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K Turnheim

Publications and source records attributed to K Turnheim.

At least 19 recordsLinked to original sources
PubMed1992

Vanadium-induced Cl(-)-secretion in rabbit descending colon is mediated by prostaglandins.

Vanadium in the 4+ (vanadyl-ion) and 5+ (vanadate-ion) oxidation state stimulates furosemide-sensitive electrogenic Cl- secretion in isolated epithelia of rabbit descending colon. This effect is associated with an increased release of prostaglandin E2 from the tissue. Inhibitors of phospholipase A2 or cyclooxygenase abolish both vanadium-induced release of prostaglandin E2 and Cl- secretion. Neuronal mechanisms are not likely to be involved, as tetrodotoxin does not affect the vanadate induced Cl- secretion. Although vanadate is known to inhibit Na+,K(+)-ATPase activity, no inhibition of active Na+ transport was observed in intact colonic epithelia suggesting a rapid intracellular reduction of vanadate ions to vanadyl ions which have no inhibitory effect on the Na+,K(+)-ATPase. The present findings therefore indicate that vanadate stimulated colonic Cl- secretion involves intracellular conversion of vanadate to vanadyl and release of prostaglandin E2.

Animals↗
PubMed1992

Endothelin-1 stimulates chloride and potassium secretion in rabbit descending colon.

The vasoactive peptide endothelin-1 (ET-1) which is present in high concentrations in the colon, causes concentration-dependent electrogenic Cl- secretion in rabbit descending colon. This effect is half-maximal at 0.11 mumol/l. Like other secretagogues, ET-1 also stimulates K+ secretion. The secretory effect of ET-1 is associated with increased release of prostaglandin E2 from the serosal surface of the mucosa. ET-1-induced Cl- secretion is completely inhibited by the loop diuretic bumetanide and by indomethacin and quinacrine, inhibitors of prostaglandin synthesis. Neuronal mechanisms do not seem to be involved, as tetrodotoxin did not affect the secretory response to ET-1 significantly. On the other hand, neither the catalytic activity nor the transport function of the Na+/K(+)-ATPase of rabbit colon epithelium is affected by endothelin-1 (ET-1) in concentrations up to 10 mumol/l. It is concluded that ET-1 causes Cl- and K+ secretion by stimulating phospholipase A2 and release of prostaglandins, whereas Na+ transport is not altered.

Animals↗
PubMed1991

Intrinsic regulation of apical sodium entry in epithelia.

In the past 30 years the basic features of Na+ absorption by epithelia have been unraveled and generally accepted cell models have been established. However, these cell models of transepithelial Na+ transport represent, for the most part, a static view of cell function, i.e., all transport parameters are assumed to be in a steady state. Today the focus is on the dynamic properties of epithelia, the non-steady-state condition, and the adaptation to environmental or transport changes. This review deals with mechanisms intrinsic to the epithelium that regulate apical membrane Na+ permeability in response to changes in transport load and ambient conditions. Together with parallel autoregulatory events concerning the basolateral K+ conductance, the described mechanisms controlling apical membrane Na+ permeability serve to maintain the intracellular ionic composition within the limits that are compatible with cell function and survival. Extraepithelial factors that influence epithelial Na+ transport such as mineralocorticoids and glucocorticoids, ADH, catecholamines, and other neurotransmitters are discussed elsewhere. Apical membrane Na+ permeability appears to be determined by several intrinsic or autoregulatory mechanisms. The PmNa of epithelia with channel-mediated apical Na+ entry is downregulated by increases in the Na+ concentration of the apical bathing solution (self-inhibition) and by procedures that inhibit basolateral Na+ extrusion (feedback inhibition). The underlying mechanisms of both regulatory systems are unclear. With the use of current-noise (fluctuation) analysis, on the one hand, and single-channel recordings, on the other hand, conflicting results were obtained concerning the saturability of single-channel conductance with increasing external Na+ concentrations. Results from Na(+)-uptake studies in apical membrane vesicles from amiloride-sensitive epithelia render it unlikely that cell Na+ itself is the mediator of feedback inhibition. Both self-inhibition and feedback inhibition of PmNa are prevented by titrating superficial sulfhydryl groups in the apical membrane. Elevations of cell Ca2+ decrease apical Na+ entry, possibly via an indirect mechanism involving protein kinase C. The PmNa is markedly dependent on cell metabolism and pHc; inhibition of ATP supply and lowering cell pH reduce PmNa. Additionally, PmNa may be altered by exocytotic expansion and endocytotic retrieval of the apical membrane area or by insertion of channel proteins into the apical membrane without increasing the apical membrane area. The diversity of regulatory systems may insure the high degree of flexibility and plasticity of epithelia in their response to environmental changes.

