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Biomedical subjects

W M Armstrong

Publications and source records attributed to W M Armstrong.

At least 37 records · Page 2Linked to original sources
PubMed1981

Cyclic AMP and intracellular ionic activities in Necturus gallbladder.

Open-tip and liquid ion-exchanger microelectrodes were used to study the effects of cAMP (6mM, added to the serosal medium) on apical membrane potential (Em) and intracellular sodium, potassium, and chloride activities (alphaiNa, alphaiK, alphaiCl) in Necturus gallbladder under open-circuit conditions. Transepithelial potential difference (ETr) was also measured. In the presence of cAMP, alphaiCl fell from about 1.5 times its equilibrium value to a level that corresponded to electrochemical equilibrium across the apical and basolateral cell membranes. Under these conditions alphaiNa decreased and alphaiK increased, Em was unchanged and ETr increased from virtually zero to a small but significant serosal positive value. The cAMP-induced increase in alphaiK was abolished when Cl- -free incubation media were used. Addition of the Ca++-ionophore A23187 (0.5 microgram/ml) to the serosal medium had no effect on Em, ETr, or alphaiCl. When A23187 was added to the mucosal medium, Em and the basolateral membrane potential hyperpolarized by about 20 mV and an increase in the outwardly directed electrochemical driving force for Cl- was observed. These results indicate that cAMP inhibits coupled transapical Na-Cl entry into epithelial cells of Necturus gallbladder and suggest that this inhibition may not be mediated by an increase in intracellular Ca++ concentration.

Animals↗
PubMed1981

Intracellular ionic activities in frog skin.

Intracellular Na+, K+, and Cl- activities (aiNa, aiK, aiCl) and transapical membrane potentials (V0) were measured with liquid ion-exchanger and open-tip microelectrodes in isolated short-circuited frog skins (R. pipiens) incubated at 23 degrees C in normal amphibian Ringer's solution. Under control conditions aiNa = 14 +/- 3 mM, aiK = 132 +/- 10 mM and aiCl = 18 +/- 3 mM (SD). The value of aiCl is 4.4 times the value corresponding to electrochemical equilibrium for this ion. Thus, Cl- is actively accumulated by epithelial cells of the frog skin. Shortly after addition of amiloride (2--5 microM) to the apical bathing medium, aiK, aiNa, and aiCl were essentially unchanged although V0 had hyperpolarized by about 30--40 mV. During long-term exposure to amiloride aiK and aiCl did not change significantly, V0 depolarized by about 16 mV from the maximal value and aiNa decreased to 8 +/- 3 mM. Immediately after exposure to amiloride the transmembrane driving force for Na+ increased from 124 to 154 mV. During further exposure to amiloride, despite changes in both V0 and aiNa, this driving force remained virtually constant. Since Isc during this period was close to zero, it is suggested that the observed driving force for Na+ under these condition approximates the maximal driving force generated by the Na+--K+ ATP-ase pump in the basolateral cell membrane.

Amiloride↗
PubMed1981

Intracellular sodium activity and transcellular sodium transport in gallbladder.

As stated in the introduction, the purpose of this report has been to illustrate how the measurement of steady-state intracellular ionic activities, a technique that has proved to be of great importance in studying the energetics of transmembrane ionic transfer processes, can, with appropriate assumptions, be used to obtain information concerning the kinetics of these processes. Specifically, our analysis has focussed on transcellular Na+ transport in Necturus gallbladder and has shown that, given the steady-state values of Em, a1Na, and apical Na+ conductance for a particular set of conditions, it is possible to obtain estimates of net baso-lateral Na+ efflux (Na+ pump rate), net (and/or unidirectional) diffusive apical Na+ influx, and net coupled NaCl influx. It should be emphasized that the analysis outlined above is a preliminary essay in this direction. We present it here in the hope that, wit appropriate refinements, it may prove useful in unraveling the mechanisms by which drugs, hormones and other specific agents affect membrane function in epithelial and other systems.

Animals↗
PubMed1980

Cytochalasin B does not stimulate sugar uptake into small intestine of necturus or chick.

