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Biomedical subjects

M Morad

Publications and source records attributed to M Morad.

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

A uniform enzymatic method for dissociation of myocytes from hearts and stomachs of vertebrates.

A method is presented that consistently yields a large number of calcium-tolerant myocytes from mammalian, amphibian, and elasmobranch hearts and from mammalian stomach. The use of incubating solutions or cell harvesting techniques was not required. The time needed to isolate cells was shorter than previously reported values. Action potentials recorded from each cell type appear similar in configuration to that of the intact multicellular tissue. The isolated myocytes appear to tolerate long periods of electrophysiological experimentation using the "giga-seal" suction electrode technique of Hamill et al. (Pfluegers Arch. 391: 85-100, 1981). This method is ideally suited for comparative electrophysiological studies, since the procedure for cell isolation was not seriously modified according to the preparation or species used.

Animals↗
PubMed1984

Optical measurements of extracellular calcium depletion during a single heartbeat.

The impermeant dye antipyrylazo III was used to measure depletion of extracellular calcium and net influx of calcium through the sarcolemma during the cardiac action potential. It was found that calcium entry occurs continuously during the action potential and is under direct control of the membrane potential. The inotropic action of epinephrine is accompanied by increased influx of calcium, while strophanthidin enhances the twitch without altering calcium influx during the action potential.

Action Potentials↗
PubMed1984

Ionic currents responsible for the generation of pace-maker current in the rabbit sino-atrial node.

The ionic nature of the pace-maker current (delta Ip, If, Ih) was investigated in rabbit sino-atrial node using a single sucrose-gap voltage-clamp technique. The pace-maker current was activated by hyperpolarizing clamp steps negative to -50 mV and the pace-maker potential was activated by an action potential or a depolarizing clamp step. Neither pace-maker current nor pace-maker potential were altered by addition of tetrodotoxin, but a tetrodotoxin-sensitive channel could be activated in sino-atrial nodal strips following hyperpolarizing clamp steps. Ca2+-channel blockers did not affect the voltage dependence of delta Ip or the maximum diastolic potential (m.d.p.) significantly. Removal of Ca2+ did not affect the pace-maker current at clamp potentials near the pace-maker potential range (-60 to -80 mV), but it did reduce the potential dependence of the m.d.p. Removal of Na+ suppressed completely the pace-maker current and hyperpolarized the membrane. Removal of Na+ also increased membrane conductance, most likely through an increase in resting K+ permeability. Low concentration of Cs+ (less than 5 mM), but not Ba2+ or tetraethylammonium, markedly suppressed activation delta Ip and reduced the rate of pacing slightly. Cs+ also decreased the membrane conductance and hyperpolarized the membrane. In 50% of experiments designed to determine contribution of IK to pace-maker current, a double-pulse procedure revealed a time-dependent component of delta Ip which reversed near the K+ equilibrium potential, EK. Release of depolarizing or hyperpolarizing test clamps was followed by pace-maker potentials, the magnitudes of which were dependent on the test-clamp potential. The m.d.p. approached values near EK following depolarizing clamps and near -45 mV following hyperpolarizing clamps. The results suggest that delta Ip is carried primarily by Na+ and is blocked by Cs+. It is likely, however, that Ca2+ alters the rate of pacing not only through its contribution to the Isi system, but also through activation of a K+ conductance.

Action Potentials↗
PubMed1984

A transient outward current related to calcium release and development of tension in elephant seal atrial fibres.

Membrane currents and development of tension in atrial trabeculae from elephant seal hearts were studied using a single sucrose-gap voltage-clamp technique. A transient outward current (Ito) was observed with kinetics, voltage and beat dependence, similar to those of tension. Ito had a bell-shaped voltage dependence similar to that of tension and the slow inward current (Isi). Ito, unlike Isi, showed beat dependence quite similar to developed tension. Increases in [Ca]o, frequency of stimulation, and addition of adrenaline enhanced Ito and developed tension. Ito was suppressed by addition of Mn2+, tetracaine, or by depolarizing pre-pulses (to -40 mV for 250 ms). Caffeine at low concentrations (1 mM) blocked beat dependence of Ito. At higher concentrations (greater than 5 mM) caffeine suppressed the activation of Ito, phasic tension, and the second component of the birefringence signal (related to Ca2+-releasing activity of the sarcoplasmic reticulum (s.r.]. Similar to Isi phasic tension and Ito, the voltage dependence of the second component of the birefringence signal was bell-shaped. Our studies suggest that activation of Ito is related to triggered release of Ca2+ from the s.r. which generates the phasic tension. An excitation-contraction coupling scheme is presented which incorporates these findings and suggests that Ito may be responsible for shorter action potentials found in atrial fibres.

