Biomedical subjects

D G Allen

Publications and source records attributed to D G Allen.

At least 109 records · Page 6Linked to original sources

PubMed1993

Intracellular calcium and myocardial function during ischemia.

Cardiac ischemia causes a rapid decline in mechanical performance and, if prolonged, myocardial cell death occurs on reperfusion. The early decline in mechanical performance could, in principle, be caused either by reduced intracellular calcium release or by reduced responsiveness of the myofibrillar proteins to calcium. It is now known that intracellular calcium rises during ischemia and that the early decline in mechanical performance is caused largely by the inhibitory effects of phosphate and protons on the myofibrillar proteins. The rise of intracellular calcium during ischemia is related to the acidosis and is probably caused by calcium influx on the Na/Ca exchanger. This is triggered by a rise in intracellular sodium which enter the cell in exchange for protons on the Na/H exchanger. Intracellular calcium rises still further on reperfusion the elevation of calcium and the degree of muscle damage are closely correlated.

Animals↗

PubMed1993

Changes of tension and [Ca2+]i during beta-adrenoceptor activation of single, intact fibres from mouse skeletal muscle.

beta-Adrenergic agonists increase tension production in fast-twitch skeletal muscle, but the underlying mechanism is unknown. In the present study we have exposed intact, single fibres from a mouse muscle to the beta 2-adrenergic agonist terbutaline. Fibres were stimulated to produce 350-ms tetani at 20-100 Hz while measuring the myoplasmic Ca2+ concentration ([Ca2+]i) and tension. The fluorescent indicator Indo-1 was used to measure [Ca2+]i. Application of terbutaline resulted in marked increases of both tetanic [Ca2+]i and tension. Terbutaline had no significant effect on myofibrillar function as judged from normal Ca2+ sensitivity and tension production at saturating [Ca2+]i. The rate of [Ca2+]i and tension decline during relaxation was not affected by terbutaline, thus indicating a normal function of the sarcoplasmic reticulum (SR) Ca2+ pumps. The effect of terbutaline developed gradually over 5-10 min when fibres were stimulated each minute; the full effect of terbutaline was also obtained after a 10-min rest period in terbutaline. The [Ca2+]i at rest was not affected by terbutaline. In conclusion, beta-adrenergic stimulation increases tetanic tension by enhancing SR Ca2+ release.

Animals↗

PubMed1993

Control of intracellular Ca2+ by adrenergic and muscarinic agonists in mouse mandibular ducts and end-pieces.

The changes in free Ca2+ ([Ca2+]i) in the cells of the secretory end-pieces and intralobular ducts of mouse mandibular glands exposed to adrenergic or cholinergic agonists were measured using fluorescence imaging techniques. [Ca2+]i in both cell types increased in a dose-dependent manner during both adrenergic and cholinergic stimulation. The duct cells responded to noradrenaline and to acetylcholine over the same concentration range (30 nmol/l to 3 mumol/l) although the maximum increase in [Ca2+]i above resting levels evoked by noradrenaline (ca. 137 nmol/l) was about twice that evoked by acetylcholine. The response to acetylcholine was blocked by atropine (0.1 mumol/l) and the response to noradrenaline was blocked by the alpha 1-adrenergic antagonist, prazosin (0.1 mumol/l), but not by the alpha 2-adrenergic antagonist, yohimbine. The alpha-adrenergic agonist, phenylephrine, mimicked the action of noradrenaline but the beta-adrenergic agonist, isoproterenol, had no effect. In contrast to the duct cells, the end-piece cells responded to acetylcholine at much lower concentrations (threshold << 1 nmol/l) than to noradrenaline (threshold ca. 300 nmol/l) and the size of the increase in [Ca2+]i above resting levels evoked by acetylcholine (216 nmol/l) was nearly 5-times greater than for noradrenaline. VIP and substance P failed to evoked a Ca2+ response in either end-piece or duct cells.