3-O-Methylglucose accumulation by Necturus intestine was studied in the presence and absence of cytochalasin B. The effect of this agent on 3-O-methylglucose-induced changes in transepithelial potential difference was also investigated. Cytochalasin B (50 microM) blocked the rise in potential difference induced by 4 mM 3-O-methyglucose. 3-O-Methyglucose accumulation by Necturus intestine was measured; it was inhibited by cytochalasin B at concentrations of 50 and 100 microM. These results indicate that, in Necturus intestine, sodium-dependent sugar transport across the apical cell membrane is blocked by cytochalasin B.

Animals↗
PubMed1980

Calcium regulation during stimulus-secretion coupling: continuous measurement of intracellular calcium activities.

Accurate measurements of intracellular calcium activities in salivary gland epithelial cells of the insect Phormia regina were obtained with microelectrodes in which N,N'-di(11-ethoxycarbonyl)undecyl-N,N'-4,5-tetramethyl-3,6-dioxaoctane diacid diamide wsa incorporated in a liquid membrane system. When calibrated in solutions approximating the ionic concentration of the cell interior, these microelectrodes gave rapid stable responses that were linear functions of the logarithm of calcium activities and were not affected by potassium, sodium and magnesium. Continuous monitoring of calcium activities during serotonin-induced saliva release provided direct evidence of hormonal influence on transmembrane calcium movement and spontaneous regulation of intracellular calcium by stimulated cells.

Animals↗
PubMed1980

The role of bicarbonate ions and of adenosine 3',5'-monophosphate (cAMP) in chloride transport by epithelial cells of bullfrog small intestine.

In an HCO3-free medium, isolated segments of bullfrog small intestine, stripped of their external muscle layers, displayed a small, serosal positive PD that did not, on the average, differ significantly from zero. Similarly, in this medium, the mean values of Isc and of net Na+ and Cl- absorption under short-circuit conditions did not differ significantly from zero. External HCO3- (25 mM) induced a highly significant serosal negative PD and Isc and a large net absorption of Cl-. Net Cl- absorption exceeded Isc, i.e., there was a significant net flux, JR, which was consistent with a net secretion of HCO3-. The ratio of the internal Cl-activity of the absorptive cells (alpha Cli) to its equilibrium value was larger in the presence than in the absence of HCO3-. In the presence of HCO3-, cAMP, added to the serosal medium, reversed the serosal negative PD and Isc, and inhibited, though it did not completely abolish, net Cl- absorption. JR was unchanged; tissue Cl- and alpha Cli were reduced, and tissue Na+ decreased and tissue K+ increased. When HCO3- and Cl- were removed from the bathing medium, the electrical response of the tissue to cAMP, though greatly attenuated, was not completely abolished. Under these conditions, cAMP induced a significant net Na+ absorption. A model for ion transport in the absorptive cells of the small intestine is proposed that is consistent with these findings.

Animals↗
PubMed1980

Ion-selective microelectrodes: theory and technique.

This report reviews the use of ion-selective microelectrodes to measure intracellular ionic activities and ionic electrochemical potential differences across cell membranes. Particular emphasis is placed on the electrochemical characteristics of liquid ion-exchanger microelectrodes. Methods for assessing the effect of interfering ions on the electrode potential are discussed and analyzed. An equivalent electrical circuit model is proposed in which deviations from their theoretical values of the slopes and selectivities of liquid ion-exchanger microelectrodes are analyzed in terms of surface conductance phenomena. A quantitative expression is developed that permits the transmembrane electrochemical potential difference for an ion, which is measured with an ion-selective microelectrode, to be corrected for deviations from ideality in the slope of the electrode response. The effect of lipophilic anions, dissolved in the organic ion-exchanger solution, on the electrochemical characteristics of cation selective liquid ion-exchanger microelectrodes containing neutral ionophores is discussed.

Animals↗
PubMed1979

Sodium-selective liquid ion-exchanger microelectrodes for intracellular measurements.

