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A J Levi

Publications and source records attributed to A J Levi.

At least 73 records · Page 4Linked to original sources
PubMed1994

Relation between reverse sodium-calcium exchange and sarcoplasmic reticulum calcium release in guinea pig ventricular cells.

Exchange-inhibitory peptide (XIP) can inhibit sodium-calcium exchange without inhibiting L-type calcium current (ICa). We therefore used this compound to test the hypothesis that reverse sodium-calcium exchange can trigger contraction in guinea pig ventricular myocytes. When cells were dialyzed with 20 mmol/L sodium, rapid blockade of ICa with nifedipine had little effect on cell shortening. However, if reverse exchange was inhibited by first dialyzing the cells with XIP, blockade of ICa largely inhibited cell shortening. In cells dialyzed with 10 mmol/L sodium, about 51% of the maximum cell shortening remained after ICa was blocked. When both ICa and reverse exchange were significantly inhibited with nifedipine and XIP, only 24% of the cell shortening remained; ie, 27% was XIP inhibitable. Cells dialyzed with solutions deficient in sodium exhibited contractions that were largely dependent on ICa (ie, not XIP inhibitable). If the sarcoplasmic reticulum (SR) was disabled with ryanodine and thapsigargin, reverse exchange could not cause contraction. We therefore conclude that with intact SR, reverse sodium-calcium exchange activates contraction by triggering calcium release from the SR in cells dialyzed with either 10 or 20 mmol/L sodium. A scrambled sequence of XIP, sXIP, caused no measurable effect on contraction.

Amino Acid Sequence↗
PubMed1993

Characterization of the inhibition by stilbene disulphonates and phloretin of lactate and pyruvate transport into rat and guinea-pig cardiac myocytes suggests the presence of two kinetically distinct carriers in heart cells.

1. The kinetics of transport of pyruvate (Km 0.20 mM), L-lactate (Km 2.2 mM) and D-lactate (Ki 10.2 mM) into rat cardiac myocytes were studied and compared with those for guinea-pig heart cells [Poole, Halestrap, Price and Levi (1989) Biochem. J. 264, 409-418] whose equivalent values were 0.07, 2.3 and 6.6 mM respectively. Maximal rates of transport were about 5-fold higher in the rat heart cells. 2. 4,4'-Dibenzamidostilbene-2,2'-disulphonate (DBDS), a powerful inhibitor of monocarboxylate transport into erythrocytes [Poole & Halestrap (1991) Biochem. J. 275, 307-312], was found to be a potent but apparently partial inhibitor of lactate and pyruvate transport, with an apparent Ki value at 0.5 mM L-lactate of about 16 microM in both species. Maximal inhibition was 50% and 80% in rat and guinea-pig cells respectively. 3. The maximal extent of inhibition and apparent Ki values were dependent on both the substrate transported and its concentration. Maximum inhibition was less and the Ki was greater at higher substrate concentrations. 4. A variety of other stilbene disulphonates were studied which showed different Ki values and maximal extents of inhibition. 5. Phloretin was a significantly less potent inhibitor of transport into both rat (Ki 25 microM) and guinea-pig (Ki 16 microM) heart cells than into rat erythrocytes (Ki 1.4 microM). In the rat but not the guinea-pig heart cells, inhibition appeared partial (maximal inhibition 84%). 6. We demonstrate that our results can be explained by the presence of two monocarboxylate carriers in heart cells, both with Km values for L-lactate of about 2 mM and inhibited by alpha-cyano-4-hydroxycinnamate, but with different affinities for other substrates and inhibitors. One carrier is sensitive to inhibition by stilbene disulphonates and has lower Km values for pyruvate (0.05-0.10 mM) and D-lactate (5 mM), whereas the other has higher Km values for pyruvate (0.30 mM) and D-lactate (25 mM), and is relatively insensitive to stilbene disulphonates. Rat heart cells possess more of the latter carrier and guinea-pig heart cells more of the former. 7. The significance of these results for the study of lactate transport in the perfused heart is discussed.

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗
PubMed1993

One hump or two? The triggering of calcium release from the sarcoplasmic reticulum and the voltage dependence of contraction in mammalian cardiac muscle.