Animals↗
PubMed1990

Calcium-activated potassium channels in basolateral membranes of colon epithelial cells; reconstitution and functional properties.

Using differential sedimentation, isopycnic and Ficoll-400 barrier centrifugation, basolateral membrane vesicles of surface and crypt cells of the rabbit distal colon were enriched 34- and 9-fold, respectively. 86Rb(+)-uptake into these vesicles, driven by an electrical potential difference, was stimulated by submicromolar Ca2+ activities and inhibited by Ba2+. These findings indicate the presence of Ca2(+)-activated K+ channels. The K+ channels in surface and crypt cell membranes differed with respect to inhibition by the bee venom apamin, the scorpion venom charybdotoxin and tetraethylammonium and exhibited a different pH dependence. Fusion of basolateral membrane vesicles with planar phospholipid bilayers revealed the presence of high-conductance Ba2(+)-sensitive K+ channels which were activated by micromolar Ca2+ and inhibited by crude scorpion venom and trifluoperazine. These K+ channels may be involved in the coupling of apical and basolateral membrane conductances during Na+ absorption and Cl- secretion, but they may also play a role in cell volume regulation.

Animals↗
PubMed1978

pH and temperature dependence of adenosine uptake in human erythrocytes.

Kinetic analysis of the saturable adenosine uptake in human erythrocytes suggests the existence of two saturable components, distinguished by different Km values (1.4 and 260 micron, respectively, at pH 7.4 and 25 degrees C). Both components were abolished by p-nitrobenzylthioguanosine or dipyridamole. Total uptake was significantly higher at pH 8 than at pH 7 at adenosine concentrations above 2 micron. The increase in uptake at the higher pH was brought about mainly by an increase in the maximum rate of transport of the low-affinity uptake system. With rising temperature the Km and the V of both uptake components increased. No transition temperature was observed between 12 and 37 degrees C.

Adenosine↗
PubMed1978

Intracellular chloride activities in rabbit gallbladder: direct evidence for the role of the sodium-gradient in energizing "uphill" chloride transport.

Intracellular chloride activities, (Cl)c, in rabbit gallbladder were determined by using conventional (Kcl-filled) microelectrodes and Cl-selective, liquid ion-exchanger, microelectrodes. The results indicated that in the presence of a normal Ringer's solution, (Cl)c averages 35mM; this value is 2.3 times that predicted for an equilibrium distribution across the mucosal and baso-lateral membranes. On the other hand, when the tissue is bathed by Na-free solutions, (Cl)c declines to a value that does not differ significantly from that predicted for an equilibrium distribution. These results, together with those of Frizzell et al. (J. Gen. Physiol. 65:769, 1975) provide, for the first time, compelling evidence that (i) the movement of Cl from the mucosal solution into the cell is directed against an electrochemical potential difference (23mV); and (ii) this movement is energized by coupling to the entry of Na down a steep electrochemical potential difference. Finally, our data suggest that (i) Cl exit from the cell across the basolateral membrane may be coupled to the co-transport of a cation or the countertransport of an anion; and (ii) the mechanism responsible for active Na extrusion from the cell across the baso-lateral membrane is rheogenic (electrogenic), and is not the result of a neutral Na-K exchange.

Animals↗
PubMed1978

Ion transport by rabbit colon: II. Unidirectional sodium influx and the effects of amphotericin B and amiloride.