Acetylcholine↗

PubMed1993

Advanced epithelial ovarian cancer: 1993 consensus statements.

BACKGROUND: Over the last few days of a 5-day international workshop held in June 1993, a group of specialists in the field of advanced epithelial ovarian cancer tried to reach consensus on a number of issues with implications for standard practice and for research. METHODS: Five groups of experts considered several issues which included: biologic factors, prognostic factors, surgery, management recommendations, dose intensity, supportive care, drug resistance, second-line treatment, investigational drugs, and tumour markers. Discussing the management recommendations, the group attempted to arrive at answers to four questions: Is there in fact a cure rate for advanced ovarian carcinoma? Are there prognostic factors which help to identify patients who will not do well with current therapy? What is the current best therapy for advanced ovarian carcinoma? What directions should research take in advanced ovarian cancer? In a plenary meeting these issues were discussed. RESULTS: Consensus statements were achieved on all topics mentioned above. This article reports on the statements written by the chairmen and approved by the consensus group.

Antineoplastic Agents↗

PubMed1993

Changes in myoplasmic pH and calcium concentration during exposure to lactate in isolated rat ventricular myocytes.

1. We investigated the mechanisms involved in the rise of myoplasmic calcium concentration ([Ca2+]i) when isolated rat ventricular myocytes were exposed to lactate. The intracellular pH (pHi) and [Ca2+]i were measured using the fluorescent indicators 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) and fura-2, respectively. Cell shortening was used as a measure of contractile performance. 2. Exposure to 20 mM lactate at the normal extracellular pH (pHo 7.4) for 10 min caused the pHi to fall rapidly by 0.24 pH units and cell shortening was reduced. Thereafter, pHi partially recovered by 0.16 pH units, which was paralleled by a recovery of shortening. 3. Exposure to lactate at a reduced extracellular pH (pHo 6.4) induced a very large acidosis of 0.70 pH units and cell shortening was abolished. During maintained exposure to lactate the pHi remained constant and cell shortening did not recover. 4. Application of Na(+)-H+ exchanger inhibitors, amiloride or ethylisopropyl-amiloride (EIPA), abolished the recovery of pHi and shortening during maintained exposure to lactate at pHo 7.4 and caused an additional acidosis during maintained application of lactate at pHo 6.4. 5. Application of lactate at both the normal and reduced pHo resulted in a rapid, followed by a slower, rise in [Ca2+]i. The diastolic and systolic [Ca2+]i and the amplitude of the systolic rise in the [Ca2+]i (the Ca2+ transient) all increased in both the rapid and the slow phase. 6. When lactate was applied at pHo 7.4, in the presence of EIPA, the initial rise of [Ca2+]i still occurred but the slower increase was abolished. This suggests an involvement of the Na(+)-H+ exchanger in the slower rise of [Ca2+]i. 7. In conclusion, the Na(+)-H+ exchanger is an important regulator of pHi during a lactate-induced intracellular acidosis. The rise of [Ca2+]i involves at least two mechanisms: (i) a rapid component which may represent reduced myoplasmic Ca2+ buffering, impaired Ca2+ removal by the sarcoplasmic reticulum or a direct inhibitory effect of protons on the Na(+)-Ca2+ exchanger; (ii) a slower component linked to stimulation of Na(+)-H+ exchanger which causes an increased [Na+]i and stimulates the Na(+)-Ca2+ exchanger, resulting in an enhanced Ca2+ influx.

Animals↗

PubMed1993

The contribution of [Ca2+]i to the slowing of relaxation in fatigued single fibres from mouse skeletal muscle.