We have examined the hypothesis that the sarcolemmal Na/Ca exchanger is able to trigger calcium release from the sarcoplasmic reticulum in a direct fashion. We propose that when the cardiac muscle membrane is depolarised, for instance during the upstroke of the action potential or a square voltage clamp pulse, the voltage dependent Na/Ca exchanger generates an initial "spike" of calcium entry which is sufficient to trigger a fraction of the normal sarcoplasmic reticular calcium release, via calcium induced calcium release. For the last 20 years, it has been widely considered that calcium entry through L-type calcium channels is the only trigger for calcium release from sarcoplasmic reticulum in cardiac muscle. In the first section of this review, we examined some of the earlier studies of excitation-contraction coupling which used multicellular preparations of cardiac muscle. We suggested that these earlier studies do not support the idea that calcium entry via the calcium current (ICa) is the only trigger for sarcoplasmic reticular release. In contrast, more recent studies using isolated myocytes have supported ICa as the only trigger. However, these were performed mostly with a low or absent sodium inside the cell, or with an increased intracellular calcium buffering, or with other altered internal ions (eg, high magnesium or caesium in the pipette) or at a relatively low temperature. All these factors may have reduced or abolished the initial spike of calcium entry which the Na/Ca exchanger is expected to generate at the start of depolarisation. New studies on myocytes are presented, using conditions where cells are dialysed minimally, or where a normal level of internal sodium is preserved deliberately.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1993

A role for sodium/calcium exchange in the action potential shortening caused by strophanthidin in guinea pig ventricular myocytes.

OBJECTIVE: The aim was to investigate the nature of the membrane currents which underlie the shortening of the action potential in guinea pig cardiac myocytes upon exposure to the digitalis analogue strophanthidin. METHODS: Ventricular myocytes were isolated enzymatically from the guinea pig heart and impaled with conventional microelectrodes to measure action potentials. Cells were voltage clamped and the change in membrane current upon strophanthidin exposure was recorded. Contractile activity was assessed optically as cell shortening. RESULTS: Strophanthidin caused an initial lengthening followed by a progressive shortening of the action potential. The initial lengthening was due to an inhibition of outward Na/K pump current caused by strophanthidin. The subsequent action potential shortening was associated with the progressive activation of a membrane current that reversed at -54.5(SD 7.5) mV, n = 8. Since this current was outward over the potential range of the action potential plateau, it participated in causing the action potential shortening with strophanthidin. This component of membrane current was not sensitive to potassium channel blockers, but it was blocked by removing external Ca and applying 5 mM nickel externally. Removal of external Ca inhibits outward current generated by the Ca entry/Na extrusion mode of the Na/Ca exchange, whereas nickel is known to block the Na/Ca exchange. CONCLUSIONS: The voltage dependence of the membrane current associated with progressive action potential shortening, and its sensitivity to external Ca and nickel, suggest that it is carried on the Na/Ca exchange. It is proposed that the Na/Ca exchange generates this membrane current in response to the combined rise of intracellular sodium and calcium that occurs with strophanthidin. Theoretical calculations simulating the effect of a combined rise of intracellular sodium and calcium on Na/Ca exchange predict closely the reversal potential and characteristics of the experimentally measured current. The results of this study suggest that a membrane current generated by the Na/Ca exchange makes an important contribution to the action potential shortening that occurs with digitalis compounds.

4-Aminopyridine↗
PubMed1993

Investigation of the mechanisms underlying the increased contraction of hypertrophied ventricular myocytes isolated from the spontaneously hypertensive rat.

