(-3) cm/s) is high enough and the measured k-1 is low enough to explain ammonia secretion rates seen in previous studies. We conclude that proton secretion in the CCD generates an acidic luminal disequilibrium pH, associated with an absolute lack of luminal carbonic anhydrase, which enhances the net rate of NH3 secretion.

Acid-Base Equilibrium↗
PubMed1987

Urea permeability of mammalian inner medullary collecting duct system and papillary surface epithelium.

To compare passive urea transport across the inner medullary collecting ducts (IMCDs) and the papillary surface epithelium (PSE) of the kidney, two determinants of passive transport were measured, namely permeability coefficient and surface area. Urea permeability was measured in isolated perfused IMCDs dissected from carefully localized sites along the inner medullas of rats and rabbits. Mean permeability coefficients (X 10(-5) cm/s) in rat IMCDs were: outer third of inner medulla (IMCD1), 1.6 +/- 0.5; middle third (IMCD2), 46.6 +/- 10.5; and inner third (IMCD3), 39.1 +/- 3.6. Mean permeability coefficients in rabbit IMCDs were: IMCD1, 1.2 +/- 0.1; IMCD2, 11.6 +/- 2.8; and IMCD3, 13.1 +/- 1.8. The rabbit PSE was dissected free from the underlying renal inner medulla and was mounted in a specially designed chamber to measure its permeability to urea. The mean value was 1 X 10(-5) cm/s both in the absence and presence of vasopressin (10 nM). Morphometry of renal papillary cross sections revealed that the total surface area of IMCDs exceeds the total area of the PSE by 10-fold in the rat and threefold in the rabbit. We conclude: the IMCD displays axial heterogeneity with respect to urea permeability, with a high permeability only in its distal two-thirds; and because the urea permeability and surface area of the PSE are relatively small, passive transport across it is unlikely to be a major source of urea to the inner medullary interstitium.

Animals↗
PubMed1987

Effects of atrial natriuretic factor on cyclic guanosine monophosphate and cyclic adenosine monophosphate accumulation in microdissected nephron segments from rats.

Atrial natriuretic factor (ANF) (1 microM) markedly increased cyclic guanosine monophosphate (cGMP) content in microdissected glomeruli (35-fold) and in microdissected inner medullary collecting ducts (IMCD) (20-fold). ANF caused little or no increase in cGMP content in other nephron segments. The threshold concentration for increased cGMP accumulation by ANF was 0.1-1 nM in IMCD, which is in the range reported for rat plasma. Sodium nitroprusside (1 mM), which selectively stimulates soluble guanylate cyclase, increased cGMP content in glomeruli but not in IMCD. ANF did not alter cAMP accumulation in the absence or presence of vasopressin (AVP) or parathyroid hormone (PTH) in outer and inner medullary tubule suspensions, or in microdissected proximal convoluted tubules (PCT), medullary thick ascending limbs (MAL) or IMCD. These data are compatible with the hypothesis that cGMP is a second messenger for a physiologic action of ANF in the inner medullary collecting duct. ANF apparently activates membrane-bound guanylate cyclase in this segment.

Animals↗
PubMed1986

Na-K-Cl cotransport in apical membrane of rabbit renal papillary surface epithelium.

The renal papillary surface epithelium is exposed to pelvic urine on its apical surface and to inner medullary interstitium on its basolateral surface. To investigate transport in this epithelium, we dissected it free from the renal papilla of rabbits and mounted it in a chamber that allowed both sides to be bathed independently. Cell volume was measured at 25 degrees C utilizing computerized quantitative microscopy. Addition of ouabain (10(-4) M) to the basolateral solution induced a 20% volume increase. This volume increase was completely inhibited by the removal of apical bath NaCl, Na+, K+, or Cl- but not by the removal of urea. Bumetanide, down to 10(-9) M in the apical bath, completely inhibited the ouabain-induced swelling. Changes in apical bath osmolality, resulting from addition or removal of NaCl, caused cell volume changes that were greater than could be accounted for by osmotic water flow alone. This hyperresponse was blocked by bumetanide and was stimulated by vasopressin (10(-8) M). These observations are consistent with the presence of Na-K-ATPase in the basolateral membrane and a bumetanide-sensitive, vasopressin-responsive Na-K-Cl co-transporter in the apical membrane.

Animals↗
PubMed1986

Effects of vasopressin and bradykinin on anion transport by the rat cortical collecting duct. Evidence for an electroneutral sodium chloride transport pathway.

Our previous studies in cortical collecting ducts isolated from rat kidneys have shown that vasopressin increases both sodium absorption and potassium secretion, while bradykinin inhibits sodium absorption without affecting potassium transport. To determine which anions are affected by these agents, we perfused cortical collecting ducts from rats treated with deoxycorticosterone and measured net chloride flux, net bicarbonate flux (measured as total CO2), transepithelial voltage, and the rate of fluid absorption. Arginine vasopressin (10(-10) M in the peritubular bath) caused a sustained sixfold increase in net chloride absorption and a two- to threefold increase in the magnitude of the lumen negative transepithelial voltage. Before addition of vasopressin, the tubules secreted bicarbonate. Vasopressin abolished the bicarbonate secretion, resulting in net bicarbonate absorption (presumably due to proton secretion) in many tubules. Bradykinin (10(-9) M added to the peritubular bath) caused a reversible 40% inhibition of net chloride absorption, but did not affect the transepithelial voltage or the bicarbonate flux. We concluded: (a) that arginine vasopressin stimulates absorption of chloride and inhibits bicarbonate secretion (or stimulates proton secretion) in the rat cortical collecting duct; and (b) that bradykinin inhibits net chloride absorption in the rat cortical collecting duct without affecting transepithelial voltage or bicarbonate flux. Combining these results with the previous observations on cation fluxes described above, we conclude that bradykinin inhibits electroneutral NaCl absorption (or stimulates electroneutral NaCl secretion) in the rat cortical collecting duct.

Animals↗