scholarly journals Calcium Ions and the Action Potential in Nitella

1962 ◽  
Vol 15 (1) ◽  
pp. 69 ◽  
Author(s):  
GP Findlay
Keyword(s):  
Cell Membrane ◽  
Action Potential ◽  
Electrical Activity ◽  
Calcium Ions ◽  
Current Flow ◽  
External Medium ◽  
Feedback Circuit ◽  
Predetermined Level ◽  
Voltage Clamping ◽  
Main Factor

Experiments are described in which a "voltage� clamping" technique has been applied to large ecorticate internodal cells of the freshwater alga Nitella. In this technique, a feedback circuit is used to change the potential difference between the vacuole of the cell and the external medium to some predetermined level and maintain it as close as possible to this level during the electrical activity of the cell. It is shown that the main factor in the phenomena of potential change and current flow, during the initial stages of the action potential in Nitella, is a tran-sient increase in the permeability of the cell membrane to calcium ions, and a consequent flow of these ions into the cell from the external medium.

The Water Balance of a Serpulid Polychaete, Mercierella Enigmatica (Fauvel)

1974 ◽  
Vol 60 (2) ◽  
pp. 351-370
Author(s):  
HELEN LE B. SKAER
Keyword(s):  
Action Potential ◽  
Electrical Activity ◽  
Muscle Cell ◽  
Cell Membranes ◽  
External Medium ◽  
Ionic Composition ◽  
Chloride Concentration ◽  
Chloride Ions ◽  
Resting Potential ◽  
Inward Current

1. The electrical activity of the two types of longitudinal muscles of an osmoconforming polychaete worm, Mercierella enigmatica, have been studied in media of widely varying osmotic and ionic composition. Activity persists practically unaltered in both types of muscle cell. 2. The possible effects of osmotically induced changes in cell volume on the ionic gradients across the cell membranes are considered. It is concluded that the normal gradients are unlikely to be maintained as a result of such changes. 3. The involvement of ion pumps in the maintenance of the normal gradients across the muscle cell membranes has been studied using specific and metabolic poisons. It is evident that the persistence of electrical activity in media of altered ionic content does not depend on the sodium-potassium exchange pump. 4. The ionic basis of the overshoot of action potentials recorded from cells of the small resting potential type has been studied. It is concluded that calcium ions but not sodium ions are responsible for the inward current although there is a component of the inward current carried by some other as yet unidentified ion. 5. Alterations in the external concentrations of chloride ions are found to alter both the height of the overshoot and the length of the action potential. 6. Profound alterations in the overshoot height are produced only when the normal ratio of calcium to chloride concentration in the external medium is altered. Possible mechanisms to explain these effects are discussed. 7. It is suggested that the stability of the action potential in the muscle cells of M. enigmatica, despite large fluctuations in the salinity of the external medium, depends on the constancy of the ratios between the concentrations of the ions in the fluids bathing the cells and not on the absolute concentrations of the ions.

Effects of octanol on canine subendocardial Purkinje-to-ventricular transmission

1985 ◽  
Vol 249 (6) ◽  
pp. H1228-H1231 ◽  
Author(s):  
R. W. Joyner ◽  
E. D. Overholt
Keyword(s):  
Action Potential ◽  
Conduction Velocity ◽  
Current Flow ◽  
Conduction Delay ◽  
Papillary Muscles ◽  
Maximal Rate ◽  
Gap Junctional ◽  
Junctional Resistance ◽  
P Cells

The effects of 0.2 mM octanol on action potential propagation were investigated using in vitro preparations of canine papillary muscles. In these preparations an action potential initiated in the superficial Purkinje (P) layer propagates across specific Purkinje-ventricular junction (PVJ) sites into the underlying ventricular (V) layer. The conduction delay at PVJ sites increased from 4.85 +/- 1.55 to 8.85 +/- 3.34 (mean +/- SD) ms (n = 10, P less than 0.005), an 82% increase. However, propagation within the V syncytium was much less affected, with a decrease of conduction velocity by only 10% and a decrease in the maximal rate of rise of the action potential of 23%. The results indicate that octanol, which has previously been shown to increase gap junctional resistance, has a preferential effect on PVJ sites, as predicted by the hypothesis that there is a restricted pathway for intracellular current flow from P cells to V cells at these sites.

