Inactivation Of Isi Research Articles

Electrophysiological Effects of CD-349, a Dihydropyridine-Type Calcium Antagonist, on Goat Cardiac Purkinje Fibers

We examined the calcium antagonistic action of CD-349, a dihydropyridine derivative, on goat cardiac Purkinje fibers using the two-microelectrode voltage-clamp method. CD-349 at a concentration of 10(-5) M shortened the action potential duration without changing the maximum rise in the action potential (Vmax) in goat Purkinje fibers. CD-349 at 3 x 10(-7) to 3 x 10(-6) M inhibited the slow inward current (Isi) in a concentration-dependent manner. At the holding potential of -55 mV, CD-349 exerted a tonic block of Isi, and, furthermore, it exerted a use-dependent block at a frequency range of 1 Hz, but it did not exert a use-dependent block at 0.5 and 0.2 Hz. This may be because CD-349 delayed the recovery process from the inactivation of Isi. The amplitude of the block of Isi was larger at the holding potential of -45 mV than at -55 mV. The inactivation curve of Isi shifted toward a negative potential in the presence of CD-349. Nifedipine also exerted a tonic block of Isi. The onset of the action of nifedipine was quicker than that of CD-349 or nitrendipine. A use-dependent block at 1 Hz and delay of the recovery process from inactivation was also observed with nifedipine. The inactivation curve shifted toward the negative potential with nifedipine. On washout of the drugs, the effects of CD-349 or nitrendipine were not readily reversed compared with those of nifedipine. CD-349 had no effect on either inward rectifying (IK1) or delayed outward potassium (IK) or hyperpolarization-activated inward (If) currents. These observations suggest that, in cardiac tissues, CD-349 selectively inhibits the calcium current, presumably by acting on the inactivated channel.

Electrical and mechanical effects of strontium in sheep cardiac Purkinje fibres.

The electrical and mechanical effects of strontium were studied in sheep cardiac Purkinje fibres perfused in vitro. In a nominally calcium free solution, strontium (1.35-10.8 mmol.litre-1): (1) caused a time, rate and concentration dependent shift of the plateau to more positive potentials, prolonged the action potential and decreased the maximum diastolic potential; (2) increased the time to peak and amplitude of the twitch and caused a tonic force which relaxed only on repolarisation; (3) was rapidly overcome in its effects by calcium (1.35-2.7 mmol.litre-1); (4) was antagonised by manganese (1 mmol.litre-1) and cadmium (0.1-0.2 mmol.litre-1); (5) was potentiated by noradrenaline (0.1 mumol.litre-1); (6) could induce action potentials in 27 mmol.litre-1 [K]o; (7) induced a tail following the action potential when the pacemaker potential had been blocked by caesium; (8) could induce a tail in 8 mmol.litre-1 [K]o which sustained force development and was reduced by calcium antagonists; (9) if applied to a quiescent fibre, induced a prolongation of the first resumed action potential and tonic force but a small twitch, and these effects were antagonised by calcium and manganese; and (10) induced a strong twitch after a period of quiescence in low [Na]o. It is concluded that the pronounced and progressive electrical and mechanical effects of strontium in cardiac Purkinje fibres are due to an enhanced strontium influx (due to inability of strontium to substitute for calcium in the inactivation of Isi) and to strontium extrusion through an electrogenic Na-Sr exchange.

Electrophysiologic and mechanical properties of single feline RV and LV myocytes

With the advent of techniques to isolate large numbers of single adult mammalian ventricular myocytes, it has become possible to determine whether, as a result of the different pressure loading of the right and left ventricles (RV and LV), RV and LV myocytes differ in electromechanical properties. We studied the morphology, contraction and electrophysiology of the L-type slow inward calcium current (Isi) in isolated adult feline RV and LV myocytes. The maximum width of LV myocytes was slightly greater than for RV myocytes (25.9 ± 7.0 μm vs. 25.1 ± 7.9 μm, P = 0.05), but RV and LV myocytes did not differ significantly in maximum length or two-dimensional surface area. RV and LV myocytes did not differ significantly in the extent of shortening or rates of shortening and relaxation. The voltage dependence of activation and inactivation and the time course of activation and recovery from inactivation of Isi also did not differ significantly between RV and LV myocytes. We conclude that despite the different pressure loads on the RV and LV, single myocytes from either ventricle have similar physiologic properties.

Electrical activity and contraction in cells isolated from rat and guinea‐pig ventricular muscle: a comparative study.

1. Contraction in single ventricular muscle cells from rat and guinea-pig heart was measured using an optical technique, while at the same time either action potentials were recorded or transmembrane currents were measured under voltage-clamp conditions. 2. When the membrane was depolarized to 0 mV, there was a phasic and a tonic component of the contraction in guinea-pig cells, whereas in rat cells only the phasic component was obvious. In both species the depolarizations evoked the second inward current (Isi). 3. In rat cells, when the membrane potential during a depolarization was varied over the range -40 to +60 mV, the amplitude of contraction first increased to a peak at a potential close to 0 mV, and then declined as the membrane potential became more positive. In contrast, contraction in guinea-pig cells measured under similar conditions continued to increase as the depolarization was increased, and the tonic component of contraction became more obvious at more positive potentials. Contraction amplitude in guinea-pig cells could also be increased by increasing pulse duration under conditions where the tonic component of contraction was prominent. 4. Contraction during depolarization was suppressed by ryanodine in rat cells, whereas in guinea-pig cells contraction persisted, but with a modified time course. Ryanodine did inhibit spontaneous contractions of guinea-pig cells during exposure to low extracellular sodium. 5. Nifedipine suppressed Isi and phasic contraction in both rat and guinea-pig cells. In guinea-pig cells these effects developed contemporaneously, but in rat cells substantial reduction of Isi occurred before marked suppression of contraction. 6. In rat cells exposed to strontium in place of external calcium, inactivation of Isi was slowed and contraction was prolonged, with a slower time-to-peak and relaxation. The time course of the action potential was modified and ryanodine no longer inhibited contraction of rat cells in the presence of strontium. 7. It is concluded that the amplitude of contraction in rat and guinea-pig ventricular cells is determined by calcium both entering through the surface membrane and released from internal stores, and that under normal conditions the balance is towards release from stores in rat cells, and towards entry through the surface in guinea-pig cells.