OBJECTIVE: The aim was to investigate the cellular mechanisms responsible for the increased contraction of left ventricular myocytes isolated from the spontaneously hypertensive rat (SHR). METHODS: Single myocytes were isolated enzymatically from the left ventricles of SHR, Wistar-Kyoto (WKY), and Wistar rats. WKY and Wistar myocytes were used as normotensive controls. Cytoplasmic calcium was measured with Fura-2 and contraction was measured optically. Membrane potential was measured with microelectrodes and cells were voltage clamped to measure the amplitude of L-type calcium current (iCa). RESULTS: Under action potential conditions, SHR myocytes had a larger calcium transient and an increased sarcoplasmic reticular calcium content compared to normotensive myocytes. There was no detectable difference in the resting cytoplasmic calcium concentration between SHR and control myocytes. SHR myocytes also had a prolonged action potential compared to normotensive cells. However, when cells were voltage clamped and short pulses of 120 ms duration were applied (a similar duration of depolarisation to the action potential), the difference in the calcium transient or contraction between SHR and normotensive myocytes was abolished. SHR myocytes had an unchanged amplitude of ICa in comparison to control myocytes, and there was no detectable difference in the myofilament response to calcium between SHR and control myocytes. CONCLUSIONS: (1) Hypertrophied SHR myocytes stimulated with action potentials had an increased calcium transient compared to normotensive cells. The greater calcium transient in the SHR cells is likely to be a major factor responsible for their increased contraction. (2) SHR myocytes had a prolonged action potential in comparison to normotensive cells. (3) The amplitude of ICa and myofilament response to calcium were unchanged in SHR myocytes, suggesting that these factors do not play a role in the increased contraction of these cells. (4) Since the difference between SHR and control cells was abolished by voltage clamping the cells to prevent the difference of action potential, it is unlikely that an alteration of intrinsic mechanisms in SHR myocytes is responsible for their increased contraction. Rather, it suggests that the prolonged action potential of SHR myocytes plays a important role in causing their increased calcium transient and contraction. Our results indicate that the prolonged action potential in SHR cells results in an increased calcium content of the sarcoplasmic reticulum, which leads to a greater sarcoplasmic reticular calcium release upon stimulation and an increased contraction.

Action Potentials↗
PubMed1993

A role for depolarisation induced calcium entry on the Na-Ca exchange in triggering intracellular calcium release and contraction in rat ventricular myocytes.

OBJECTIVE: The aim was to test whether depolarisation-induced calcium entry on the Na-Ca exchange is able to trigger calcium release from the sarcoplasmic reticulum in rat ventricular myocytes. METHODS: Myocytes were isolated enzymatically from the left ventricle of the rat heart. Cells were impaled with narrow tipped microelectrodes to minimise intracellular dialysis and maintain normal internal ionic conditions. Cells were voltage clamped, contraction was measured optically, and in some experiments intracellular calcium was measured with Fura-2. RESULTS: When the fast Na current was inactivated by using a holding potential of -40 mV, Ca entry via L-type Ca channels was expected to be the only mechanism capable of triggering sarcoplasmic reticular Ca release. In this situation, blocking L-type Ca channels should have abolished sarcoplasmic reticular release and the phasic twitch. However, after 2 min exposure to 20 microM nifedipine, which abolished the Ca current (ICa) completely, voltage clamp depolarisation from -40 mV to 0 mV still elicited 41(SEM 8.9)% of the control phasic twitch (n = 22 cells). This shows that there must be another mechanism, besides Ca entry via Ca channels, by which membrane depolarisation can trigger sarcoplasmic reticular release and the phasic twitch. The phasic twitch that remained in the presence of nifedipine increased progressively with the magnitude of step depolarisation, required a functional sarcoplasmic reticulum, was abolished by 5 mM external nickel, and was sensitive to both the Na and Ca transmembrane gradients. CONCLUSIONS: The voltage dependent sarcolemmal Na-Ca exchange is predicted theoretically to generate a transient Ca entry at the start of a step membrane depolarisation, when membrane potential suddenly becomes more positive than the reversal potential of the Na-Ca exchange. The results of this study indicate that in rat myocytes with normal internal ions, physiological levels of membrane depolarisation generate a sufficient Ca entry on the exchange to trigger sarcoplasmic reticular calcium release and contraction. In the absence of ICa, this mechanism is capable of triggering a calcium release which leads to about 40% of the phasic contraction in cells depolarised from -40 mV to 0 mV. The existence of this sarcoplasmic triggering mechanism may have significance for the normal control of cardiac muscle contraction.

Animals↗
PubMed1993

The electrophysiological characteristics of hypertrophied ventricular myocytes from the spontaneously hypertensive rat.