Rhythmic hyperpolarizations and depolarization of sympathetic ganglion cells induced by caffeine

1976 ◽  
Vol 39 (3) ◽  
pp. 547-563 ◽  
Author(s):  
K. Kuba ◽  
S. Nishi
Keyword(s):  
Action Potential ◽  
Sympathetic Ganglion ◽  
External Medium ◽  
Ganglion Cells ◽  
Membrane Resistance ◽  
Free Medium ◽  
Dibutyryl Cyclic Amp ◽  
High K ◽  
Hyperpolarizing Response ◽  
K Permeability

Superfusion of the isolated sympathetic ganglion of the bullfrog with a caffeine-containing (1-6 mM) solution caused in many cells an initial slow hyperpolarization which was followed by a subliminal depolarization interruped by rhythmic hyperpolarizations. A hyperpolarization, similar to one of the rhythmic hyperpolarizations, could be triggered by an action potential in the presence of caffeine. The action potential itself was not markedly affected by caffeine except for its afterhyperpolarization which was prolonged. All these caffeine-induced hyperpolarizations were associated with a marked reduction of the membrane resistance, their amplitude was increased in a K+-free solution and decreased in a high-K+ solution, and their polarity was reversed at the same level at which the afterhyperpolarization was also inverted. This reversal level was not altered by omission of Na+ or C1- from the external medium. These hyperpolarizations were reversibly abolished by depletion of external Ca2+ or replacement of external Ca2+ by Mg2+. Excess of external Ca2+ caused a shortening of the interval between rhythmic hyperpolarizations. Furthermore, iontophoretic injection of EDTA into the cytoplasm markedly depressed the initial caffeine hyperpolarizatin and abolished both the rhythmic and evoked caffeine hyperpolarizations. The caffeine-induced depolarization was not affected by omission of external Cl-. It was decreased in a Na+-free medium, but completely eliminated by omission of both Na+ and Ca2+ from the external medium. Tetrodotoxin did not impair the production of the initial and the rhythmic hyperpolarizations. A strong depolarizing pulse could evoke a typical hyperpolarizing response in the presence of this compound. Dibutyryl cyclic AMP, d-tubocurarine, atropine, and phenoxybenzamine were without effect on the caffeine-induced hyperpolarizations and depolarization. It was concluded that each caffeine-induced hyperpolarization is the result of an increased K+ permeability, which is probably caused by a rise in the internal Ca2+ concentration. It was also concluded that the caffeine-induced depolarization is due to an increased membrane permeability to Ca2+ and Na+.

Regression of LV hypertrophy with captopril normalizes membrane currents in rabbits

1998 ◽  
Vol 275 (4) ◽  
pp. H1216-H1224 ◽  
Author(s):  
Seth J. Rials ◽  
Xiaoping Xu ◽  
Ying Wu ◽  
Roger A. Marinchak ◽  
Peter R. Kowey
Keyword(s):  
Cell Membrane ◽  
Action Potential ◽  
Renal Artery ◽  
Current Density ◽  
Action Potential Duration ◽  
Left Ventricular ◽  
Membrane Current ◽  
Membrane Capacitance ◽  
Membrane Currents ◽  
Cell Membrane Capacitance

Recent studies indicate that regression of left ventricular hypertrophy (LVH) normalizes the in situ electrophysiological abnormalities of the left ventricle. This study was designed to determine whether regression of LVH also normalizes the abnormalities of individual membrane currents. LVH was induced in rabbits by renal artery banding. Single ventricular myocytes from rabbits with LVH at 3 mo after renal artery banding demonstrated increased cell membrane capacitance, prolonged action potential duration, decreased inward rectifier K+ current density, and increased transient outward K+ current density compared with myocytes from age-matched controls. Additional rabbits were randomized at 3 mo after banding to treatment with either vehicle or captopril for an additional 3 mo. Myocytes from LVH rabbits treated with vehicle showed persistent membrane current abnormalities. However, myocytes isolated from LVH rabbits treated with captopril had normal cell membrane capacitance, action potential duration, and membrane current densities. Captopril had no direct effect on membrane currents of either control or LVH myocytes. These data support the hypothesis that the action potential prolongation and membrane current abnormalities of LVH are reversed by regression. Normalization of membrane currents probably explains the reduced vulnerability to ventricular arrhythmia observed in this LVH model after treatment with captopril.