Action Potentials↗
PubMed1983

Effect of caffeine on the birefringence signal in single skeletal muscle fibers and mammalian heart. Possible mechanism of action.

Effects of caffeine on single fibers of frog (Rana pipiens) toe muscle and rabbit atrial trabeculae were investigated by measuring action potential, tension and the E-C coupling-related birefringence signal. Caffeine in concentrations of 1.0 to 2.0 mmol/l potentiated the twitch, prolonged the action potential, delayed the onset and suppressed the rate of the birefringence signal in skeletal muscle fibers. In heart muscle, caffeine at concentrations of 2-10 mmol/l potentiated tension and suppressed the second component of the birefringence signal. Although theophylline also potentiated tension and suppressed the rate of rise of the birefringence signal, other diesterase inhibitors, Ro7-2956, or stimulators of adenylate cyclase system such as dibutyryl cAMP or adrenaline failed to alter the birefringence signal while potentiating tension. Since the second component of the birefringence signal has been associated with Ca2+-releasing activity of the sarcoplasmic reticulum, the suppression of this signal and the simultaneous potentiation of tension by caffeine suggests that either caffeine depresses the rate of Ca2+ release or that the birefringence signal is not associated directly with Ca2+ release, rather with Ca2+ uptake. In either case, the results suggest that potentiation of tension by caffeine may be mediated by suppression of the Ca2+ reuptake process.

Absorption↗
PubMed1983

Does the "pacemaker current" generate the diastolic depolarization in the rabbit SA node cells?

Small preparations of spontaneously beating rabbit sino-atrial node (SA node) were voltage clamped with the two-microelectrode technique. The effects of 0.25-5 mM Cs+ on the spontaneous pacing rate and the time-dependent inward "pacemaker" current, ih, were studied. In the presence of 2 mM Cs+, the spontaneous pacing rate decreased only slightly even though ih was strongly depressed at potentials negative to -60 mV Cs+ had little or no effect on other time-dependent currents observed with clamp pulses less negative than -50 mV. Since no voltage-dependence to the Cs+ effect on ih could be measured (between -90 mV and -20 mV), it was considered unlikely that the lack of Cs+ effect on the rate of diastolic depolarization results from a voltage-dependent effect of Cs+ on the ih channel. Adrenaline produced a marked positive chronotropic effect in Cs+-treated SA node cells. This effect was accompanied by marked enhancement of the slow inward current (isi) with no change in the Cs+-blocked ih current. These results are consistent with the idea that ih plays a minor role in generation of pacemaker depolarization, and suggest a more prominent role of isi in the generation of diastolic depolarization in SA nodal cells.

Animals↗
PubMed1983

Voltage- and frequency-dependent block of diltiazem on the slow inward current and generation of tension in frog ventricular muscle.

The effects of a new "Ca2+ -antagonist", diltiazem, were studied in frog ventricular myocardium. The single sucrose gap voltage clamp technique was used to control membrane potential and measure membrane current and tension. Diltiazem had no effect on the resting potential, but reduced the slow inward current (Isi) and increased the net outward current. Membrane conductance measurements suggest that the increase in the total membrane current may be due to the suppression of a maintained inward current. The blocking action of diltiazem is frequency-dependent such that at higher frequencies of stimulation, the steady-state amplitude of twitch is reduced. The diltiazem-induced suppression of tension and Isi could be partially reversed by hyperpolarizing the surface membrane for brief intervals. These results suggest that diltiazem blocks Isi in a voltage- and time-dependent manner. These effects of diltiazem on Isi seem to be, in part, responsible for the tension-suppressant effect of the drug.

Action Potentials↗
PubMed1983

The effects of Ni2+ on ionic currents and tension generation in frog ventricular muscle.

The effect of Ni2+ on E-C coupling events of the frog ventricular muscle were studied using a single sucrose gap voltage clamp technique. The results showed that Ni2+ increased the overshoot potential and depressed and prolonged the plateau of the action potential. Ni2+ also increased the dependence of the overshoot potential on [Na]0 from 18 to 58 mV per decade. In the presence of Ni2+, TTX blocked both the upstroke and the plateau of the action potential. The combination of TTX and Ni2+ suppressed the tension-voltage relation, the time-dependent outward currents and K+ efflux. While suppression of the tension-voltage relation by Ni2+ alone was reversed by increasing [Ca]0, the effects of Ni2+ plus TTX are not reversed by addition of Ca2+. The results suggest that Ni2+ may alter the action potential by slowing the inactivation of the Na+ current and blocking the inward Ca2+ current. Although the tension-suppressant effects of Ni2+ could be attributed to the inhibition of a slowly inactivating Ca2+ current, the effects of Ni2+ in the presence of TTX were less readily explained. Several possible mechanisms are considered which are all consistent with the hypothesis that development of tension in ventricular strips is mediated by both a Ca2+ current and a Ca2+ counter-transport system.

