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PubMed1962

Contraction and action potentials of frog heart muscles soaked in sucrose solution.

Isolated auricles or ventricles from the frog continue to contract, either spontaneously or when stimulated, for from 2 to 4 hours after they are placed in isotonic sucrose solution. After the muscles stop contracting in sucrose solution, contractility is partially restored when the muscles are placed in chloride Ringer's. However, contractility is usually not restored if the muscles are placed in sulfate Ringer's. Ventricles soaked in sucrose solution at 4-7 degrees C continue to contract for 12 to 24 hours and during the first few hours in sucrose solution the contractions often are enhanced. Several types of experiment indicate that the sucrose solution does replace the Ringer's in the extracellular space. Auricles and ventricles also continue to conduct action potentials, with an overshoot, for from 30 to 360 minutes after being placed in sucrose solution. Muscles soaked in sucrose until they are inexcitable rapidly recover in chloride Ringer's but often fail to recover in sulfate Ringer's. The results are discussed in relation to theories about the generation of the action potential in cardiac muscle, and the role of the extracellular fluid in contraction.

Action Potentials↗
PubMed1984

Behavior of the Plasma Membrane of Isolated Protoplasts during a Freeze-Thaw Cycle.

Cryomicroscopy of protoplasts isolated from nonacclimated (NA) rye leaves (Secale cereale L. cv Puma) revealed that the predominant form of injury following cooling to the minimum temperature for 50% survival (LT(50)) (-5 degrees C) was expansion-induced lysis of the plasma membrane during warming and thawing of the suspending medium when the decreasing osmolality resulted in osmotic expansion of the protoplasts. When cooled to temperatures below the LT(50), the predominant form of injury was loss of osmotic responsiveness following cooling so that the protoplasts were osmotically inactive during warming. Only a low incidence (<10%) of expansion-induced lysis was observed in protoplasts isolated from acclimated (ACC) leaves, and the predominant form of injury following cooling to the LT(50) (-25 degrees C) was loss of osmotic responsiveness. The tolerable surface area increment (TSAI) which resulted in lysis of 50% of a population (TSAI(50)) of NA protoplasts osmotically expanded from isotonic solutions was 1122 +/- 172 square micrometers. Similar values were obtained when the protoplasts were osmotically expanded from hypertonic solutions. The TSAI determined from cryomicroscopic measurements of individual NA protoplasts was similar to the TSAI(50) values obtained from osmotic manipulation. The TSAI(50) of ACC protoplasts expanded from isotonic solutions (2145 +/- 235 square micrometers) was approximately double that of NA protoplasts and increased following osmotic contraction. Osmotic contractions were readily reversible upon return to isotonic solutions. During freeze-induced dehydration, endocytotic vesicles formed in NA protoplasts whereas exocytotic extrusions formed on the surface of ACC protoplasts. During osmotic expansion following thawing of the suspending medium, the endocytotic vesicles remained in the cytoplasm of NA protoplasts and the protoplasts lysed before their original volume and surface area were regained. In contrast, the exocytotic extrusions were drawn back into the surface of ACC protoplasts as the protoplasts regained their original volume and surface area.

Journal Article↗
PubMed1980

Hexamethonium modification of cardiovascular adjustments during combined static-dynamic arm exercise in monkeys.

In weight lifting and rowing, essentially the same groups of muscles contract in isometric (static) and isotonic (dynamic) fashion. To approximate the combined static-dynamic arm movements involved in rowing or lifting weights, four rhesus monkeys were trained to pull a T-bar and thereby avoid tail shock. Each animal received 8 daily test sessions in which loads (0.4, 0.8, 1.2, 1.6 kg), total pulls (3, 6, 9, 12 at a constant pull frequency, 0.5 Hz) and alternate sessions of pulling after injection of hexamethonium chloride (7 mg/kg) or saline were factorially combined. Our data indicate that heart rate in this model is primarily influenced by the duration of the dynamic exercise component (number of pulls) in this specific exercise task whereas both dynamic and static components affect systolic and diastolic blood pressure. After ganglionic blockade, heart rate and diastolic pressure do not change appreciably during T-bar pulling while the rise in systolic pressure is attenuated and varies primarily as a function of the static exercise component. The clinical implications of these experiments are discussed.

Animals↗
PubMed1971

The effect of calcium on the force-velocity relation of briefly glycerinated frog muscle fibres.