An electrophysiological study of amiloride on sino-atrial node cells and ventricular muscle of rabbit and dog.

Effects of amiloride were studied on canine and rabbit sino-atrial node cells and the canine ventricular cells. In the canine sino-atrial node selectively perfused with Tyrode solution through the sinus node artery, amiloride (4.4 X 10(-4)-1.3 X 10(-2)mol/l) induced a negative chronotropic and a positive inotropic effect in a concentration-dependent manner. In canine ventricular muscle, the amplitude of the action potential and the resting membrane potential were decreased in the presence of amiloride (4.4 X 10(-6)-8.7 X 10(-4) mol/l). The maximum rate of rise was also depressed in a use-dependent manner as well as a concentration-dependent manner. Simultaneously the action potential duration was markedly prolonged. Low concentrations of amiloride decreased the contractile force, but higher concentrations (over 8.7 X 10(-5) mol/l) enhanced it. In the spontaneously beating rabbit sino-atrial node, amiloride (8.7 X 10(-5)-2.2 X 10(-3) mol/l) lengthened the action potential duration and the cycle length accompanied with the decreases in the amplitude and the maximum rate of rise of the action potential. The maximum diastolic potential was slightly depolarized. In voltage clamp experiments of the rabbit sino-atrial node, amiloride (4.4 X 10(-4)-1.3 X 10(-3) mol/l) suppressed the slow inward current (Isi) and the outward current (Ik) concentration-dependently, without altering steady-state inactivation of Isi (f infinity) and the activation of Ik (p infinity). The hyperpolarization-activated inward current (Ih) was also reduced. These results suggest that amiloride decreased Isi, Ik and Ih by a reduction in the conductance of these current systems.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Effects of alpha-adrenoceptor stimulation with phenylephrine in the presence of propranolol on force of contraction, slow inward current and cyclic AMP content in the bovine heart.

The mechanism of the cyclic AMP-independent positive inotropic effect of cardiac alpha-adrenoceptor stimulation was studied by analyzing the effects of phenylephrine on force of contraction, calcium-dependent slow action potentials and the slow inward current (Isi) in bovine ventricular trabeculae. The preparations were electrically driven at 0.3 Hz in the presence of propranolol 1 mumol 1(-1). Phenylephrine increased the force of contraction in a concentration-dependent manner (maximum about 200% of control at 30 mumol 1(-1). The effect was surmountably antagonized by phentolamine. The positive inotropic effect of phenylephrine was accompanied by a concentration-dependent increase in time to peak force and occurred without any detectable increase in cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels. The positive inotropic effect of phenylephrine was accompanied by an increase in action potential duration both at 20% and 90% repolarization. Calcium-dependent slow action potentials were also prolonged by phenylephrine and there was a distinct increase in the maximal rate of depolarization (dV/dtmax) of these slow potentials. These effects were also completely reversible on washing and surmountably blocked by phentolamine. However, the increase in dV/dtmax was smaller than that of isoprenaline in concentrations producing similar inotropic effects. Voltage-clamp experiments with the single sucrose-gap method showed that the phenylephrine-induced increase in force of contraction was associated not only with an increase in peak slow calcium inward current, Isi max, but also with a delay in the inactivation of Isi. Outward currents were not detectably altered by phenylephrine. It is concluded that the alpha-adrenoceptor mediated, cyclic AMP-independent positive inotropic effects of phenylephrine in bovine cardiac muscle are associated with an increase in slow inward current. Additionally, the amount of calcium influx during excitation is probably increased by a delay in the inactivation of Isi. Both effects can explain the phenylephrine-produced prolongation of the action potential, and probably contribute to the positive inotropic effect of alpha-adrenoceptor stimulation. However, as the effect on dV/dtmax is smaller than that of isoprenaline, other (still unknown) mechanisms may also be involved.

Frequency-dependent depression of Vmax of slow response action potentials by a high concentration of papaverine.

The maximum upstroke velocity (Vmax) of Ca2+-dependent action potentials was used to measure relative changes in the slow inward current (Isi) caused by 10(-4) M papaverine as a function of pacing frequency in K-depolarized guinea pig ventricular strips. Papaverine caused a pronounced frequency-dependent depression of Vmax presumed to be correlated with a decrease in the rate of recovery from inactivation of Isi. Papaverine (10(-4) M) tripled the time constants for the biexponential recovery from depression of Vmax (determined by a paired pulse protocol) relative to the control. Depression of Vmax by papaverine during a 1 Hz stimulus train preceded by a long rest period reached a new steady state within three depolarizations. Complete poststimulation recovery from the depression of Vmax was also relatively rapid, unlike that described for verapamil. Preparations exposed to Mn2+ (after pretreatment with Ba2+ to restore excitability) showed a depressed steady-state Vmax independent of frequency. It is concluded that the frequency-dependent blockers of Vmax (papaverine and verapamil) differ in their rates of association and dissociation with the slow inward channel.