1. The contribution of Ca2+ vs. cross-bridges to the slowing of relaxation in fatigue was studied in intact, single fibres dissected from a mouse foot muscle. Fatigue was produced by repeated 350 ms tetani. The free myoplasmic Ca2+ concentration ([Ca2+]i) was measured with indo-1. 2. The rate of [Ca2+]i decline after a tetanus was reduced in fatigue but this reduction appeared smaller than the decrease in relaxation speed. 3. Under control conditions, steady-state [Ca2+]i-tension curves were constructed from measurements of tetani at various stimulus frequencies. Measurements from the final part of fatigue runs, where both tetanic [Ca2+]i and tension fell relatively fast, were used to construct similar curves in fatigue. A comparison of these curves revealed a reduction of both the maximum Ca(2+)-activated tension and the myofibrillar Ca2+ sensitivity in fatigue. 4. Calcium-derived tension records were generated by converting [Ca2+]i signals during tetani into tension by means of the steady-state [Ca2+]i-tension relation. These records would represent a situation where tension responds to changes of [Ca2+]i without delays due to, for instance, cross-bridge attachment or detachment. The relaxation speed of the calcium-derived tension was similar in control and fatigue; the lag between the calcium-derived and the real tension was about twice as long in fatigue. 5. Both the decline of [Ca2+]i and tension were markedly faster after a 10 s pause elicited during fatigue runs at the time of maximum slowing of relaxation. The relaxation of the calcium-derived tension was also faster after the pause. 6. The function of the sarcoplasmic reticulum (SR) Ca2+ pumps was analysed by plotting the rate of [Ca2+]i decline vs. [Ca2+]i during the tail of elevated [Ca2+]i after tetani. This analysis showed approximately a sevenfold reduction of the pump rate in fatigue. 7. In conclusion, the rate of [Ca2+]i decline after a tetanus is reduced in fatigue probably due to impaired SR Ca2+ pumping. This is not the cause of the slowed relaxation because it is counteracted by a reduced myofibrillar Ca2+ sensitivity. Thus, the slowing of relaxation in fatigued mouse muscle fibres would reflect slowed cross-bridge kinetics.

Animals↗

PubMed1993

Characterization of bacteria isolated from a bleached kraft pulp mill wastewater treatment system.

Water samples from the wastewater treatment system of a bleach kraft mill and from the river that supplies the mill were plated on six different media and culturable isolates were screened for substrate utilization patterns, taxonomic characters, plasmid content, and resistance to ampicillin, streptomycin, kanamycin, tetracycline, naldixic acid, mercury, nickel, copper, cobalt, cadmium, and zinc. A cluster analysis of the substrate utilization profiles and taxonomic characters revealed that Pseudomonas, Acinetobacter, and Acidovorax spp. were common among the culturable isolates from the river, while Ancylobacter aquaticus, Klebsiella spp., and an unknown group of pleomorphic Gram-negative methylotrophs were common among the culturable isolates from the mill treatment system. Of isolates from the settling pond and aerated lagoon, 78 and 64% carried plasmids, while only 56% of isolates from the river carried plasmids. Plasmids were significantly associated with resistance to cadmium but not with any other resistance characters. Large numbers of plasmid-carrying A. aquaticus strains and pleomorphic methylotrophs accounted for high plasmid incidence levels in the mill treatment system, and the ability to dechlorinate simple aliphatic substrates was found in these two groups as well as in one Pseudomonas strain.

Acinetobacter↗

PubMed1993

Intracellular calcium concentration during low-frequency fatigue in isolated single fibers of mouse skeletal muscle.

