Electromechanical relationships of rabbit papillary muscle under interpolated extrasystole conditions and after a pause

The aim of the study was to describe and attempt to explain certain specific features of electromechanical coupling in the rabbit myocardium. Electromechanical correlations in the papillary muscles of the right ventricle of adult rabbit hearts were studied by a programmed stimulation technique. The duration of action potentials (AP) was measured in the plateau phase (Do, ms) and at -80 mV level (D80, ms), together with the intensity of the corresponding isometric contractions (MG, arbitrary units). After twenty AP of 1 Hz frequency, we interpolated an extrasystole with a variable interval (TE = 100-900 ms) and measured D0 and MG of the premature AP and the first AP of the subsequent cycle. When a steady state at 1 Hz frequency had been reached, we interpolated pauses (Tp) of 5 to 600 s and read D80 and MG of the first to the tenth contraction after the pause. The extrasystole D0 attained the maximum at TE = 260 ms and then fell abruptly. The MG of extrasystoles with a longer TE grew from the lowest value (0.3), attained 1 at TE = 700 ms and then remained stable. The first contraction after extrasystole displayed distinct postextrasystolic potentiation (MG = 2), while D0 was unwontedly short. Prolongation of TE was accompanied by an increase in its D0 value and by a steep drop in MG (to as little as 0.2). D80 of the first AP evoked after the pause fell proportionally to log Tp and then, from Tp = 60 s, gradually rose. The MG value of the first contraction after the pause fell proportionally to log Tp. The AP recovered much more rapidly than contractility from the effect of the pause. In the discussion, an attempt is made to explain the found correlations on the basis of differences in the behaviour of the calcium current channel system in the rabbit myocardium and a commentary on electromechanical correlations is based on the hypothesis that the free sarcoplasmic calcium level determines both membrane electrogenesis and the inotropic state.     

Action potential duration and force-frequency relationship in isolated rabbit,guinea pig and rat cardiac muscle

The effect of action potential duration and elevated cytosolic sodium concentration on the forcefrequency relationship in isolated rabbit, guinea pig and rat papillary muscle preparations was studied. Shortening of action potential duration in guinea pig and rabbit from 150–200 ms to values characteristic of rat (20–40 ms), using the K_(ATP) channel activator levkromakalim (15 mol·l^–1), markedly reduced the force of contraction and converted the positive force-frequency relationship into negative one at longer pacing cycle lengths. This conversion was greatly enhanced in the presence of acetylstrophanthidin (0.2–1 mol·l^–1), an inhibitor of the Na-K pump. Acetylstrophanthidin (1 mol·l^–1) alone, however, had no effect on the forcefrequency relationship. Prolongation of action potential duration in rat with inhibitors of cardiac K channels (4-aminopyridine [10 mmol·l^–1] plus tetraethylammonium [2 mmol·l^–1) increased the force of contraction and abolished the negative force-frequency relationship observed in rat at longer pacing-cycle lengths. It is concluded that both action potential duration and cytosolic sodium concentration are major determinants of the force-frequency relationship in mammalian myocardium.Abbreviations AC acetylstrophanthidin - APD action potential duration - APD ₅₀ and APD ₉₀ action potential duration measured at 50% and 90% level of repolarization, respectively - SR sarcoplasmic reticulum     

Action of thiamine on auditory evoked potentials in the guinea pig

A technique is proposed for quantifying the effects of physiologically active substances at the periphery of the auditory analyzer. It was found that applying 1×10^–11 to 1×10^–3 M thiamine to the membrane of guinea pig cochlear round window (fenestra rotunda) produces a rise in the amplitude and a reduction in the latency of the N₁ and N₂ components of auditory nerve action potentials, waves I and II of brainstem auditory evoked potentials occurring in response to an acoustic stimulus. It is suggested that this effect is produced by facilitated synaptic transmission at synapses between hair cells and spiral ganglia neurons under the action of thiamine penetrating into the cochlea.A. V. Palladin Institute of Biochemistry, Academy of Sciences of the Ukrainian SSR, Kiev. A. I. Kolomiichenko Research Institute of Otolaryngology, Ministry of Public Health of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 5, pp. 654–660, September–October, 1986.     