The unidirectional influx of Na from the mucosal solution into the epithelium of in vitro descending rabbit colon (JName) determined under short-circuit conditions, is comprised of two components: one represents entry of Na into transporting epithelial cells and is abolished by amiloride which also abolishes Na absorption (JNanet). The other represents diffusional Na entry into paracellular pathways traversing the epithelium. In all instances, exposure of the mucosal surface to amphotericin B increased tissue conductance and JName and elicited K secretion. Tissues showing a spontaneous Isc of approximately 4 microneq/cm2hr did not respond to amphotericin B with increased Isc and JNanet. However, in tissues characterized by a lower Isc under control conditions, amphotericin B increased Isc and JNanet to approximately 4 microneq/cm2 hr. These findings suggest that amphotericin increases JNanet and elicits K secretion by disrupting the normal perm-selectivity of the mucosal membrane. Under these conditions the extrusion of Na from cell-to-serosal solution becomes the rate limiting step in transepithelial Na transport. Finally, a close correlation between JName and JNanet was was observed when the rate of Na absorption varied either spontaneously or experimentally with amiloride, suggesting that the backflux of Na from cell-to-mucosal solution is undetectably small.

Amiloride↗
PubMed1978

Interaction between cell sodium and the amiloride-sensitive sodium entry step in rabbit colon.

Ouabain abolishes the short-circuit current (Isc) and decreases the transepithelial conductance (Gt) of rabbit colon. In contrast, amphotericin B elicits a maximum Isc and markedly increases Gt. However, in both instances the amiloride-sensitive Na entry step is completely blocked, presumably due to an increase in cell Na. Conversely, when Na-depleted tissues are suddenly exposed to 140 mM Na, the amiloride-sensitive Isc and the amiloride-sensitive component of Gt (alphaGNa) increase abruptly to their maximum values and then decline to steady-state plateaus with a half time of approximately 6 min; throughout the decline (Isc/alphaGNa) = ENa is constant at a value of 95 mV. In the presence of amphotericin B, the Isc abruptly rises to the same maximum but does not decline. These findings indicate that in the presence of 140 mM Na the conductance of the amiloride-sensitive Na entry step can vary from a maximum value of approximately 1.6 mmhos/cm2 when cell Na is depleted, to zero when cell Na is abnormally elevated (e.g., in the presence of ouabain or amphotericin B). Our findings are consistent with a system in which the pathway responsible for transcellular Na transport parallels another cellular compartment with which it communicates. The Na capacity of the active transport pathway appears to be very small so that this compartment fills rapidly after exposure of Na-depleted cells to 140 mM Na, and active transepithelial Na transport is initiated and reaches steady-state levels quickly. The Na capacity of the second compartment is much larger; the Na content of this compartment appears to be responsible for the negative feedback effect on the permeability of the amiloride-sensitive entry step.

Amiloride↗
PubMed1978

P-chloromercuribenzene sulfonate blocks and reverses the effect of amiloride on sodium transport across rabbit colon in vitro.

The addition of 10(-3) M p-chloromercuribenzene sulfonate (PCMBS) to the solution bathing the mucosal surface of rabbit colon has no effect on the rate of active Na transport but blocks or reverses the inhibitory action of amiloride. The tissue must be exposed to PCMBS for 20-30 min for a complete blocking effect, and removal of PCMBS from the mucosal solution after this period of exposure does not restore the sensitivity of the tissue to amiloride. The slow time-courses of the blocking and reversal effects suggest that PCMBS does not irreversibly interact with groups directly involved in the binding of amiloride.

4-Chloromercuribenzenesulfonate↗
PubMed1978

Influence of hexobendine, dipyridamole, dilazep, lidoflazine, inosine and purine riboside on adenosine uptake by the isolated epithelium of guinea pig jejunum.

The uptake of (14C)adenosine by isolated epithelium of guinea pig jejunum, administered on the blood side, was inhibited by hexobendine, dipyridamole, dilazep and lidoflazine. On the lumen side, however, weak inhibition was observed with lidoflazine only and no significant change was recorded with hexobendine, dipyridamole or dilazep. This difference was not altered when the degradation of hexobendine by the jejunal epithelium was blocked by physostigmine. When adenosine uptake was already reduced by purine riboside, further addition of hexobendine, dipyridamole, dilazep or lidoflazine caused divergent changes depending on the side of administration. Adenosine uptake was further diminished on the blood side, but raised towards control values on the lumen side. By contrast, inosine inhibited adenosine uptake on both sides of the epithleium. The results suggest that the mechanism of adenosine uptake is different on either side with respect to inhibition characteristics, corresponding to differences in morphology and function of the two sides of the intestinal epithelium.