OBJECTIVE: Previous studies on multicellular preparations have shown that hypertrophied cardiac muscle from the spontaneously hypertensive rat (SHR) has a prolonged action potential. The first aim of the present study was to determine whether the action potential of isolated left ventricular myocytes was similarly prolonged and to study the underlying membrane currents that might be responsible. The second aim was to evaluate the L-type calcium current amplitude of SHR myocytes, as we have recently shown that they have an increased contraction and an increase in the calcium trigger entering via the L-type calcium channel might be one possible mechanism for this. METHODS: The electrophysiological characteristics of left ventricular myocytes isolated from the SHR were compared with those from normotensive control rats. Action potentials were recorded with microelectrodes. Cells were voltage-clamped and the membrane currents elicited by steps to different potentials were analysed. Blockers of potassium and calcium currents were used to reveal the contribution made by these currents to net membrane currents. RESULTS: SHR myocytes had prolonged action potentials. The action potential duration of SHR myocytes at 90% repolarization was found to be longer, although at 20% and 50% repolarization no difference was found. There was no difference in the resting membrane potential between SHR and control myocytes. Using a voltage clamp we studied the L-type calcium current and potassium currents. The major change in SHR myocytes was a decrease in the magnitude (normalized to the membrane capacitance) of the inward rectifier potassium current elicited by negative potentials. There was no detectable difference in either the transient outward or delayed rectifier potassium currents. We also found no difference in the magnitude, time course or voltage dependence of L-type calcium current in hypertrophied SHR myocytes. CONCLUSIONS: First, the action potential of SHR myocytes was prolonged compared with control myocytes. Secondly, the main change in SHR myocytes was that pulses to negative potentials elicited a lower inward rectifier potassium current. A reduction in the density of inward rectifier channels might play a role in prolonging the SHR action potential, since a lower outward repolarizing current will flow through inward rectifier potassium channels during the SHR action potential repolarization. Thirdly, there was no difference in L-type calcium current density or time course between SHR and control myocytes. Thus, a change in L-type calcium current probably plays no role in causing the prolonged SHR action potential or the increased contraction of hypertrophied SHR ventricular myocytes. Finally, the prolonged action potential in SHR myocytes may itself be one factor responsible for the increased contraction of these cells.

Action Potentials↗
PubMed1993

Food intolerance and Crohn's disease.

It has been claimed that prolonged remissions of Crohn's disease can be achieved after enteral or parenteral nutrition, by identifying and excluding foods that exacerbate a patient's symptoms. The occurrence of food intolerances were assessed after induction of remission with elemental diet in 42 eligible patients to whom single foods were introduced over five days. Suspect foods were reinvestigated with open and if possible, double blind rechallenge. Fourteen patients (33%) dropped out of the study because of relapse of disease unrelated to food (n = 8) or because of difficulties in complying with the regimen (n = 6). Twenty (48%) of the patients identified food sensitivities whereas eight (19%) did not. Seventeen of the patients who identified food sensitivities had an open rechallenge with recurrence of symptoms in 10 (24% of total). Food sensitivity was confirmed in three patients on double blind challenge. There was no significant difference in the duration of remission between patients who did or did not identify food sensitivities. During the study three cases of intolerance to the formula diet, and one of severe salicylate sensitivity were encountered. In conclusion food sensitivities are evident after treatment of Crohn's disease with elemental diet but are variable, often do not persist, and are of insufficient importance to warrant putting all patients through elimination diets.

Adolescent↗
PubMed1993

A method for isolating rabbit atrioventricular node myocytes which retain normal morphology and function.

This report describes a method for isolating single rabbit atrioventricular (AV) node myocytes which retain their normal morphology when exposed to millimolar levels of calcium. Previous attempts to isolate cells from the AV node have produced myocytes that "round up" (i.e., go into contracture) when exposed to calcium. We show that the cells isolated with our technique possess properties similar to those described for intact AV nodal tissue. We find that single AV node myocytes are shorter and thinner (mean dimension = 103.5 +/- 2.3 by 7.8 +/- 0.2 microns; mean +/- SE, n = 90) than atrial or ventricular cells. Many of the cells produced by this isolation procedure generate spontaneous action potentials (188 +/- 9 beats/min; n = 6), which resemble action potentials recorded previously from the intact AV node. Voltage-clamp recordings from spontaneously active cells revealed similar membrane currents to those seen in intact tissue: fast sodium current and a L-type calcium current, followed by a delayed outward current. However, we found little evidence for the hyperpolarization-activated current (I(f)). Because the cells responded normally to concentrations of acetylcholine and isoproterenol within the physiological range, their cholinergic and adrenergic receptors appear to be well preserved by the isolation procedure. The ability to isolate morphologically and functionally normal AV myocytes may represent a significant advance for the investigation of nodal physiology at the cellular level.