1. Twitch fibres were isolated from the semitendinosus muscles of frogs. The sarcolemma was made more permeable by a 30 min soak in a solution containing 47.3% glycerine (v/v), 2 mM-EGTA and 10 mM phosphate buffer, pH 7. This was followed by a 30-60 min soak in a solution containing the non-ionic detergent Lubrol-WX. The fibres were then placed in a relaxing medium containing (in mM): KCl, 100; MgCl(2), 1; ATP, 4; EGTA, 2; imidazole buffer, 10; pH 7.0.2. A piece of fibre about 1-2 mm long treated as described in (1) was attached to a servo apparatus. This apparatus made it possible either to hold fibre length constant giving isometric conditions, or alternatively to hold the force constant while measuring isotonic length changes. A special network made it possible to switch control from isometric to isotonic conditions so that afterloaded contractions with a shortening stop could be carried out.3. Contractions were induced at about 4 degrees C by lowering the pCa in the relaxing solution to various levels determined by the ratio of calcium and EGTA added. Contractions were never observed above pCa 7. The steady force generated reached a maximum over the range of pCa 6.09 to 5.49. The relationship between steady force generated and pCa is S-shaped and very steep, implying that multiple interacting binding sites for calcium are involved in the force generating process.4. The relative force-velocity relation is the same at pCa 6.09 and 5.49 where the steady force is at a maximum. The data points can be well fitted by a hyperbola in which the extrapolated value for V(max) is 2.39 muscle lengths/sec. The values obtained for the Hill parameters a/P(0) and b are within the range of those reported for living electrically excited frog muscle.5. The relative force-velocity points obtained at higher pCa values at which the steady force was on average 37% of that developed at pCa 5.49 can also be fitted by a hyperbola. However, the extrapolated value for V(max) is only 1.12 muscle lengths/sec. The value for a/P(0) is increased slightly and the value for b is markedly decreased.6. Evidence is presented against the possibility that an unrecognized fixed internal load is responsible for the change in the relative force-velocity relation obtained at high pCa.7. The relative force-velocity relation does not change appreciably over at least part of the range of sarcomere lengths in which the force generated varies linearly with overlap provided the pCa is held constant.8. The results support the view that lowering the pCa produces a mechanical state equivalent to that produced by tetanic electrical stimulation.9. Some models for calcium activation are discussed. It is concluded that a model based on calcium binding to troponin on the thin filaments is difficult to reconcile with all of the experimental evidence. There is additional evidence for believing that activating calcium may directly influence the cross-bridges.

Adenosine Triphosphate↗
PubMed1991

Differential ontogeny of in vitro vascular responses to three categories of calcium channel antagonists in rats.

We examined the ontogeny of relaxation responses to three categories of calcium channel antagonists, represented by verapamil, diltiazem, and nifedipine, for both potential-operated (KCl-mediated) and receptor-operated channels [norepinephrine (NE)-mediated] in rat thoracic aorta. Aortic rings from 2- to 3-d, 1-wk, and 12-wk-old Sprague Dawley rats were mounted in an organ bath, bathed in Krebs' solution, and connected to a force-displacement transducer to measure isometric tension. Endothelium intact vessels at optimal passive force were exposed to a single ED50 of isotonic KCl or NE, equilibrium contraction was measured, then vessels were washed and exposed for 30 min to 1 microM verapamil, 1 microM diltiazem, or 0.1 microM nifedipine, followed by another dose of KCl or NE. Verapamil and diltiazem demonstrated significant (p less than 0.05) age-related increases in effectiveness for blocking KCl-mediated contraction [(% reduction of control contraction +/- SEM) (Verapamil: 2-3 d, 67.7 +/- 4.2; 1 wk, 72.5 +/- 1.8; 12 wk, 89.5 +/- 1.0. Diltiazem: 2-3 d, 64.6 +/- 2.9; 1 wk, 73.5 +/- 3.0; 12 wk, 83.1 +/- 1.8]. Nifedipine was equally effective at all ages: 2-3 d, 85.6 +/- 1.3; 1 wk, 90.0 +/- 1.6; and 12 wk, 91.3 +/- 1.4. Verapamil and diltiazem also showed significant age-related increases in effectiveness for blocking NE-mediated contraction (Verapamil: 2-3 d, 6.2 +/- 3.9; 1 wk, 28.0 +/- 4.8; 12 wk, 44.1 +/- 6.0. Diltiazem: 2-3 d, 8.0 +/- 3.1; 1 wk, 20.5 +/- 3.9; 12 wk, 46.5 +/- 4.8).(ABSTRACT TRUNCATED AT 250 WORDS)

Age Factors↗
PubMedDate not supplied

[Influence of mechanical conditions on myocardial chronoinotropy].