Low-frequency fatigue is a form of muscle fatigue that follows intense muscle activity and is characterized by reduced tetanic tension at low frequencies of stimulation while tetanic tension at high stimulus frequencies is close to normal. The present experiments were performed on isolated single fibers of mouse in which tension and intracellular calcium concentration ([Ca2+]i) were measured. Fatigue was produced by intermittent short tetani continued until tension had declined to 30% of control. Comparison of low- (30- and 50-Hz) and high- (100-Hz) frequency tetani under control conditions and after 30 min of recovery from fatigue showed that low-frequency fatigue was present. During low-frequency fatigue, tetanic [Ca2+]i was substantially reduced at all stimulus frequencies but there was no change in Ca2+ sensitivity or maximum Ca(2+)-activated tension. One possible cause of the reduced tetanic [Ca2+]i is failure of conduction of the action potential in the T tubule, leading to reduced [Ca2+]i in the center of the fiber. However, imaging of [Ca2+]i across the fiber during low-frequency fatigue did not show any such gradient, suggesting that Ca2+ release is uniform across the fiber. Another possible mechanism is that changes in the Ca2+ pumping ability of the sarcoplasmic reticulum might affect tetanic [Ca2+]i. Measurements of the sarcoplasmic reticulum pump function showed a small slowing of Ca2+ uptake rate during low-frequency fatigue, which is unlikely to cause the reduced tetanic [Ca2+]i. In conclusion, the immediate cause of low-frequency fatigue appears to be a reduced tetanic [Ca2+]i, which is probably a consequence of a reduced Ca2+ release from the sarcoplasmic reticulum.

Action Potentials↗

PubMed1993

The influence of intracellular pH on contraction, relaxation and [Ca2+]i in intact single fibres from mouse muscle.

1. The effects of intracellular pH (pHi) on myoplasmic free calcium concentration ([Ca2+]i) and contractile performance were studied in intact single fibres dissected from mouse skeletal muscle. Indo-1 was used to measure [Ca2+]i and pHi was altered by changing perfusate CO2. 2. Tetanic tension was decreased at acidic pHi and increased at alkaline pHi whereas the rate of mechanical relaxation was showed at both acidic and alkaline pHi. Resting and tetanic [Ca2+]i were increased at acidic pHi and decreased at alkaline pHi while the final rate of decline of [Ca2+]i after a tetanus was markedly slowed at acid pHi but only marginally accelerated at alkaline pHi. 3. Steady-state [Ca2+]i-tension curves were constructed from measurements of tetani at different stimulus frequencies. The curves at acid pHi showed a reduced maximum Ca(2+)-activated tension and a reduced Ca2+ sensitivity, and curves at alkaline pHi showed the opposite changes. 4. Two methods were devised to determine the contribution of [Ca2+]i to the rate of relaxation. In one method the instantaneous tension was plotted as a function of instantaneous [Ca2+]i throughout a tetanus and compared with the steady-state [Ca2+]i-tension relation. In a second method the [Ca2+]i signal during a tetanus was converted to a Ca(2+)-derived tension record by means of the steady-state [Ca2+]i-tension relation and this Ca(2+)-derived tension was then compared to the true tension. 5. The sarcoplasmic reticulum (SR) pump function was analysed by plotting -d[Ca2+]i/dt against [Ca2+]i during the final slow decline of [Ca2+]i after a tetanus. This analysis shows that the Ca2+ uptake by the SR is a third- or fourth-power function of [Ca2+]i and that acidosis substantially slows the rate of SR Ca2+ pumping. 6. In conclusion, the slowing of relaxation at acidic pHi could be attributed to a slowing of cross-bridge detachment rather than the observed slowing of the rate of uptake of Ca2+. Conversely the slowing of relaxation in alkaline pHi could be attributed to the increase of Ca2+ sensitivity of the myofibrillar proteins.

Acidosis↗

PubMed1992

Tetraethylammonium blocks muscarinically evoked secretion in the sheep parotid gland by a mechanism additional to its blockade of BK channels.

Since the secretory cells of the sheep parotid gland contain large numbers of high-conductance, voltage- and Ca(2+)-activated K+ channels (BK channels), we have used tetraethylammonium (TEA), a commonly employed blocker of BK channels, to investigate their role in secretion by this gland. In patch-clamp studies we found that 10 mmol/l TEA applied extracellularly inhibits the BK channel but not a 30-pS K+ channel also seen in this gland. We then showed by in-vivo perfusion that muscarinically evoked secretion is inhibited almost completely by 10 mmol/l TEA. We next used microspectrofluorimetry with fura-2 to demonstrate that muscarinic agonists cause the intracellular free Ca2+ concentration to increase. Unexpectedly, however, we found that 0.3-10 mmol/l TEA inhibited the increase in intracellular free Ca2+ induced by 5.0 mumol/l bethanechol or by 0.1 mumol/l acetylcholine. Consequently we conclude that the inhibition of muscarinically evoked secretion by the sheep parotid gland by TEA cannot be attributed solely to blockade of the BK channel--rather it must be attributed, at least in part, to blockade of some step in muscarinic signal transduction, for instance, receptor-agonist binding or Ca2+ release into the cytosol.