Action Potentials↗
PubMed1983

Excitation-contraction coupling in frog ventricle. Possible Ca2+ transport mechanisms.

In frog ventricular muscle generation of tension was found to be under direct and continuous control of membrane potential. No phasic component of tension was found at any membrane potential. Developed tension depended only on the duration and amplitude of depolarization and was independent of previous contractile history. Developed tension, in part, depended on Ca2+ influx through a slowly inactivating component of Isi. Using long voltage clamp steps to achieve steady-state tension, no decline or reversal of developed tension was found at ECa. Increasing the [Ca]o shifted the tension-voltage relation to more negative potentials and increased the net outward current at potentials positive to -10 mV. The increase in tension seemed to be related to the increase in outward membrane current and K+ efflux, as estimated from post clamp K+ accumulations. Increasing [K]o, either by clamp-induced K+-accumulation or by increasing the [K] of the bathing solution, decreased the developed tension. These results suggest that in frog ventricular muscle Ca2+ for activation of tension is transported primarily from the extracellular space. There was no trigger-release of internal stores or recirculation of sequestered Ca2+. Activator Ca2+ was transported in part by a slowly inactivating Isi channel and a coupled transport mechanism. The exact mechanism by which Ca2+ transport and K+ efflux were related could not be identified.

Animals↗
PubMed1983

Diffusion profiles of Na+-fluorescein in frog ventricular muscle.

Frog ventricular muscle strips were placed in a single sucrose-gap chamber to measure the interdiffusion of solutes across the sucrose-Ringer's solution partition. Steady-state diffusion profiles of fluorescein sodium developed along the axis of the muscle in the physiological node by continuously perfusing the sucrose pool with 210-mM sucrose plus fluorescein (5-10 mM). Fluorescein was found to diffuse freely through the extracellular space of the ventricular muscle without binding to the tissue. The fluorescence of Na+-fluorescein in the muscle (measured at 530 +/- 30 nm) varied linearly with the dye concentration in the sucrose perfusate. The diffusion profiles of dye in the test node depended on the tightness or snaring of the muscle strip by the latex diaphragms, the diameter of the muscle strip, and changes in hydrostatic pressure between the sucrose and Ringer's solution pools. Fluorescein concentration in the cross section of test node closest to the latex partition (sucrose-Ringer's solution interface) ranged between 4-13% of the dye concentration in the sucrose pool. These values are more than five times smaller than those estimated theoretically, assuming free diffusion. The experimental findings indicate that the presence of a physical barrier, such as a rubber diaphragm, limits free interdiffusion of solutes across the sucrose gap. The presence of such a barrier thus prevents large concentration gradients from occurring in the extracellular spaces along the physiological node.

Animals↗
PubMed1983

Birefringence signals in mammalian and frog myocardium. E-C coupling implications.

Birefringence signals from mammalian and frog hearts were studied. The period between excitation and the onset of contraction in which optical signals were free of movement artifact was determined by changes in scattered incandescent light and changes in laser diffraction patterns. The birefringence signal preceding contraction was found to behave as a change in retardation and was not contaminated measurably by linear dichroic or isotropic absorption changes. There were two components of the birefringence signal in mammalian heart muscles but only one component in the frog heart. The first component of the birefringence signals in both mammalian and frog hearts had a time course coincident with the action potential upstroke. The second component in mammalian preparations was sensitive to inotropic interventions, such as variation of extracellular Ca2+, stimulation frequency, temperature, and epinephrine, in a manner that correlated with the maximum rate of rise of tension. Caffeine (2-10 mM) not only failed to generate a second component in the frog heart, but also suppressed the second component in the mammalian heart while potentiating twitch tension. The results suggest that the second component of the birefringence signal in the mammalian myocardium is related to Ca2+ release from the sarcoplasmic reticulum.

Action Potentials↗
PubMed1982

Ionic events responsible for the cardiac resting and action potential.