Adenosine↗
PubMed1977

Effects of anions on amiloride-sensitive, active sodium transport across rabbit colon, in vitro. Evidence for "trans-inhibition" of the Na entry mechanism.

Replacement of Cl in the solutions bathing partial mucosal strips of rabbit descending colon with sulfate, isethionate, hydroxypropane-sulfonate and, to a lesser degree, ethanesulfonate stimulates active Na absorption (JNanet) when the baso-lateral pump mechanism is not saturated. These effects are rapid in onset and are readily reversible. Our findings indicate that these stimulatory anions decrease the resistance of the amiloridesensitive Na entry step at the mucosal membrane (RmNa). However, when the active Na pump mechanism at the baso-lateral membrane is saturated these stimulatory anions do not decrease the resistance of the Na entry process. These findings suggest the presence of a negative feedback between the activity of the pump mechanism and the resistance of the Na entry step which may be mediated by the size of the intracellular Na transport pool. In other words, it seems that when the baso-lateral pump is operating at its maximal rate the resistance to Na entry across the mucosal membrane through the amiloride-sensitive pathway is at a minimum and cannot be further decreased.

Amiloride↗
PubMed1977

Relaxation of coronary artery strips by adenosine and acidosis.

Cumulative dose-response curves of Ca2+-induced tension increments were studied in K+-depolarized helical strips of dog coronary arteries. Adenosine 10(-4) M reduced the Ca2+ sensitivity of the strips without altering the maximal tension with full Ca2+ activation. In contrast, acidosis of pH 7.05 significantly diminished the maximal tension with full Ca2+ activation. The relaxing effect of acidosis was almost completely abolished by 10(-4) M adenosine. It is concluded that adenosine inhibits Ca2+ influx, whereas acidosis depresses the contractile process of vascular smooth muscle directly.

Acidosis↗
PubMed1977

Secretion of monoquaternary ammonium compounds by guinea pig small intestine in vivo.

In anesthetized guinea pigs N-(3H)methylscopolamine (NMScop), N1-(14C)methylnicotinamide (NMN), and (14C)tetraethylammonium (TEA), administered intravenously, were secreted against a concentration gradient into the lumen of the small intestine. The concentration ratio of unmetabolized ammonium base in the intestinal lumen to that in the plasma was 4.3 and 6.5 for NMScop and NMN, respectively, 75 min after the intravenous injection of 1 nmole/g body weight of the individual compounds. The corresponding value for TEA after 180 min was 2.0. The establishment of the concentration gradient between intestinal lumen and plasma was diminished with increasing doses. An excess of NMN inhibited the uphill transport of NMScop. Since the electrical potential difference across the intestinal epithelium and a 'fluid circuit' mechanism cannot solely account for the observed accumulation of the monoquaternary ammonium compounds in the intestinal lumen, the evidence presented supports previous in vitro findings that the small intestine is capable of actively secreting organic cations.

Animals↗
PubMed1977

Weak electrolyte transfer in the guinea pig jejunum: secretion of trimethoxybenzoic acid.

In isolated epithelia of guinea pig jejunum the transcellular permeation of 10(-4) M (carboxyl-14C)-3,4,5-trimethoxybenzoic acid (TMBA) in the direction blood-lumen was more than 10 times greater the transcellular permeation of 10(-4) M (carboxyl-was reduced to less than 2 by anaerobiosis or by increasing TMBA concentrations of up to 10(-2) M. Under aerobic conditions the cellular uptake of TMBA (10(-4) M) from the blood side was twice as high as that from the lumen side. In anaerobiosis the percentage of TMBA taken up into the epithelium was enhanced, when TMBA was administered on the lumen side, while the percentage was unchanged after administration on the blood side; thereby the difference in cellular TMBA concentrations was abolished. Similar results were obtained under aerobic conditions, if the TMBA concentration was increased up to 10(-2) M. The results are consistent with a three-compartment model with an intermediate compartment distinguished by a high pH as compared to that of the outer compartments and by a luminal boundary highly permeable for the ionized form of the substrate in contrast to the contraluminal boundary.

Aerobiosis↗