Animals↗

PubMed1992

Subcellular gradients of intracellular free calcium concentration in isolated lacrimal acinar cells.

The spatial distribution of intracellular free calcium concentration ([Ca2+]i) was measured in small clusters of isolated rat lacrimal acinar cells by imaging the fluorescence of the Ca(2+)-sensitive dye fura-2. In the absence of extracellular Ca2+, stimulation with acetylcholine (ACh) caused an increase in [Ca2+]i, due to release of intracellular Ca2+ stores, which was maximal at the luminal pole of the cell. In contrast, the organellar Ca(2+)-ATPase inhibitor 2,5-di(tert-butyl)-hydroquinone caused an increase in [Ca2+]i, which was most marked in the basolateral region of the cell. When the cells were stimulated with ACh in a medium containing Ca2+, the gradients of [Ca2+]i (with [Ca2+]i most elevated at the luminal pole) were maintained for the duration of agonist stimulation. The possible implications of these results concerning the location and identity of intracellular Ca2+ stores, and the location of the sites that underlie agonist-stimulated Ca2+ influx, are considered. In particular, it seems likely that intracellular inositol-1,4,5-trisphosphate (InsP3) binding sites may be concentrated in the luminal region of the cell. It is not clear, however, whether this implies that there is a distinct luminally located InsP3-sensitive organelle.

Acetylcholine↗

PubMed1992

Maximum effort in the management of ovarian cancer, including pelvic and para-aortic lymphadenectomy.

Patients treated for ovarian cancer at the Mercy Hospital for Women, Melbourne over a 5 1/2 year period were studied with an emphasis on the need for lymphadenectomy. There were 80 patients identified with ovarian cancer. Forty patients underwent pelvic and/or para-aortic lymphadenectomy and 25 (62.5%) were found to have lymph node metastases, in 7 of the 40 women the lymphadenectomy resulting in upstaging of the disease. FIGO has adopted a surgicopathological approach to the staging of ovarian cancer and this requires lymphadenectomy to be performed. The importance of accurate staging in clinically early ovarian cancer and maximum surgical effort in advanced disease is discussed with particular regard to the place of lymphadenectomy.

Carcinoembryonic Antigen↗

PubMed1992

Myoplasmic Mg2+ concentration in Xenopus muscle fibres at rest, during fatigue and during metabolic blockade.

Intracellular free Mg2+ concentration ([Mg2+]i) was measured in isolated single fibres of Xenopus muscle using the fluorescent Mg2+ indicator furaptra. In resting muscle the [Mg2+]i was 1.7 mM in a Mg(2+)-free Ringer solution. There was no significant change in [Mg2+]i over 2 h in Mg(2+)-free Ringer solution. Elevating extracellular [Mg2+] to 40 mM for 5 min caused a small rise (0.13 mM) in [Mg2+]i. There was no detectable rise in [Mg2+]i after 5 min in Na(+)-free Ringer solution. These results suggest that the membrane is relatively impermeable to Mg2+ and that there was no detectable Na(+)-Mg2+ exchange over 5 min. When muscle fibres were fatigued by repeated tetani continued until force declined to about 40% of control, [Mg2+]i showed characteristic changes. During the early period of fatigue when force first showed a small decline and then became almost stable, [Mg2+]i was unchanged; during the final period of fatigue when force declined more rapidly, [Mg2+]i increased by 0.8 mM. Recovery of [Mg2+]i took about 30 min. Recovery of force was complex: tetanic force first declined (post-contractile depression) and then slowly recovered to control. Since the minimum force occurred at about the time when [Mg2+]i had recovered, it seems unlikely that post-contractile depression is caused by elevated [Mg2+]i. Rigor, produced by inhibiting oxidative phosphorylation and glycolysis, was associated with a larger increase (1.6 mM) in [Mg2+]i than fatigue. The rise in [Mg2+]i during fatigue and metabolic blockade could be explained as release of Mg2+ normally bound to ATP. A model of the metabolic changes and the resulting increase in [Mg2+]i explains our results reasonably well.