Acetylcholine↗
PubMed1992

Contractile properties of ventricular myocytes isolated from spontaneously hypertensive rat.

OBJECTIVE: To determine whether there are any differences in contractile properties of individual cardiac myocytes isolated from the spontaneously hypertensive rat (SHR) in comparison with its normotensive control--the Wistar-Kyoto (WKY) rat. DESIGN: The effects of cardiac hypertrophy upon individual myocytes from SHR have not been studied previously. Isolated cardiac myocytes do not suffer from a number of problems inherent in experiments on multicellular preparations. METHODS: Seven SHR and eight WKY animals were studied. Age-matched animals were compared at 60 and 100 days old. Ventricular myocytes were isolated enzymatically. Myocyte length and width was measured. The cells were stimulated with extracellular electrodes and contraction was measured optically. The effects of altering stimulus rate and extracellular calcium concentration upon contraction were studied. RESULTS: SHR myocytes were found to be significantly wider than WKY myocytes. The contraction (i.e. unloaded cell shortening) of SHR myocytes at stimulation rate of 0.3, 1, 2 and 3 Hz was significantly increased. The time-course of contraction was altered, with SHR myocytes having an increased maximal velocity of shortening and relaxation. The response to changes in bathing calcium was similar in both strains. CONCLUSIONS: Individual cardiac myocytes isolated from SHR have an increased contraction. This indicates that cardiac hypertrophy, at least in the early stages, is a protective adaptation allowing the heart to overcome the increased afterload resulting from hypertension.

Animals↗
PubMed1992

Intestinal permeability in patients with Crohn's disease and their first degree relatives.

It has been reported that intestinal permeability to polyethylene glycol 400 is increased in patients with Crohn's disease and their apparently unaffected first degree relatives. Because of the implications that these findings have for the aetiology of Crohn's disease these studies were repeated. Patients with Crohn's disease (n = 28) and 32 first degree relatives from 11 families underwent a polyethylene glycol 400 (PEG400) intestinal permeability test and a hyperosmotic (1500 mosmol/l) absorption/permeability test using 3-0-methyl-D-glucose, D-xylose, L-rhamnose, lactulose, and 51chromium labelled ethylenediamine-tetraacetate. The five hour urine excretion of polyethylene glycol 400 did not differ significantly between controls (n = 25) and first degree relatives, 25.5 (3.3)% v 24.6% (4)% (mean(SD)) p greater than 0.1, respectively. Patients with small bowel involvement excreted significantly less (p less than 0.01) polyethylene glycol 400 (16.3 (4.6)% than controls while those with Crohn's colitis did not (26.4 (3.9)% p greater than 0.1). The permeation of the monosaccharides in patients with Crohn's disease and their first degree relatives did not differ from normal subjects. The permeation of lactulose and 51chromium ethylenediaminetetraacetate was not significantly altered in first degree relatives but was significantly increased in the patients, as was the lactulose/L-rhamnose urine excretion ratio which is a specific measure of small intestinal permeability. These studies show normal absorption and permeability in first degree relatives of patients with Crohn's disease. A genetically determined abnormality of intestinal permeability is not likely to be an important aetiological factor in Crohn's disease.

Adolescent↗
PubMed1991

The effect of elemental diet on intestinal permeability and inflammation in Crohn's disease.

This study examines whether treatment of acute Crohn's disease with an elemental diet improves intestinal integrity and inflammation as assessed by a 51Cr-labeled ethylenediaminetetraacetatic acid (EDTA) permeability test and the fecal excretion of 111In-labeled autologous leukocytes, respectively. Thirty-four patients with active Crohn's disease completed a 4-week treatment course with an elemental diet. Active disease was characterized by increased intestinal permeability [24-hour urine excretion of orally administered 51Cr-EDTA, 6.4% +/- 0.6% (mean +/- SE); normal, less than 3.0%] and by high fecal excretion of 111In-labeled leukocytes (14.2% +/- 1.1%; normal, less than 1.0%). Twenty-seven (80%) went into clinical remission, usually within a week of starting treatment. After 4 weeks of treatment, there was a significant decrease in both the urine excretion of 51Cr-EDTA (to 3.4% +/- 0.5%; P less than 0.01) and the fecal excretion of 111In (to 5.7% +/- 1.0%; P less than 0.001), indicating that such treatment is not just symptomatic. A framework for the mechanism by which elemental diet works, centering around the importance of the integrity of the intestinal barrier function, is proposed, and also appears to provide a logical explanation for some relapses of the disease.