The sodium-selective ligand 1,1,1-tris[1(1)-(2(1)-oxa-4(1)-oxo-5(1)-aza-5(1)-methyl)dodecanyl]propane dissolved in 3-nitro-o-xylene containing a small amount of the lipophilic anion tetrachlorophenyl borate was used as a liquid ion-exchanger in sodium-selective microelectrodes. The microelectrodes gave rapid, stable responses that were linear functions of the logarithm of sodium activity. They were tested under conditions approximating those to be expected in the cell interior, and the results indicated that they can be used to measure intracellular sodium activity without significant interference from intracellular potassium.

Animals↗
PubMed1979

Energetics of coupled Na+ and Cl- entry into epithelial cells of bullfrog small intestine.

Na+, K+ and Cl- concentrations (cij) and activities (aij), and mucosal membrane potentials (Em) were measured in epithelial cells of isolated bullfrog (Rana catesbeiana) small intestine. Segments of intestine were stripped of their external muscle layers, and bathed (at 25 degrees C and pH 7.2) in oxygenated Ringer solutions containing 105 mM Na+ and Cl- and 5.4 mM K+. Na+ and K+ concentrations were determined by atomic absorption spectrometry and Cl- concentrations by conductometric titration following extraction of the dried tissue with 0.1 M HNO3. 14C-labelled inulin was used to determine extracellular volume. Em was measured with conventional open tip microelectrodes, aiCl with solid-state Cl-selective silver microelectrodes and aiNa and aiK with Na+ and K+-selective liquid ion-exchanger microelectrodes. The average Em recorded was -34mV. ciNa, ciK and ciCl were 51, 105 and 52 mM. The corresponding values for aiNa, aiK and aiCl were 18, 80 and 33 mM. These results suggest that a large fraction of the cytoplasmic Na+ is 'bound' or sequestered in an osmotically inactive form, that all, or virtually all the cytoplasmic K+ behaves as if in free solution, and that there is probably some binding of cytoplasmic Cl-. aiCl significantly exceeds the level corresponding to electrochemical equilibrium across the mucosal and baso-lateral cell membranes. Earlier studies showed that coupled mucosal entry of Na+ and Cl- is implicated in intracellular Cl- accumulation in this tissue. This study permitted estimation of the steady-state transapical Na+ and Cl- electrochemical potential differences (deltamuNa and deltamuCl). deltamuNa (-7000 J . mol-1; cell minus mucosal medium) was energetically more than sufficient to account for deltamuCl (1000--2000 J . mol-1).

Animals↗
PubMed1978

Calcium fluxes and contractility in isolated guinea pig atrium: effect of A23187.

The Ca2+ ionophore A23187 (10(-6) to 3 X 10(-5) M) increased the force of contraction is isolated guinea pig atria. In individual twitches, peak tension, maximum rate of tension development, time to peak tension, and total twitch duration were all increased by A23187. Tripelennamine, indomethacin, and atropine did not significantly alter the inotropic effect of A23187. Serotonin produced changes in individual twitches that differed qualitatively and quantitatively from those of A23187. Therefore, the inotropic action of A23187 is probably not mediated by release of endogenous histamine, prostaglandins, acetylcholine, or serotonin. 45Ca influx and efflux were increased by A23187. The enhanced 45Ca efflux exceeded that which would be predicted if the ionophore acted only to increase the passive Ca2+ permeability of the myocardial cell membrane. These results suggest that A23187 facilitates the entry of extracellular Ca2+ into the myocardial cell and the release of intracellular Ca2+ stores into the myoplasm. The resultant increase in intracellular Ca2+ activity could account for the positive inotropic action of A23187.

Animals↗
PubMed1977

A new solid-state microelectrode for measuring intracellular chloride activities.

Solid-state microelectrodes from measuring intracellular Cl activity (alphaiCl) were made by sealing the tips of tapered glass capillaries (tip diameter 0.3 mum), coating them under vacuum with a 0.2-0.3 mum thick layer of spectrscopic grade silver, and sealing them (except for the terminal 2-5 mum of the tip) inside tapered glass shields. 106 microelectrodes had an average slope of 55.0+/- 0.6 m V (S,E,) per decade c hange in alphaCl. Tip resistance was (77.1+/- 3.1) x 10(9) omega(n=30). Electrode response was rapid (10-20 s), was unaffected by HCO3, H2PO4, HPO42 or protein, and remained essentially unchanged over a 24-h period. AlphaiCl in frog sartorius muscle fibers and epithelial cells of bullfrog small intestine was measured in vitro. In both tissues, alphaiCl significantly exceeded the value corresponding to equlibrium ditribution of Cl across the cell membrane.