The force-frequency relationships in isometric, isotonic and auxotonic regimes of contractions have been analysed in right rabbit papillary muscles. The uncorrelated Hauss input sequence of interstimulus intervals was used. For studied mechanical regimes a number of differences have been shown. These differences were in: 1) coefficients of contraction amplitude variation (with the same input stimulus dispersion); 2) dependence of mechanical activity upon the amplitude of previous contraction; 3) force-frequency relationships calculated with regression equations. The nature of revealed differences and usage of chronotropic indexes as a measure of contractility have been discussed.

Animals↗
PubMed1977

Effects of voluntary isometric and isotonic activity on late transcortical reflex components in normal subjects and hemiparetic patients.

When the median nerve is stimulated at the wrist during voluntary contraction of m. abductor pollicis brevis (APB) EMG activity of this muscle contains different distinct responses: a direct muscular response (M response), a spinal reflex response (S response) and a third late reflex response, first described by Upton et al. (1971) as a second volitional reflex response (V2). Comparison of this very likely long-loop reflex involving pathways to and from the cortical level (therefore labelled C response) under isometric (hold) and isotonic (move) contraction revealed a strong facilitation of the response under the latter condition. Repetitive trains of stimuli had stronger facilitatory effect on the occurrence of this late reflex response than single stimuli. Cutaneous stimuli of the median nerve afferents did not exhibit M and S responses but definite late excitatory responses with similar latencies to the C responses. In cases of hemiparesis (mild to severe loss of sensorimotor control of finger movements) the C response was diminished, delayed or absent. These findings together with recent primate and human experimental work support the hypothesis of a transcortical stretch reflex. On the premise that muscular as well as cutaneous afferents contribute to the afferent cortical input the late C response can be interpreted as the efferent component. The C response may be a valuable aid in the diagnosis of small lesions of the sensorimotor cortex and internal capsule.

Adult↗
PubMed1979

Analysis of the length response to a force step in smooth muscle from rabbit urinary bladder.

Responses to isotonic quick release of AC-stimulated smooth muscle strips from rabbit urinary bladder were analysed. Releases were performed at the peak of contraction and at a preset tension level in the contraction and relaxation phase. In other expts. responses at 37 degrees C and 27 degrees C were compared. The length response always consisted of 3 parts: (1) elastic recoil, (2) rapid length change (isotonic transient), (3) steady length change. Qualitatively, phases (1)-(3) could be distinguished also in responses to isotonic quick stretch. The immediate elastic recoils, phase (1), were described by exponential stress-strain relations. Stiffness was found to be somewhat lower during relaxation than during contraction. No effect of temperature on the elastic recoil was seen. The initial velocity in phase (2) was 2-3 times greater than the velocity 100 ms after release. By means of computer analysis of the length records during phases (2) and (3) two decaying exponential processes with widely different time constants could be separated. The time constant of the faster process was of the order of 15-30 ms at 37 degrees C. It decreased with increasing force steps and with increasing temperature. The amount of shortening associated with this process was correlated with the size of the force step, reaching a maximum of about 1.2% of the muscle length. The shortening velocities in phase (3), measured 100 ms after release, were described by Hill's equation. Vmax in the rising part and at the peak of contraction were 0.7 and 0.6 L/s respectively at 37 degrees C. Lower values were found during relaxation and at 27 degrees C. We suggest that part of the elastic recoil in phase (1) occurs in structures associated with the individual cross-bridges, that phase (2) is dominated by a change in the distribution of conformations of bridges in the attached position and that the shortening rate in phase (3) is determined by the entire cycle of events during turnover of bridges after the muscle has adapted to the new load. Observations on the force response to length steps and on shifts from isometric to afterloaded isotonic contraction and vice versa are consistent with this interpretation.

Animals↗
PubMed1992

Interlimb coordination in Parkinson's disease.