Effect of twitch interval duration on the contractile function of subsequent twitches in isolated rat, rabbit, and dog myocardium under physiological conditions

Many studies have shown that a change in stimulation frequency leads to altered contractility of the myocardium. However, it remains unclear what changes occur directly after a change in frequency and which ones are a result of the slow processes that lead to the altered homeostasis, which develops after a change in stimulation frequency. To distinguish the immediate from the slow responses, we assessed contractile function in two species that have distinctively different calcium (Ca(2+))-handling properties using a recently developed, randomized pacing protocol. In isolated dog and rat right ventricular trabeculae, twitch contractions at five different cycle lengths within the physiologic range of each species were randomized around a steady-state frequency. We found, in both species, that the duration of the cycle length just prior to the analyzed twitch (primary) positively correlated with the increased force of the analyzed twitch. In sharp contrast, the cycle lengths, one and two more removed from the analyzed twitch ("secondary" and "tertiary"), displayed a negative correlation with force of the analyzed twitch. In additional experiments, assessment of intracellular Ca(2+) transients in rabbit trabeculae revealed that diastolic Ca(2+) levels were closely correlated to contractile function outcome. The relative contribution of the primary cycle length was different between dog (51%) and rat (71%), whereas in neither species was a significant effect on relaxation time observed. With the use of randomized cycle lengths, we have distinguished the intrinsic response from the signaling-mediated effects of frequency-dependent activation on myofilament properties and Ca(2+) handling.     

Intracellular proton mobility and buffering power in cardiac ventricular myocytes from rat,rabbit, and guinea pig

Intracellular pH (pHi) is an important modulator of cardiac function. The spatial regulation of pH within the cytoplasm depends, in part, on intracellular H+ (Hi+) mobility. The apparent diffusion coefficient for Hi+, DHapp, was estimated in single ventricular myocytes isolated from the rat, guinea pig, and rabbit. DHapp was derived by best-fitting predictions of a two-dimensional model of H+ diffusion to the local rise of intracellular [H+], recorded confocally (ratiometric seminaphthorhodafluor fluorescence) downstream from an acid-filled, whole cell patch pipette. Under CO2/HCO3--free conditions, DHapp was similar in all three species (mean values: 8-12.5 x 10-7 cm2/s) and was over 200-fold lower than that for H+ in water. In guinea pig myocytes, DHapp was increased 2.5-fold in the presence of CO2/HCO3- buffer, in agreement with previous observations in rabbit myocytes. Hi+ mobility is therefore low in cardiac cells, a feature that may predispose them to the generation of pHi gradients in response to sarcolemmal acid/base transport or local cytoplasmic acid production. Low Hi+ mobility most likely results from H+ shuttling among cytoplasmic mobile and fixed buffers. This hypothesis was explored by comparing the pHi dependence of intrinsic, intracellular buffering capacity, measured for all three species, and subdividing buffering into mobile and fixed fractions. The proportion of buffer that is mobile will be the main determinant of DHapp. At a given pHi, this proportion appeared to be similar in all three species, consistent with a common value for DHapp. Over the pHi range of 6.0-8.0, the proportion is expected to change, predicting that DHapp may display some pHi sensitivity.     

Lectin receptors in the salivary glands of the rat during postnatal ontogenesis

Distribution of lectin-binding sites in rat submandibular and sublingual salivary glands during postnatal development has been investigated. Lectin preparations include con A, lentin lectin, castor beans agglutinin, peanut, soybean and Sophora japonica agglutinins, wheat germ agglutinin and lectin from the bark of Laburnum anagyroides. The direct and indirect peroxidase techniques are used. According to the similarities of histochemical patterns, all lectins are divided into four groups. Besides the general patterns of lectin binding sites, some details are noted. Lectins of peanut and Sophora japonica possess an extremely high affinity to mast cells, con A, lens lectin, castor beans and wheat germ agglutinins--to serous demilunes cells. Laburnum lectin--to salivary ducts epithelia in adult rat salivary glands. Lentin lectin, con A and Laburnum lectin preferentially stain cells with specific granularity in granular ducts at early stages of postnatal development. Considering the character of staining, we propose for further histochemical investigations of the salivary glands lentin lectin, peanut agglutinin, wheat germ agglutinin and Laburum anagyroides lectin.     

Effect of stretching on the action potentials of the human and rabbit ventricular myocardium

The effect of stretching from L0 to Lmax on the electrical activity was studied on human myocardial preparations from patients with heart disease and on strips of rabbit ventricular myocardium. Muscular deformation was shown to decrease the amplitude and velocity of depolarization in slow action potentials. The action potentials (AP) possessing a fast depolarization phase were not sensitive to physiological stretching. Antiarrhythmic drugs--ethmozin (2 X 10(-5) M) and ethacizin (2 X 10(-6) M)--caused a decrease in the rate of AP depolarization, thus increasing AP sensitivity to deformation. It is suggested that stretching under the action of ethmozin and ethacizin reduced cardiomyocyte excitability due to suppression of slow Ca-current.     