Adenosine Triphosphate↗

PubMed1992

Changes of intracellular pH due to repetitive stimulation of single fibres from mouse skeletal muscle.

1. The performance of skeletal muscle during repetitive stimulation may be limited by the development of an intracellular acidosis due to lactic acid accumulation. To study this, we have measured the intracellular pH (pHi) with the fluorescent indicator BCECF (2',7'-bis(carboxyethyl)-5(6)- carboxyfluorescein) during fatigue produced by repeated, short tetani in intact, single fibres isolated from the mouse flexor brevis muscle. 2. The pHi at rest was 7.33 +/- 0.02 (mean +/- S.E.M., n = 29, 22 degrees C). During fatiguing stimulation pHi initially went alkaline by about 0.03 units (maximum alkalinization after about ten tetani). Thereafter pHi declined slowly and at the end of fatiguing stimulation (tetanic tension reduced to 30% of the original; 0.3Po), pHi was only 0.063 +/- 0.011 units (n = 14) more acid than in control. 3. We considered three possible causes of acidosis being so small in fatigue: (i) a high oxidative capacity so that fatigue occurs without marked production of lactic acid; (ii) an effective transport of H+ or H+ equivalents out of the fibres; a high intracellular buffer power. 4. The oxidative metabolism was inhibited by 2 mM-cyanide in three fibres. After being exposed to cyanide for 5 min without stimulation, the tetanic tension was reduced to about 0.9 Po and pHi was alkaline by about 0.1 units. The fibres fatigued faster in cyanide and the pHi decline in fatigue was more than twice as large as that under control conditions. 5. Inhibition of Na(+)-H+ exchange with amiloride resulted in a slow acidification of rested fibres; resting pHi was not affected by either inhibition of HCO3(-)-Cl- exchange with DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid) or inhibition of the lactate transporter with cinnamate. 6. Fibres fatigued in cinnamate displayed a markedly larger acidification (approximately 0.4 pH units) and tension fell more rapidly than under control conditions; inhibition of Na(+)-H+ and HCO3(-)-Cl- exchange did not have any significant effect on fatigue. 7. The intracellular buffer power, assessed by exposing fibres to the weak base trimethylamine, was about 15 mM (pH unit)-1 in a HEPES-buffered solution (non-CO2 or intrinsic buffer power) and about 33 mM (pH unit)-1 in a bicarbonate-buffered solution. Somewhat higher values of the intrinsic buffer power was obtained from changes of the partial pressure of CO2 (PCO2) of the bath solution. Application of lactate or butyrate frequently gave an infinite buffer power, which indicates that powerful pH-regulating mechanisms operate in these cases.(ABSTRACT TRUNCATED AT 400 WORDS)

Acid-Base Equilibrium↗

PubMed1992

Myoplasmic free Mg2+ concentration during repetitive stimulation of single fibres from mouse skeletal muscle.