The cardiac action potential is distinguished from other excitatory phenomena by a prominent plateau and by the latent pacemaking capability of cardiac muscle. A review of experimental data suggests that ionic fluxes through gated membrane channels are the primary determinants of the shape of the cardiac action potential. The rapid depolarization phase of the action potential is mediated in part by an ionic channel that resembles the sodium ion (Na+) channel of nerve. A slower channel capable of carrying both calcium ion (Ca2+) and Na+ currents (Isi) also contributes to the upstroke of the action potential. The Ca2+ current through this channel is partly responsible for maintaining the plateau phase of the action potential. Moreover, because this slower channel activates at more positive potentials in partly depolarized myocardium or in specialized conduction tissue such as the sinoatrial and atrioventricular nodes. Two distinct transport systems appear to be the principal regulators of potassium ion (K+) in myocardium. An inwardly rectifying, voltage-dependent K+ channel apparently maintains all K+ conductance at rest and at all potentials negative to --20 mV. A second channel, which is both voltage- and time-dependent, evidently mediates K+ flux during the plateau phase of the action potential. This K+ current activates at potentials positive to --20 mV. The role of coupled transport mechanisms is now well established. The low intracellular concentrations of Na+ and Ca2+ prevailing in the myocardium are maintained by an electrogenic Na+ pump and a Na+-Ca2+ counter-transport system. Current data on carrier-mediated transport systems are insufficient to delineate the role of such mechanisms in the control of cardiac action potential. Further studies are required to provide details of the voltage, temporal and ionic dependence of gated channels, as well as of ionic counter-transport carriers, so that a quantitative reconstruction of cardiac action potential may be attempted.

Animals↗
PubMed1982

Activity-induced potassium accumulation and its uptake in frog ventricular muscle.

1. Extracellular K+ activity in frog ventricular muscle was monitored with a K+-selective micro-electrode during and following periods of rapid stimulation. 2. During activity K+ accumulated in the paracellular space, declined with continued beating and became depleted below bathing K+ concentrations, [K+], when activity was terminated. 3. The re-uptake and depletion of K+ was inhibited by ouabain, Li+ and lowering bathing [K+], and was enhanced by prolonged stimulation, raising bathing [K+], and by addition of adrenaline. These ionic and drug dependencies of the K+ re-uptake process are similar to the ionic and drug dependencies of the Na+-K+-ATPase system. 4. Frequency-induced K+ accumulation appears to result from a delay in the activation of the Na+ pump. 5. Possible changes in intracellular sodium concentration, [Na+]i, in the response to changes in frequency, appear to be a more powerful stimulant of the K+ re-uptake process than changes in extracellular potassium concentration, [K+]o. 6. Frequency-induced changes in [K+]o were also detected by measurements of resting potential. Alterations in membrane potential and action potential duration observed during and following electrical stimulation are suggestive of an electrogenic K+ re-uptake process. 7. Aside from their direct effects on the action potential, Ca2+ and Mg2+ had little or no effect on Na+ pump activity. While Ni2+ suppressed pump activity, Ba2+ indirectly enhanced the K+ uptake process by blocking the resting K+ conductance. 8. K+ uptake rate was estimated to range between 3 and 8 p-mole/cm2.sec. Since diffusion in and out of the paracellular space was a much slower process (t1/2 60-90 sec), it contributes little to the beat-to-beat control of paracellular [K+].

Animals↗
PubMed1981

A new laser scanning system for measuring action potential propagation in the heart.

A rapid laser scanning system was developed to map the spread of excitation in amphibian and mammalian hearts stained with fluorescent dye. Isochronic maps of conduction were constructed by timing the upstroke of the optical action potential; 128 sites could be scanned in 4 milliseconds. The accuracy of this technique was verified by recording simultaneously from 16 unipolar electrodes placed in different areas of the heart. Conducted action potentials in normal frog heart propagated at 0.1 meter per second. Propagation of action potentials was also monitored in ischemic cat heart, in which both driven and arrhythmic action potential upstrokes could be tracked. The results suggest that this system is capable of scanning the normal and abnormal spread of electrical activity in the heart.

Action Potentials↗
PubMed1981

Intrinsic birefringence signal preceding the onset of contraction in heart muscle.

An intrinsic birefringence signal with two components occurring before sarcomere shortening was measured in mammalian cardiac muscle. The second component was sensitive to the inotropic state of the muscle as affected by external calcium concentration and epinephrine but not by changes of resting length. The second component was absent in frog heart. These results suggest that the second component of the birefringence signal reflects the activity of the sarcoplasmic reticulum related to excitation-contraction coupling processes occurring prior to onset of contraction in mammalian cardiac muscle.

Action Potentials↗
PubMed1981

Potassium chloride versus voltage clamp contractures in ventricular muscle.

In frog ventricle, developed tension was markedly larger in response to depolarization caused by a voltage clamp step than to depolarization induced by high concentrations of potassium chloride. Measurement of extracellular potassium activity at the surface and at the depth of muscle during the development of contractures showed that the diffusion of potassium is much slower than the spread of depolarization through the cross section of muscle. These two observations suggest that competition between the depolarizing and the negative inotropic effects of an increase in the extracellular potassium ion concentration may determine the time course and magnitude of contractile tension in heart muscle.

Animals↗