Analysis of catecholamine effects in single atrial trabeculae of the frog heart

1977 ◽  
Vol 197 (1128) ◽  
pp. 333-362 ◽  
Keyword(s):  
Cell Membrane ◽  
Action Potential ◽  
Time Course ◽  
Drug Exposure ◽  
Drug Application ◽  
Latency Period ◽  
Frog Heart ◽  
Phosphodiesterase Activity ◽  
Inward Current ◽  
Calcium Inward Current

A study was made of the time course of the effects of adrenaline and isoprenaline on both twitch tension and the intracellular action potential of single atrial trabeculae from frog heart, under a variety of experimental conditions. Twitch tension and overshoot of action potentials rose and subsided in a parallel fashion during build-up and decline of catecholamine action. Cessation of stimulation during drug application had little effect on the tension responses to the drugs. These, and also results obtained with step changes of external calcium concentration during drug exposure, suggest that tension enhancement is a direct consequence of the increased calcium inward current produced by the catecholamines. Exceptional results from trabeculae of ‘hypodynamic’ hearts are described and interpreted on the basis of previous findings obtained in the ‘hypo-dynamic’ condition. Under suitable conditions, including the use of brief periods of drug exposure (≤20 s), three phases of ( β -catecholamine action were discernible: (1) a latency period, of up to 15 s, which preceded tension and potential rise after drug application. Results are presented suggesting that this latency mainly reflects the time which it takes for drug-combined receptors to activate adenylate cyclase in the cell membrane. (2) A sub­sequent phase was critically dependent, in both its magnitude and time course, on phosphodiesterase activity, as was shown by the application of the specific inhibitors papaverine, ICI 63 197, and Ro 20-1724. This phase is probably controlled by the build-up and decline of cAMP within the cells and the subsequent activation and deactivation of a protein kinase. (3) A third phase, associated with the final portion of the decline of catecholamine action, was relatively insensitive to moderate inhi­bition of phosphodiesterase activity. It is attributed to a change of phosphorylation of sites at the internal surface of the cell membrane, the process which, it is assumed, determines the size of calcium inward current during an action potential. Tension decline after a short staircase occurred with a time course closely similar to that of the final phase of the declining catecholamine response. A common final step in the sequential cellular processes under­ lying the two responses is proposed. In some 40% of the trabeculae examined, adrenaline responses were of ‘mixed’ origin: in addition to the relatively slow β -adrenergic action, an initial rapid tension change was present, and experimental tests suggest that this is mediated by α -type receptors.

Phasic efflux of potassium from frog ventricle

1962 ◽  
Vol 203 (2) ◽  
pp. 253-257 ◽  
Author(s):  
Victor Lorber ◽  
John L. Walker ◽  
Ernest A. Greene ◽  
Margaret H. Minarik ◽  
Moon Jae Pak
Keyword(s):  
Cell Membrane ◽  
Action Potential ◽  
Cardiac Cycle ◽  
Pronounced Effect ◽  
Voltage Gradient ◽  
Perfusion Medium ◽  
Actual Increase ◽  
Frog Ventricle ◽  
Turtle Heart ◽  
Do So

A phasic effiux of K42 during the cardiac cycle, first described by Wilde in the turtle heart, has been observed in perfused strips of frog ventricle. This was shown to represent an actual increase in the outward movement of K, occurring during the action potential, and of approximately the same duration as the electrical transient. Perfusion with K-free Ringer's prolonged both events, and, within the limits of resolution of the method, appeared to do so to about the same extent. The level of K in the perfusion medium was found to have a pronounced effect on the K efflux, a result which may be interpreted in terms of an effect of extracellular [K+] on the permeability of the cell membrane to K. The finding that the increase in K efflux observed during the action potential is much smaller than that predicted from the increase in the voltage gradient (assuming the voltage gradient to be the only variable) is consistent with a diminished K conductance during the action potential.

scholarly journals Gq-activated fibroblasts induce cardiomyocyte action potential prolongation and automaticity in a three-dimensional microtissue environment

2017 ◽  
Vol 313 (4) ◽  
pp. H810-H827 ◽  
Author(s):  
C. M. Kofron ◽  
T. Y. Kim ◽  
M. E. King ◽  
A. Xie ◽  
F. Feng ◽  
...  
Keyword(s):  
Action Potential ◽  
Electrical Activity ◽  
Rise Time ◽  
Connexin 43 ◽  
Cardiac Fibroblasts ◽  
Three Dimensional ◽  
Neonatal Rat ◽  
Resting Membrane Potential ◽  
Western Blots ◽  
Cell Cell

