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Biomedical subjects

T Powell

Publications and source records attributed to T Powell.

At least 109 records · Page 6Linked to original sources
PubMed1985

Electrophysiological properties of isolated ventricular myocytes.

There is no doubt that major advances have been made in cardiac electrophysiology using single heart cells. Accurate analysis of rapid inward sodium current and the second inward current carried by calcium has resulted in a major re-examination of the steps involved in excitation-contraction coupling in the heart. Future work using glass microelectrodes, suction pipettes and patch pipettes will yield a vast amount of information highly relevant to mechanisms involved in the initiation of the heartbeat.

Animals↗
PubMed1985

Morphometric analysis of the isolated calcium-tolerant cardiac myocyte. Organelle volumes, sarcomere length, plasma membrane surface folds, and intramembrane particle density and distribution.

Using morphometric analysis of thin sections and freeze-fracture replicas, the ultrastructure of isolated rat myocytes prepared by collagenase digestion (Powell et al. 1980) was compared with that of myocytes fixed by perfusion of intact myocardium. The volumes of myofibrils, mitochondria, nuclei, sarcoplasmic reticulum and lipid droplets in the isolated myocytes did not differ from those of their counterparts in the intact heart, but the volume occupied by transverse tubules was apparently reduced. The isolated cells had significantly shorter sarcomeres than did cells in the intact tissue, and this was associated with an altered topography of plasma membrane surface folds at the level of the Z-lines. Plasma membrane intramembrane particles were randomly distributed and showed the same numerical density on the E-faces of both isolated and intact-heart myocytes. However, P-face particle density was slightly reduced in the isolated cells. It is concluded that the few differences detected in the isolated cells do not reflect any fundamental derangement of their properties.

Animals↗
PubMed1985

Influence of a change in stimulation rate on action potentials, currents and contractions in rat ventricular cells.

The effects of a change in stimulation rate on electrical activity and accompanying contraction were investigated in ventricular cells isolated from rat heart; the cells were stimulated to contract either by brief depolarization pulses which evoked action potentials, or, under voltage-clamp conditions, by step depolarizations. An increase in stimulation rate from 0.3 to 3 Hz resulted in a gradual reduction in the amplitude of contraction and attenuation of the late phase of the action potential. These changes were less marked at more depolarized potentials. The ventricular cells were voltage clamped at -40 mV and initially stimulated at 0.3 Hz by step depolarizations to 0 mV for 10 or 100 ms, which activated the second inward current (Isi) and an accompanying contraction. The amplitude and time course of contraction were similar with the two pulse durations. When the duration of the depolarization was 100 ms, an increase in stimulation rate to 3 Hz caused a gradual decline in the amplitude of Isi and of the evoked contraction; at the same time extra contractions and small, transient inward currents appeared in addition to the evoked contractions and Isis. There was a reduction in the early component of decay of Isi at 3 Hz. With a depolarizing pulse duration of 10 ms, an increase in stimulation rate to 3 or to 4.2 Hz did not change the amplitude of the evoked Isi or contraction and no extra contractions or currents appeared. Intracellular EGTA abolished all contractions in the cells and an increase in the rate of stimulation with 100 ms pulses did not then induce transient inward currents. There was some decrease in the Isi amplitude but this was not as marked as in the absence of EGTA and the time course of current decay was similar at the two rates. Ryanodine prevented the appearance of extra contractions and currents when the stimulation rate was increased to 3 Hz and, as in the presence of intracellular EGTA, there was a small decrease in Isi amplitude while the time course of decay was similar at the two stimulation rates. The time course of recovery of Isi from inactivation, as shown by a double-pulse procedure, was altered when the duration of the first pulse was reduced from 100 to 10 ms, an extra inactivation of Isi being seen at pulse intervals of 20-100 ms. This extra component of inactivation was not seen with intracellular EGTA or in the presence of ryanodine.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1985

Morphometric analysis of calcium-tolerant myocytes isolated from the adult rat heart.