This study examined the degree to which Parkinson's disease (PD) patients could "spatially link" the upper limbs to facilitate the performance of bimanual simultaneous movements. Six right-handed PD patients, and seven normal age- and sex-matched controls performed three different tasks: (a) an isotonic elbow flexion as rapidly as possible through an angle of 30 degrees; (b) an isometric contraction of the flexor muscles at the elbow joint to 40% and 60% of maximal volitional force (MVF) for a period of 5 s; (c) an isometric contraction for 2.5 s with one limb, then simultaneously performing an isotonic flexion with the contralateral limb while maintaining the isometric contraction for 2.5 s more. As expected, PD patients were significantly slower in performing the isotonic movement and produced lower peak velocities than the controls. More importantly, the two groups were differentially affected during the bimanual condition. In normals, movement time decreased and peak velocity increased in the bimanual condition. In contrast, PD patients showed increased movement times and sometimes decreased peak velocities in the bimanual condition. The results suggest that normal subjects utilize bilateral outflow to symmetrical muscle groups to synchronize the two limbs in the bimanual task, whereas PD patients dissociate the two limbs.

Aged↗
PubMed1993

Cross-bridge kinetics during shortening in early and sustained contraction of intestinal smooth muscle.

Mechanisms responsible for the decrease in shortening velocity after prolonged contraction ("latch" state) were investigated at identical force during early (20 s, "phasic") and sustained (5 min, "tonic") phases of high-K+ (25-30 mM) contractions in smooth muscle of guinea pig taenia coli. Cytoplasmic Ca2+ concentration, myosin light-chain phosphorylation, and maximum shortening velocity all declined from 20 s to 5 min of contraction. The time course of shortening following isotonic quick release was biexponential, with a fastest rate constant of approximately 80 s-1 in both phasic and tonic contractions. Stiffness was identical in phasic and tonic contraction; however, after a release to slack length and unloaded shortening, stiffness during restretch was greater in tonic contraction (51 vs. 43% of isometric stiffness after 16 ms of unloaded shortening). Stiffness decreased after release with a rate constant of approximately 200 s-1, slightly greater in phasic than in tonic contraction. The results indicate that the number of attached cross bridges during unloaded shortening, while substantially reduced relative to the isometric value, is higher in latch than in nonlatch, consistent with a lower detachment relative to attachment rate.

Animals↗
PubMed1987

[Influence of preload on myocardial relaxation in the rat].

Preload, which determines the initial muscle length, has proved to be a basic determinant of the muscle relaxation phase. The mechanical properties of the papillary muscle of Wistar rats (n = 20) were studied at different initial lengths (L): Lmax and 98 p. 100, 94 p. 100, 90 p. 100, 86 p. 100 and 82 p. 100 Lmax. In isometry, the relaxation phase was proportionally less sensitive to a reduction of preload than the contraction phase. When L decreased the peaks of maximal force derivative of the contraction phase (+dF.dt-1 max) and relaxation phase (-dF.dt-1 max) were linearly and significantly lowered at L = 94 p. 100 Lmax and L = 90 p. 100 Lmax respectively. Conversely, in isotonia the relaxation phase was much more sensitive to a reduction of preload than the contraction phase. On a muscle preloaded from 82 p. 100 Lmax to Lmax, the highest max Vr value (i.e. maximal speed of isotonic relaxation) was measured at Lmax, which is the apex of the Starling's isometric curve. When L decreased max Vr decreased linearly and significantly at 94 p. 100 Lmax. In contrast, the maximal speed of contraction (max Vc) increased between Lmax and 94 p. 100 Lmax, became maximal, then significantly decreased beyond 82 p. 100 Lmax. Thus, in the physiological range of the myocardium (Lmax - 85 p. 100 Lmax) the rat papillary muscle tended to maintain its maximal isotonic speed of contraction, or even to increase that speed, whereas the isometric contraction, isometric relaxation and isotonic relaxation phases were early and significantly depressed.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

Myosin light chain phosphorylation and contraction of guinea pig gallbladder smooth muscle.