Acute Disease↗
PubMed1991

The relationship between nutritional status and serum soluble interleukin-2 receptor concentrations in patients with Crohn's disease treated with elemental diet.

Serum soluble interleukin-2 receptor concentrations (sIL-2R) were measured and correlated with indices of disease activity and nutrition in 13 patients with active Crohn's disease treated with an elemental diet. The initial serum sIL-2R concentrations were raised, 1121 +/- 181 U/ml (mean +/- SEM) compared to controls, 177 +/- 22.9 U/ml (n = 18) (p < 0.001). Four weeks' treatment resulted in significant improvement in disease activity (Harvey-Bradshaw index) and 4-day faecal (111)Indium-leucocyte excretion. Serum sIL-2R concentrations did not change significantly after treatment, 789 +/- 79.8 U/ml (p > 0.05). Serum sIL-2R concentrations were inversely correlated with albumin, pre-albumin, creatinine-height index and total body potassium. Only those patients with markedly elevated sIL-2R concentrations (>800 U/ml) and severe nutritional depletion prior to treatment, showed significant reductions in sIL-2R levels with elemental dietary treatment. These results demonstrate an association between nutritional impairment and immune activation in Crohn's disease.

Journal Article↗
PubMed1991

The role of intracellular sodium in the control of cardiac contraction.

Intracellular sodium was estimated in ventricular myocytes using the new Na-sensitive fluorescent indicator SBFI. Membrane potential and contraction were also measured simultaneously. Using an in situ calibration method, we found that intracellular sodium activity (aiNa) was 2.9 mM in quiescent rabbit cells. When the digitalis analogue strophanthidin inhibited the Na-K pump of myocytes with action potentials (APs), changes of contraction and aiNa were dissociated in time. There was also marked hysteresis between contraction and aiNa. When strophanthidin was applied to the same myocytes under voltage-clamp conditions, temporal dissociation between contraction and aiNa was dramatically reduced. This suggests that much of the dissociation and hysteresis was due the change in AP shape with strophanthidin. A small amount of residual hysteresis still existed even with voltage-clamp, and this persisted when the pump was blocked by removal of external potassium as an alternative method. We suggest that a gradient of sodium concentration from the subsarcolemmal space to the bulk cytoplasm might be responsible for hysteresis. Whereas SBFI probably signals the average Na level of the cytoplasm, subsarcolemmal Na may control Ca influx and contraction via Na-Ca exchange.

Action Potentials↗
PubMed1991

Undershoots of intracellular sodium and the strength of contraction when the sodium-potassium pump of isolated sheep Purkinje fibres is reactivated by potassium.

We have recorded changes of membrane current, intracellular Na+ activity (aiNa), intracellular pH (pHi) and the strength of contraction in voltage-clamped sheep Purkinje fibres during and after inhibition of the Na(+)-K+ pump. The pump was inhibited by the removal of K+ from the bathing solution and this resulted in a rise in aiNa. On return of K+ to the bathing medium to reactivate the Na(+)-K+ pump there was a fall of aiNa to below its control value after which aiNa slowly returned to its control value over the next 20 min. This 'undershoot' of aiNa was seen in 69% of contracting fibres and 78% of quiescent fibres, and the mean (+/- S.D.) values of the undershoot in contracting and quiescent fibres were 0.66 +/- 0.15 and 0.9 +/- 0.35 mM, respectively. The undershoot of aiNa was observed regardless of whether the Na(+)-K+ pump was reactivated with Rb+ or K+. It was not voltage dependent over the potential range studied (-95 to -45 mV) and was not accompanied by a change of intracellular pH. The undershoot of aiNa could be the result of a long-lasting increase in Na+ efflux or a long-lasting decrease in Na+ influx. Zero [K+]o resulted in the loss of one Na+ current, the pacemaker current i(f), but when K+ was returned to the bathing medium i(f) recovered rapidly and is therefore unlikely to be responsible for the long-lasting undershoot of aiNa. This conclusion was confirmed by the use of Cs+: although Cs+ blocked i(f), it did not block the undershoot of aiNa. The undershoot of aiNa was accompanied by (and, via Na(+)-Ca2+ exchange, was presumably the cause of) an undershoot of the force of contraction. Undershoots are not only seen after reactivation of the Na(+)-K+ pump: in a variety of different preparations, similar undershoots in aiNa and twitch force have been reported after a decrease in the frequency of stimulation. The undershoot of aiNa may be the result of novel feedback mechanism for the control of aiNa: the control of Na+ influx by aiNa.