The ultrastructure of calcium-tolerant isolated myocytes has been compared with that of intact heart myocytes using morphometric techniques. Isolated myocytes were prepared by the method of Powell et al. [1] from four adult rat hearts, and multiple left ventricular tissue samples were obtained from a further four rat hearts after fixation by Langendorff perfusion. The subcellular-component volumes of myofibrils, mitochondria, nuclei, transverse tubules, sarcoplasmic reticulum, lipid droplets, and cytoplasmic space have been estimated using a point-counting method on thin sections of intact myocardium and isolated myocytes. No significant differences between the two sample types were found. With the use of freeze-fracture, the numerical density and distribution of sarcolemmal intramembrane particles were analyzed by the method of Jones et al. [2]. The intramembrane particle density of isolated myocyte E-faces was very similar to that of myocytes of intact myocardium. However, isolated myocytes showed a slight reduction in P-face intramembrane particle density (less than 10%), although intramembrane particle distribution remained unaltered on both fracture faces. Our findings show that the ultrastructural features of the isolated myocytes closely resemble those of their counterparts in the intact myocardium.

Animals↗
PubMed1985

The isolation and characterization of calcium-tolerant myocytes.

Isolated cells obtained from adult myocardium are proving invaluable for a wide range of studies concerned with the mechanisms underlying many important cardiological processes. Although care must always be taken to ensure that the single myocyte is an appropriate model for the particular question to be researched, it is to be anticipated that much additional important data on heart function will be obtained using this experimental approach.

Animals↗
PubMed1985

Ultrastructure of the sarcolemma and intercalated disc in isolated rat myocytes.

The ultrastructure of the sarcolemma in isolated calcium-tolerant myocytes has been compared with that of myocytes in the intact heart, using thin section and freeze-fracture electron microscopical techniques. Data on the density and distribution of intramembrane particles and on the topography of Z-folds in the general (i.e. non-disc) sarcolemma are summarised. The fate of gap junctions on separation of the intercalated disc membranes has been studied i) at intervals after isolation, ii) using cationized ferritin as an extracellular marker and iii) by serial sectioning. The results of these studies help explain how ionic integrity of the individual isolated cells may be maintained.

Animals↗
PubMed1984

Net calcium exchange in adult rat ventricular myocytes: an assessment of mitochondrial calcium accumulating capacity.

Net calcium exchange has been measured in a suspension of cardiac myocytes after treatment with digitonin. The exchange is believed to be across the mitochondrial membranes and can be stimulated or inhibited by agents augmenting or blocking mitochondrial electron transport. The uptake of calcium shows a strong dependence on suspension pCa but is not evident below 1 microM (pCa 6.0). It is suggested that the net calcium exchange is a balance of the two processes which are equivalent at pCa 6.0. The measurement of mitochondrial specific activity for calcium uptake allows a calculation of the rapidity with which the cardiac mitochondria would affect sarcoplasmic calcium after a sudden rise. It is suggested that the organelle could partly affect relaxation especially at the peak of contraction.

Animals↗
PubMed1984

The effects of sodium, hydrogen and magnesium ions on mitochondrial calcium sequestration in adult rat ventricular myocytes.

The effect of Na+, H+ and Mg2+ ions on net calcium exchange induced in digitonin-treated myocytes has been investigated. Raising the [Na] from 1.4 to 31.4 mM revealed a sodium-sensitive fraction of net calcium exchange with a K1/2 for Na+ ions of 12 mM, alongside the respiration-dependent accumulation of calcium. An acidosis, but not an alkalosis, was found to depress both of these processes. Mg2+ ions exerted an effect solely on the respiration-dependent calcium sequestration. A simple semi-empirical model based on the experimental data was formulated to assess the effects that altering sarcoplasmic [Na+] and [H+] would have on the calcium-handling properties of cardiac mitochondria. It is concluded that part of the inotropic effects of these ions could be mediated via this organelle.