These experiments were designed to determine 1) whether acetylcholine (ACh) stimulation is accompanied by changes in myosin light chain phosphorylation in gallbladder smooth muscle and 2) whether dephosphorylated noncycling cross bridges (latch bridges) exist in gallbladder smooth muscle. Isometric stress, isotonic shortening velocity, and myosin light chain phosphorylation were determined under conditions of contraction and relaxation in ACh-stimulated guinea pig gallbladder smooth muscle. Unstimulated muscle contained 6.8 +/- 2.0% phosphorylated myosin light chain. ACh stimulation (5 x 10(-5) or 10(-4) M) was associated with a rapid increase in myosin light chain phosphorylation to a value that was maintained throughout the tonic contraction. In contrast, isotonic shortening velocity was maximal at 30 s of stimulation and then declined over time to a steady-state level that was 25-30% of the peak velocity. Upon agonist washout (relaxation), dephosphorylation of the myosin light chain occurred at about the same rate as the decline in shortening velocity and preceded the decline in isometric stress. These data suggest that ACh stimulation is accompanied by changes in myosin light chain phosphorylation but that dephosphorylation of cross bridges is not necessary for the slowing of cross-bridge cycling rates in gallbladder smooth muscle.

Acetylcholine↗
PubMed1994

The role of calcium and alpha-adrenoceptors in contractile response of chick expansor secundariorum muscle to field stimulation.

1. The effects of some alpha-adrenergic agonists and antagonists on electrically-evoked contractions and tension of chick expansor secundariorum muscle (ESM), and dependence of these events on extracellular calcium was investigated. 2. Both train and continuous electrical stimulation can produce regular contractions in preparations obtained from 40-60 day old chicks. 3. Clonidine had a biphasic action on the contractions produced by train electrical stimulation. In concentrations ranging from 10(-8) to 3 x 10(-7) M, clonidine decreased the contraction amplitude, but in higher concentrations, it caused an increase in both the muscle tension and the contraction amplitude. These effects were reversed by application of yohimbine although yohimbine by itself had no effect on the contractions. 4. Introduction of calcium free isotonic high potassium medium decreased muscle tone which was followed by further dose-dependent increase in tension, along with the addition of cumulative doses of CaCl2 (ED50 = 2.8 x 10(-3) M). 5. Nifedipine reduced the amplitude of ESM contractions produced by continuous electrical stimulation in a dose dependent manner (IC50 = 6.7 x 10(-7) M). 6. Methoxamine induced a completely dose dependent increase in muscle tension which was dependent on extracellular calcium and was inhibited by nifedipine. In the presence of 10(-8) M nifedipine, ED50 of methoxamine stimulatory effect increased from the control value of 2.2 x 10(-7) to 8.4 x 10(-7) M).(ABSTRACT TRUNCATED AT 250 WORDS)

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1972

On the inhibition of muscle contraction caused by exposure to hypertonic solutions.

The evidence supporting a site of inhibition of excitation contraction (E-C) coupling near the plasma membrane (the "glycerol effect," the K(+)-potentiating effect) for muscle in hypertonic solution was reinvestigated. It was found, using whole frog sartorii, that there was a rehydration of muscle soaked in glycerol Ringer after 30 min and a large swelling (to 140% after 1 hr soaking) upon return of the muscle to normal Ringer, suggesting that significant amounts of glycerol enter the fibers during this time. While contrary to the original report of the glycerol effect, this finding was consistent with other studies involving the use of single fibers. Also reexamined was the potentiating effect of K(+) on the hypertonic inhibition of muscle contraction. It was found that muscles exposed to this KCl pretreatment swell so that they are less dehydrated in hypertonic solutions, thus accounting for the observed potentiation. After being treated instead with a K(2)-tartrate Ringer solution, muscles did not swell and, as determined with twitch recordings, did not display any potentiation in hypertonic solutions-even though the [K(+)] was higher than an osmotically equivalent KCl solution. The evidence was thus consistent with alternative hypotheses in which inhibition of contraction occurs at a later stage in E-C coupling or involves the contractile process itself.

Animals↗
PubMed1984

Birefringence signal and early mechanical changes at normal and increased tonicities in frog skeletal muscle.

Simultaneous measurements of the time course of early mechanical events and the early large birefringence signal were performed during activation of isolated frog skeletal muscle fibres bathed in iso- and hypertonic media. A piezo-electric transducer was used which had a resonance frequency of 3.3 kHz and a sensitivity of 740 mV/mN (0.5 mV noise peak to peak) and a compliance of 0.6 mu/mN. The delay from stimulus onset to the onset of the birefringence signal was estimated to be 0.78 +/- 0.03 ms in normal Ringer solution at room temperature (20-23 degrees C). The beginning of latency relaxation appeared subsequent to the birefringence signal after a delay of 0.36 +/- 0.03 ms. Increasing the tonicity retarded the time course of both birefringence signal and latency relaxation. In solutions of twofold greater tonicity an increase in tension (denoted pre-relaxation contraction) was observed to precede latency relaxation. Pre-relaxation contraction became more prominent with increasing tonicity and appeared to coincide with its onset with the onset of the birefringence signal. The pre-relaxation contraction was shown not to be a stimulus artifact and was not reduced in amplitude by D 600 or calcium-poor solutions. Lowering the temperature to 2.2 degrees C in normal Ringer solution delayed the onsets of the birefringence signal and latency relaxation, and increased the interval between the two signals but did not result in a tension increase before latency relaxation. The coincidence of the birefringence signal onset with the onset of pre-relaxation contraction suggests a common aetiology. The aetiology remains speculative but seems to require hypertonicity for expression of the mechanical signal.