Animals↗
PubMed1976

Sugar-induced potential difference and short circuit current in bullfrog small intestine: effect of UO22+.

UO22+ 1.3 mM added as UO2(NO3)2 to the mucosal solution consistently inhibited the P.D. and ISC evoked by 11 mM glucose and 35 mM 3-O-methyl glucose across isolated strips of bullfrog small intestine bathed by symmetrical Ringer solutions in which SO42- was the major anion. The average degree of inhibition in the presence of glucose was 42 +/- 7(SEM) percent. P.D. and ISC in the absence of transported solutes were not significatnly altered by mucosal UO22+ at this concentration. Increasing the mucosal UO22+ concentration to 2.6 mM did not significantly increase its inhibitory action on glucose-evoked P.D. and ISC. Further increasing the UO22+ concentration to 13 mM completely inhibited glucose-induced P.D. and ISC but also markedly reduced these parameters in the absence of glucose. Serosal UO22+ (1.3mM) had no effect on the P.D. and ISC evoked by glucose and 3-O-methyl glucose. It is suggested that the inhibitory action of UO22+ involves binding of this ion to anionic sites located in the apical membrane of the absorptive cells. Mucosal or serosal UO22+ (1.3 mM) had no effect on the P.D. and ISC elicited by 20 mM valine, indicating that under the conditions of these experiments UO22+ selectively inhibits sugar-induced P.D. and ISC and, by implication mucosal sugar uptake.

Animals↗
PubMed1975

Osmotically induced electrical changes in isolated bullfrog small intestine.

1. Steady state values of cell water, intracellular Na+ and K+ concentrations, and the electrical parameters ETr, Em, and Isc in the mucosa of isolated bullfrog small intestine were determined following immersion in sodium sulfate Ringer solutions with identical ionic composition but different osmolalities. 2. Cell water and intracellular K+ concentration were inversely related to the osmolality of the bathing medium. During 1 h immersion, intracellular Na+ concentration was not significantly affected by an increase or decrease in external osmolality. 3. Replacement of a hypotonic or an approximately isotonic (normal) medium by a medium of greater osmolality caused statistically significant decreases in ETr, Isc and the (inside negative) magnitude of Em. Conversely, when a hypertonic or a normal medium was replaced by one of lower osmolality, significant increases in the magnitude of these parameters were observed. 4. An equivalent circuit model for the epithelial cell layer, in which the resistance of a relatively highly conducting extracellular shunt pathway is assumed to be the major determinant of the electrical responses of the small intestine to external osmolality, has been shown to account satisfactorily for the observed changes in ETr and Em. In terms of this model, the experimentally observed dependence of Isc on external osmolality requires that, even when both the mucosal and the serosal sides of the tissue are bathed by identical media, isolated bullfrog small intestine maintains a finite diffusion potential across the shunt pathway. This is consistent with current views concerning transepithelial ionic transfer mechanisms.

Animals↗
PubMed1975

A23187: a calcium ionophore that directly increases cardiac contractility (38704).

The effect of the calcium ionophore A23187 on contractility in guinea pig left atria was studied. This ionophore increased both the force of contraction and the rate of tension development in a concentration dependent manner. The positive inotropic effect of A23187 is not mediated by activation of beta-receptors or by the release of endogenous catecholamines since neither propranolol nor pretreatment with reserpine altered the inotropic effect of this agent. Our studies of the dependence of contractile force on external calcium indicate that A23187 enhances the response of atrial muscle to external calcium ions. Since A23187 is known to facilitate the movement of calcium across biological membranes, we believe the ionophore may be acting either by increasing calcium influx across the myocardial cell membrane or by facilitating calcium release from sarcoplasmic reticulum.

Animals↗