1. The role of the myoplasmic free Mg2+ concentration ([Mg2+]i) in fatigue was studied in intact single fibres isolated from mouse skeletal muscle. Fatigue was produced by repeated tetanic stimulation. The fluorescent Mg2+ indicator furaptra was pressure injected into fibres. In vivo calibrations were performed to convert fluorescence signals into [Mg2+]i. 2. [Mg2+]i at rest was 0.78 +/- 0.05 mM (mean +/- S.E.M., n = 14). An increase of the extracellular [Mg2+] from 0.5 to 20 mM resulted in a small elevation of [Mg2+]i (86 microM in 5 min). Removal of extracellular Na+ did not affect [Mg2+]i. An intracellular alkanization of about 0.6 pH units gave a [Mg2+]i reduction of 65 microM. 3. During fatiguing stimulation [Mg2+]i initially remained almost constant and it then suddenly started to rise towards the end of the stimulation period. The onset of the [Mg2+]i rise was always followed by a rapid tension decline. In fatigue [Mg2+]i was approximately twice as high as at rest. 4. Fibres were injected with MgCl2 to study if the rise in [Mg2+]i could explain the tension decline in fatigue. An elevation of [Mg2+]i was accompanied by a tension reduction but the [Mg2+]i for a given tension was generally much higher in rested fibres injected with MgCl2 than in fatigued fibres. Thus the rise in [Mg2+]i as such cannot explain the tension reduction in fatigue. 5. Injection of MgCl2 was also used to assess the intracellular Mg2+ buffering. The mean Mg2+ buffer power (i.e. the ratio of the change in [Mg2+]i to the amount of Mg2+ added) was 0.62. 6. ATP is the quantitatively most important binding site for Mg2+ at rest and ATP breakdown is then a likely source of the [Mg2+]i increase in fatigue. The role of ATP breakdown in the increase of [Mg2+]i was studied with metabolic inhibition: fibres were exposed to iodoacetic acid to inhibit glycolysis and cyanide to inhibit oxidative phosphorylation. The pattern during metabolic inhibition was similar to that observed during fatigue. After remaining almost constant during a lengthy period, [Mg2+]i rose rapidly and this rise preceded a period of rapid tension decline. The fibres thereafter went into rigor and [Mg2+]i stabilized at an elevated level; the mean [Mg2+]i increase in rigor was 1.30 mM. 7. We have used modelling to determine the likely change in the intracellular ATP concentration ([ATP]i) for the observed changes in [Mg2+]i.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Triphosphate↗

PubMed1992

Metabolic changes during ischaemia and their role in contractile failure in isolated ferret hearts.

1. The effects of global ischaemia on phosphorus metabolites, intracellular pH (pHi) and developed pressure were measured in isolated whole ferret hearts using 31P nuclear magnetic resonance (NMR) spectroscopy. 2. Brief (10 min) periods of global ischaemia reduced left ventricular developed pressure (LVDP) to undetectable levels. This fall in LVDP was accompanied by a fall in the intracellular concentration of phosphocreatine (PCr) and increases in the concentrations of inorganic phosphate (Pi) and phosphomonoesters. There was no change in the intracellular ATP concentration ([ATP]i). pHi fell approximately linearly at a rate of 0.04 pH units min-1. 3. When ferret hearts were exposed to cyanide (CN-) in the presence of alpha-cyano-4-hydroxycinnamate (CHC), a blocker of lactate efflux, the changes in pHi and [Pi]i which occurred were similar to those observed during global ischaemia. However, developed pressure only fell to around 15% of the control value. 4. Removing the intracellular acidosis (by reducing the CO2 level of the gas with which the perfusate was equilibrated) during exposure to CN- and CHC caused an increase in developed pressure, consistent with the fall in pHi being responsible for a substantial fraction of the fall in developed pressure. 5. Taken together, these results suggest that most, but not all, of the fall in developed pressure during ischaemia can be explained by the effects of the changes in pHi and [Pi]i on the contractile apparatus. 6. Action potential recordings made with a suction electrode during short periods of global ischaemia showed that there was no decrease in action potential duration over the period when developed pressure was falling, eliminating action potential shortening as a possible cause of the fall in developed pressure. 7. In hearts in which the rate of glycolysis had been reduced by glycogen depletion, global ischaemia led to a marked shortening of the action potential. NMR experiments showed that under these conditions [ATP]i decreased by around 50% over the first 10 jin of ischaemia, while the intracellular acidosis which occurred was smaller than that in a control ischaemic period. 8. The time course of the decline of [ATP]i was examined in several hearts during long (45 min and over) ischaemic periods without prior glycogen depletion. After 45 min of ischaemia [ATP]i fell to around two-thirds of the control value, while pHi declined to approximately 6.1. Resting pressure did not increase. On reperfusion pHi recovered rapidly to control levels. [ATP]i, however, did not recover. 9. If ischaemia was prolonged further, [ATP]i eventually became undetectable after 70-90 min.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1992