Action Potentials↗
PubMed1991

Activation dependence of isotonic transient in response to step tension reduction in cardiac muscle segment during barium contracture.

To clarify the activation-dependence of dynamic mechanical characteristics of contracting cardiac muscle, we analysed the healthy central segment length (SL) response to step decrease in tension at two different levels of barium contracture (0.2 mM and 0.5 mM Ba2+) in rat papillary muscles with a fixed initial SL. The time course of this response is thought to reflect the kinetics of actin-myosin interaction. The muscle was released stepwise from the steady contracture tension (Tc) to new steady tension levels (Tr) of varying magnitudes at 22 degrees C. The SL responses consisted of four phases at Tr/Tc greater than 0.3. The amplitude of shortening in the second phase, after the initial rapid and minute shortening in the first phase, increased with an increase in amplitude of step tension reduction, and was greater at the higher activation level when compared at an identical amount of Tr/Tc. The fourth phase, after the remarkable lengthening in the third phase, was an extremely slow and minute shortening toward a new steady SL under the new tension. The duration of the second and third phase was quite insensitive to activation level at Tr/Tc greater than 0.85, but became longer at the higher activation level with larger amounts of tension reduction. The velocity measured from the initial quasi-steady SL shortening in the second phase increased significantly with the increase in activation level. These results are discussed in terms of cross-bridge kinetics underlying the isotonic SL transients at two different activation levels.

Animals↗
PubMed1986

The arrhythmogenic current ITI in the absence of electrogenic sodium-calcium exchange in sheep cardiac Purkinje fibres.

Sheep cardiac Purkinje fibres were voltage clamped with a two-microelectrode technique. Under conditions that are known to elevate intracellular calcium (0 mM-external potassium), membrane currents were examined. In the above conditions, a brief depolarizing pulse leads to an oscillatory inward current (ITI) which peaks at about 300 ms after the repolarization. An after-contraction is also observed, the peak of which occurs about 80 ms after the peak of ITI. This result is in accord with the results of Kass, Lederer, Tsien & Weingart (1978a). We replaced external sodium with an isotonic CaCl2 solution to remove the sodium-calcium exchange mechanism as a possible current carrier for ITI. In the steady state under these conditions an oscillatory membrane current and after-contraction are seen following repolarization. This current was identified as ITI on the basis of its temporal relation to both the repolarization step and the after-contraction. In isotonic CaCl2, ITI has a reversal potential of -37 mV. Because of this fact ITI cannot be explained by an electrogenic sodium-calcium exchange mechanism alone. The reversal potential suggests that ITI arises from a channel which is permeable to both potassium and calcium. Fluctuations of membrane current and of tension were recorded in the steady state at different holding potentials. Power spectral analysis showed that the current fluctuations were at a minimum at a holding potential of -37 mV. Tension fluctuations were, however, relatively constant over the range of membrane potentials examined (-17 to -70 mV). The peak power of the current fluctuations occurred at about 1.5 Hz (at a holding potential of -70 mV). This peak shifted towards higher frequencies with increasing depolarization. A similar shift in frequency was observed for the tension fluctuations. Cross-correlations between membrane current and tension were calculated for various steady membrane potentials. This analysis shows that the current fluctuations are associated with the tension fluctuations, each with a principal period of about 0.5 s. This analysis also shows that at potentials more negative than the reversal potential of ITI, increasing tension is associated with increasing inward current and that the tension fluctuations follow current fluctuations by about 70 ms. At potentials positive to the reversal potential of ITI, increasing tension was associated with increasing outward current. This analysis therefore indicates that the fluctuations in membrane current reverse at a potential similar to the reversal potential of ITI.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