Animals↗
PubMed1984

Cytoplasmic free calcium measured by quin2 fluorescence in isolated ventricular myocytes at rest and during potassium-depolarization.

We have measured intracellular free Ca2+ concentration in rat and guinea pig ventricular myocytes using the fluorescent Ca2+-indicator quin2. Our results indicate a resting concentration in heart cells that is considerably lower than previous estimates. The mean value of 137 nM for rat ventricle that we have recorded is consistent with the hypothesis that resting intracellular [Ca2+] is controlled by a voltage-dependent, sarcolemmal exchange mechanism. Furthermore, we show that activation of plasma membrane Ca2+-channels is involved when intracellular free Ca2+ increases in response to K+-depolarization.

Aminoquinolines↗
PubMed1984

The effects of ryanodine, EGTA and low-sodium on action potentials in rat and guinea-pig ventricular myocytes: evidence for two inward currents during the plateau.

Action potentials were recorded from single cells isolated from rat and guinea-pig ventricular muscle. In rat cells the repolarization showed two distinct phases, referred to as the early and late phases. In guinea-pig cells there was a maintained plateau. Reducing external sodium by replacement with lithium or choline suppressed the late phase of the action potential in rat cells, and shortened the plateau of the action potential in guinea-pig cells. Intracellular EGTA abolished contraction while suppressing the late phase of the action potential in rat cells, and shortening the plateau in guinea-pig cells. Ryanodine (1 microM), which is thought to inhibit the release of calcium from internal stores, suppressed contraction and the late phase of the action potential in rat cells. In guinea-pig cells, there was no substantial effect of ryanodine (1 microM) on either contraction or the time course of the action potential. The late phase of the action potential in rat cells was suppressed by increasing the external potassium concentration to 12 mM, and enhanced by reducing external potassium to 1.2 mM. It is concluded that an inward current activated by internal calcium contributes to the late phase of the action potential in rat cells, and to the plateau in guinea-pig cells. Two possibilities are a current arising from electrogenic sodium-calcium exchange, and a current through ion channels activated by calcium. The effects of reducing external sodium would be consistent with either mechanism. The contribution of such an inward current would be expected to be modified by outward currents through a rectifying potassium conductance which varies with external potassium concentration. In the rat, but not the guinea-pig, the rise in internal calcium which activates the inward current seems to be largely dependent on ryanodine-sensitive release of calcium from internal stores.

Action Potentials↗
PubMed1984

Strontium, nifedipine and 4-aminopyridine modify the time course of the action potential in cells from rat ventricular muscle.

Action potentials, initiated by brief depolarizing pulses, were recorded from single cells isolated from rat ventricular muscle. These action potentials showed a rapid upstroke to about +30 mV, followed by two phases of repolarization referred to as the early and late phases of the action potential. Nifedipine (1 microM), which blocks the second inward current (Isi) carried by Ca in these cells, shortened the early phase. Substitution of strontium for calcium in the solution bathing the cells, a procedure which prolongs Isi, prolonged the early phase. 4-Aminopyridine (1 mM), which inhibits transient outward current, prolonged the early phase with either calcium or strontium in the external solution. It is concluded that both Isi and transient outward current contribute to the early phase of the action potential in rat ventricular muscle. It is also suggested that Isi does not directly contribute to the late phase, since the characteristics of the late phase are not compatible with such a role, and the possibility of additional inward current is investigated in the accompanying paper (Mitchell et al., 1984).

4-Aminopyridine↗
PubMed1984

Ryanodine prolongs Ca-currents while suppressing contraction in rat ventricular muscle cells.

Ryanodine (1 microM) suppressed or abolished contraction in response to step depolarization in voltage-clamped cells isolated from adult rat ventricular myocardium. The step depolarizations evoked the second inward current, which is carried largely by Ca ions under these conditions, and there was little or no change in the amplitude of this current when contraction was reduced or abolished by ryanodine. The effects of ryanodine on contraction were, however, accompanied by a prolongation of the second inward current resulting from a slowing of the apparent inactivation of this current. It is suggested that ryanodine affects steps in excitation-contraction coupling subsequent to the second inward current, perhaps Ca-release from intracellular stores, and that this slows a Ca-dependent inactivation of second inward current.