Changes in intracellular free calcium concentration during long exposures to simulated ischemia in isolated mammalian ventricular muscle.

Intracellular free calcium concentration ([Ca2+]i) was measured in isolated ferret ventricular papillary muscles during and after long exposures to ischemia. All experiments were performed at 37 degrees C, and the muscles were stimulated at 1 Hz. Ischemia was simulated by changing from superfusion with oxygenated Tyrode's solution to superfusion with water-saturated gas (95% N2-5% CO2), thus simultaneously stopping oxygenation and restricting the extracellular space. [Ca2+]i was measured with aequorin, which was microinjected into superficial cells of the preparation. Exposure to ischemia caused a complex series of changes in [Ca2+]i. In the first few minutes the changes in [Ca2+]i were variable; however, after approximately 5 minutes all preparations exhibited a progressive increase in amplitude and duration of the stimulated rise in [Ca2+]i (the calcium transient). The amplitude of the calcium transients peaked after approximately 18 minutes of ischemia, when they were 339% of the control value. After this peak, the calcium transients progressively failed to occur in response to stimulation and declined in amplitude; simultaneously, spontaneous oscillations of [Ca2+]i appeared and increased in size and frequency. The oscillations in turn then gradually became less frequent until a large, prolonged (5-10 minute) increase in [Ca2+]i occurred, after which [Ca2+]i returned to a low level. There were no further oscillations after this event, which was seen on average after 37 minutes of ischemia. A slowly progressive contracture often began to develop at about this time. A gradual rise in resting [Ca2+]i occurred during the remainder of the exposure to ischemia. When muscles were reperfused after long exposures to ischemia, there was a very large and prolonged increase in [Ca2+]i, which was usually associated with a contracture and failure of recovery of developed tension. The large increase in [Ca2+]i could be reduced by the inclusion of 3 mM nickel chloride in the reperfusing solution. Comparison between reperfusion with O2 gas versus reperfusion with anoxic Tyrode's solution indicated that reoxygenation was more beneficial to the muscle than resumption of bulk flow. These results reveal the complex spectrum of changes in [Ca2+]i that occur during ischemia and on reperfusion. These changes in [Ca2+]i are likely to play an important role in the generation of ischemic arrhythmias and muscle damage.

Animals↗

PubMed1992

Role of excitation-contraction coupling in muscle fatigue.

The force produced by muscles declines during prolonged activity and this decline arises largely from processes within the muscle. At a cellular level the reduced force could be caused by: (a) reduced intracellular calcium release during activity; (b) reduced sensitivity of the myofilaments to calcium; or (c) reduced maximal force development. Experiments involving intracellular calcium measurements in isolated single fibres show that all 3 of the above contribute to the decline of force during fatigue. Metabolic changes associated with fatigue are probably involved in each of the 3 factors. Thus the accumulation of phosphate and protons which occur during fatigue cause a reduction in calcium sensitivity and a decline in maximal force. The cause of the reduced intracellular calcium during contractions in fatigue is less clear. During prolonged tetani the conduction of the action potential in the T-tubules appears to fail leading to reduced intracellular calcium in the central part of the muscle fibre. However, during repeated tetani there is a uniform decline of intracellular calcium across the fibre and this remains one of the least understood processes which contribute to fatigue.

Action Potentials↗