Comparative study of ethynyloestradiol metabolism in the rabbit, guinea pig and rat.

Ethynyloestradiol was administered to rabbits, guinea pigs and rats, and the concentration of the steroid in blood was measured by radioimmunoassay. In both rabbits and guinea pigs, levels of conjugated steroid were much higher than those of the freely extractable form. Whereas considerable amounts of steroid were present in a congugated form in plasma 24 h after injection, none was present at this time in a freely extractable form. There were significant differences between young and adult rabbits and guinea pigs in the rate at which ethynyloestradiol was metabolized. The amounts present in the freely extractable form in rats were higher than in the other two species but no steroid was detected in the conjugated fraction. The results are compared with previous findings in humans.     

Quantitative changes of the C-cell population in the rat thyroid during postnatal ontogenesis

Summary The number and distribution of C-cells in the rat thyroid gland, have been investigated during postnatal ontogenesis from birth to 120 days of age. The argyrophilic and metachromatic properties of these cells were used to identify them. In the thyroid of newborn rats the C-cells do not exhibit argyrophilia and metachromasia. These reactions appear at 10 days and can be seen at all subsequent ages. The number of C-cells shows a parallel increase with age as demonstrated by the change in the proportion of C-cellsF-cellscolloidstroma during development. A marked increase in C-cells was found at 50 days of age when the proportion of C-cells rose to 27.67% from the value of 16.78% at 30 days. At 70 days a decrease was noted (20.50%) which hardly changed until 120 days of age (22.20%). The numerical increase in C-cells occurs at the expense of the follicular epithelium and stroma.The C-cells occupy elongated islet-like region in the central part of the lobe, decreasing in number towards the periphery where no C-cells are present. The long axis of the C-cells area is parallel with the longitudinal axis of the lobe. The area of C-cells is largest at the centre of the lobe, corresponding to the territory of the peak of the Gaussian curve for the numerical distribution of C-cells.     

The adrenergic innervation of the guinea pig ovary during prenatal and postnatal periods

The pattern of adrenergic nerves was visualized and norepinephrine (NE) quantified in guinea pig ovaries from fetal day 50 to term and on days 1, 5, 10, 20 and 28 postnatally. Before birth, there were a few perivascular adrenergic nerves and correspondingly low ovarian NE levels. After birth and through day 20, nerves gradually grew into adjacent stroma. By day 28, there was a proliferation of beaded adrenergic nerves around blood vessels and into adjacent stroma, which was reflected by significantly increased ovarian levels of NE. These findings are discussed in relation to antral follicle development, atresia, and interstitial gland formation in the ovaries of prepubertal guinea pigs.     

Comparison of brain ribonucleases of rabbit, guinea pig, rat, mouse and gerbil

The brain ribonucleases of rabbit, guinea pig, rat, mouse and gerbil were investigated by histochemical and biochemical methods. For the localization, the ribonucleases were electrophoretically transferred from cryostat sections to polyacrylamide gels. Elevated ribonuclease activities were found in the cortex, the basal ganglia, the hippocampal formation and the ventricles, whereas the corpus callosum and the internal capsule exhibited lower activities. The total RNA degrading activities of the brain extracts of the different species varied in a wide range. However, a pre-requisite for the measurement of acid soluble degradation products in the test system was the inactivation of endogeneous ribonuclease inhibitors, present in all extracts. Molecular weight analysis by means of SDS-polyacrylamide gel electrophoresis revealed a characteristic set of ribonucleases for each species, consisting of enzymes with different pH-optima.     

Comparison of brain ribonucleases of rabbit,guinea pig,rat, mouse and gerbil

Summary The brain ribonucleases of rabbit, guinea pig, rat, mouse and gerbil were investigated by histochemical and biochemical methods. For the localization, the ribonucleases were electrophoretically transferred from cryostat sections to polyacrylamide gels. Elevated ribonuclease activities were found in the cortex, the basal ganglia, the hippocampal formation and the ventricles, whereas the corpus callosum and the internal capsule exhibited lower activities. The total RNA degrading activities of the brain extracts of the different species varied in a wide range. However, a pre-requisite for the measurement of acid soluble degradation products in the test system was the inactivation of endogencous ribonuclease inhibitors, present in all extracts. Molecular weight analysis by means of SDS-polyacrylamide gel electrophoresis revealed a characteristic set of ribonucleases for each species, consisting of enzymes with different pH-optima.Dedicated to Professor Dr. T.H. Schiebler on the occasion of his 65th birthday