Alkaloids↗
PubMed1983

Characteristics of the second inward current in cells isolated from rat ventricular muscle.

The second inward current (Isi) in single cells isolated from ventricular muscle of adult rat hearts was measured in response to step depolarizations under voltage-clamp conditions. The major ion carrying this current was Ca, and Isi was reduced or abolished by Mn, Ni, Cd, nifedipine, nimodipine and D600. Sr and B could substitute for Ca as charge carriers, and reduced the rate of apparent inactivation of Isi. These effects of Sr and Ba, together with the relation between the steady level of apparent inactivation and membrane potential in Ca containing solution, were taken as evidence that inactivation was at least in part dependent on internal Ca. The reduction of external Na to 11% of normal caused a reduction in peak Isi when Ca was present in the external solution, but did not reduce Isi when Ca was replaced by Sr. It therefore seems unlikely that Na is a major charge carrier for Isi under the conditions of our experiments. The time-to-peak and rate of apparent inactivation of Isi were faster than in previous studies that used multicellular preparations. Both the kinetics and peak amplitude of Isi were markedly dependent on temperature (Q10 close to 3). Contraction of the cells, which was monitored optically, was initiated within 3 ms of the peak Isi, reached a maximum level after approximately 40-50 ms, and was about 100 ms in duration.

Animals↗
PubMed1983

[3H]nimodipine specific binding to cardiac myocytes and subcellular fractions.

[3H]Nimodipine binding was studied in isolated myocytes from rat heart and in partially purified sarcolemma, sarcoplasmic reticulum and mitochondrial fractions from dog heart. In isolated myocytes, the density of [3H]nimodipine specific sites (10(6) per cell) was close to density of [3H]QNB sites (0.8 x 10(6) per cell) and higher than that of [3H]DHA sites (0.2 x 10(6) per cell). During subcellular fractionation, [3H]nimodipine binding did not copurify with plasma membrane markers. The highest densities were found in fractions enriched in sarcolemma or in sarcoplasmic reticulum. No specific binding was found in mitochondria. These results indicate that the localization of [3H]nimodipine sites is not restricted to areas of the plasma membrane rich in beta-adrenoceptors, muscarinic receptors and sodium pump sites.

Animals↗
PubMed1983

The calcium paradox and isolated myocytes.

A brief survey is presented of methods for producing Ca-tolerant ventricular myocytes, with reference to factors which might be related to the protection of the heart from the calcium paradox. Much of the analysis is based on the assumption that maintaining extracellular calcium above a 'threshold' concentration during cell isolation is a major influence on the production of Ca-tolerant myocytes, and that a similar manoeuvre in the whole organ would prevent the paradox. It is also shown that, although single cells have yet to be shown to be a good model for the paradox, measurements already reported on enzyme release and myocyte ionic homeostasis are of relevance to both Ca-tolerance and the genesis of the paradox, and that future research on isolated cells may well clarify further mechanisms important for both of these phenomena.

Biomechanical Phenomena↗
PubMed1983

Isolated calcium-tolerant myocytes and the calcium paradox: an ultrastructural comparison.

The ultrastructure of calcium-tolerant myocytes isolated from adult rat ventricular myocardium is described, using thin section and freeze-fracture electron microscopy. Two distinct cell types are observed, rounded and rod-shaped. The former are damaged myocytes and superficially resemble the disrupted cells characteristic of the calcium paradox. Despite this resemblance however, the genesis of these isolated damaged cells is not explicable in terms of the calcium paradox. The majority of isolated cells are rod-shaped and show well preserved ultrastructural features. Our results indicate that whatever mechanisms underlie the calcium paradox it is not an indispensable condition that isolation of myocytes necessarily leads to this phenomenon.

Animals↗