Animals↗
PubMed1991

The effect of strophanthidin on action potential, calcium current and contraction in isolated guinea-pig ventricular myocytes.

1. A method is described for producing high yields of calcium-tolerant ventricular myocytes from guinea-pig hearts (73.4% rod-shaped cells, n = 19). Their action potential (AP) and membrane currents were recorded using conventional microelectrodes and cell shortening was measured optically using a linear photodiode array. 2. The sensitivity of the guinea-pig Na(+)-K+ pump to strophanthidin (a rapidly acting digitalis analogue) was determined by measuring the inhibition of outward pump current by different doses. The pump was found to have a dissociation constant (KD) for strophanthidin of 1.11 x 10(-5) M, and 5 x 10(-4) M-strophanthidin inhibited the pump maximally. 3. Exposure to strophanthidin resulted in an initial lengthening followed by a shortening of the AP, and an increased contraction. Initial AP lengthening was associated with a more positive AP plateau which became more negative as the AP shortened. 4. There was a reversible reduction of Ca2+ current (ICa) during exposure to strophanthidin. ICa changed reciprocally with contraction and with a similar time course. 5. Strophanthidin exposure caused a reduction of ICa at all activating voltages, suggesting that it resulted in a reduction of Ca2+ conductance with little change of its voltage dependence. 6. The role of an increase of intracellular calcium (Cai2+) was investigated by impaling myocytes with microelectrodes containing BAPTA 1,2-bis (2-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid, a calcium chelator) to increase Cai2+ buffering. Strophanthidin still shortened the AP when BAPTA was present, suggesting that a rise of Cai2+ is not a major cause of AP shortening. 7. Although AP shortening was little affected, the decline of ICa with strophanthidin was markedly reduced when BAPTA was present, suggesting that a rise of Cai2+ was the cause of the ICa decline with strophanthidin. 8. When barium ions carried the current through Ca2+ channels, strophanthidin did not reduce Ca2+ channel current, suggesting that this compound does not have a direct inhibitory effect on the channel. 9. The results suggest that strophanthidin causes a reduction of ICa by increasing Cai2+, via the mechanism of Cai(2+)-dependent inactivation of ICa. The reduction of ICa at least partially explains the AP shortening and more negative plateau with strophanthidin. 10. The shortening of the AP, more negative plateau and reduced ICa have negative inotropic effects which oppose the direct positive inotropic effect of strophanthidin.

Action Potentials↗
PubMed1991

Importance of local versus systemic effects of non-steroidal anti-inflammatory drugs in increasing small intestinal permeability in man.

Increased small intestinal permeability caused by non-steroidal anti-inflammatory drugs (NSAIDs) is probably a prerequisite for NSAID enteropathy, a source of morbidity in patients with rheumatoid arthritis. This increased small intestinal permeability may be a summation of a local effect during drug absorption, a systemic effect after absorption, and a local effect of the drug excreted in bile, but the relative contribution made by these factors is unknown. We assessed the effect of indomethacin and nabumetone on intestinal permeability. The principal active metabolite of nabumetone, 6-methoxy-2-naphthylacetic acid, is not subject to appreciable enterohepatic recirculation. Twelve volunteers were studied before and after one week's ingestion of indomethacin (150 mg/day) and nabumetone (1 g/day) with a combined absorption/permeability test. Neither drug had a significant effect on the permeation of 3-0-methyl-D-glucose, D-xylose, and L-rhamnose. Indomethacin increased the permeation of radioactive 51chromium ethylenediaminetetra-acetic acid (51Cr EDTA) significantly from baseline (mean (SEM) 0.63 (0.09)% v 1.20 (0.14)%, p less than 0.01) but nabumetone did not (0.70 (0.10)% p greater than 0.1). These results were supported by the 51Cr EDTA/L-rhamnose urine excretion ratios, which reflect changes in intestinal permeability. They suggest that NSAIDs increase intestinal permeability during absorption or after biliary excretion and that the systemic effect is of minor importance.

Adult↗