Cardiac fibroblasts (CFs) are known to regulate cardiomyocyte (CM) function in vivo and in two-dimensional in vitro cultures. This study examined the effect of CF activation on the regulation of CM electrical activity in a three-dimensional (3-D) microtissue environment. Using a scaffold-free 3-D platform with interspersed neonatal rat ventricular CMs and CFs, Gq-mediated signaling was selectively enhanced in CFs by Gαq adenoviral infection before coseeding with CMs in nonadhesive hydrogels. After 3 days, the microtissues were analyzed by signaling assay, histological staining, quantitative PCR, Western blots, optical mapping with voltage- or Ca2+-sensitive dyes, and microelectrode recordings of CF resting membrane potential (RMPCF). Enhanced Gq signaling in CFs increased microtissue size and profibrotic and prohypertrophic markers. Expression of constitutively active Gαq in CFs prolonged CM action potential duration (by 33%) and rise time (by 31%), prolonged Ca2+ transient duration (by 98%) and rise time (by 65%), and caused abnormal electrical activity based on depolarization-induced automaticity. Constitutive Gq activation in CFs also depolarized RMPCF from –33 to −20 mV and increased connexin 43 and connexin 45 expression. Computational modeling confers that elevated RMPCF and increased cell-cell coupling between CMs and CFs in a 3-D environment could lead to automaticity. In conclusion, our data demonstrate that CF activation alone is capable of altering action potential and Ca2+ transient characteristics of CMs, leading to proarrhythmic electrical activity. Our results also emphasize the importance of a 3-D environment where cell-cell interactions are prevalent, underscoring that CF activation in 3-D tissue plays a significant role in modulating CM electrophysiology and arrhythmias. NEW & NOTEWORTHY In a three-dimensional microtissue model, which lowers baseline activation of cardiac fibroblasts but enables cell-cell, paracrine, and cell-extracellular matrix interactions, we demonstrate that selective cardiac fibroblast activation by enhanced Gq signaling, a pathophysiological trigger in the diseased heart, modulates cardiomyocyte electrical activity, leading to proarrhythmogenic automaticity.

scholarly journals Optical mapping of the electrical activity of isolated adult zebrafish hearts: acute effects of temperature

2014 ◽  
Vol 306 (11) ◽  
pp. R823-R836 ◽  
Author(s):  
Eric Lin ◽  
Amanda Ribeiro ◽  
Weiguang Ding ◽  
Leif Hove-Madsen ◽  
Marinko V. Sarunic ◽  
...  
Keyword(s):  
Action Potential ◽  
Electrical Activity ◽  
Cardiac Electrophysiology ◽  
Optical Mapping ◽  
Cooling System ◽  
Effect Of Temperature ◽  
Adult Zebrafish ◽  
Depth Analysis ◽  
Zebrafish Heart ◽  
Av Delay

The zebrafish ( Danio rerio) has emerged as an important model for developmental cardiovascular (CV) biology; however, little is known about the cardiac function of the adult zebrafish enabling it to be used as a model of teleost CV biology. Here, we describe electrophysiological parameters, such as heart rate (HR), action potential duration (APD), and atrioventricular (AV) delay, in the zebrafish heart over a range of physiological temperatures (18–28°C). Hearts were isolated and incubated in a potentiometric dye, RH-237, enabling electrical activity assessment in several distinct regions of the heart simultaneously. Integration of a rapid thermoelectric cooling system facilitated the investigation of acute changes in temperature on critical electrophysiological parameters in the zebrafish heart. While intrinsic HR varied considerably between fish, the ex vivo preparation exhibited impressively stable HRs and sinus rhythm for more than 5 h, with a mean HR of 158 ± 9 bpm (means ± SE; n = 20) at 28°C. Atrial and ventricular APDs at 50% repolarization (APD50) were 33 ± 1 ms and 98 ± 2 ms, respectively. Excitation originated in the atrium, and there was an AV delay of 61 ± 3 ms prior to activation of the ventricle at 28°C. APD and AV delay varied between hearts beating at unique HRs; however, APD and AV delay did not appear to be statistically dependent on intrinsic basal HR, likely due to the innate beat-to-beat variability within each heart. As hearts were cooled to 18°C (by 1°C increments), HR decreased by ∼40%, and atrial and ventricular APD50 increased by a factor of ∼3 and 2, respectively. The increase in APD with cooling was disproportionate at different levels of repolarization, indicating unique temperature sensitivities for ion currents at different phases of the action potential. The effect of temperature was more apparent at lower levels of repolarization and, as a whole, the atrial APD was the cardiac parameter most affected by acute temperature change. In conclusion, this study describes a preparation enabling the in-depth analysis of transmembrane potential dynamics in whole zebrafish hearts. Because the zebrafish offers some critical advantages over the murine model for cardiac electrophysiology, optical mapping studies utilizing zebrafish offer insightful information into the understanding and treatment of human cardiac arrhythmias, as well as serving as a model for other teleosts.

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