In a variety of mammalian species, thyroid hormone regulates the contractile properties of the heart as well as the expression of the alpha and beta heavy chains of myosin. We have previously shown ...that the plasma levels of thyroid hormone reach a peak immediately after birth in guinea pigs and decline with maturation. We therefore studied age-related changes in the expression of the myosin heavy chains in the guinea pig ventricle in relation to the ventricular mechanical properties and the levels of thyroid hormone. The composition of the myosin heavy chains was characterized by gel electrophoresis and immunoblotting. Anti-beta-chain antibody stained equally myosins from newborns (0-5 days) and adults (75-90 days), while anti-alpha-chain positively decorated only the myosins of euthyroid newborns or of hyperthyroid adults, but not myosins of embryos, hypothyroid newborns or hypothyroid adults. Myosin of euthyroid adults was faintly stained by anti-alpha-chain. The alterations in the composition of myosin corresponded with the "thyroid state" of these groups. The plasma levels of total T3 were 24.3 +/- 2.7, 9.04 +/- 1.2 and 139.0 +/- 9.3 ng/dl (mean +/- SEM) in the euthyroid, hypothyroid and hyperthyroid adults, respectively. In euthyroid and hypothyroid newborns, the plasma levels of T3 were 56.5 +/- 11.9 and 26.5 +/- 9.8 ng/dl, respectively. Within each age group the thyroid state corresponded with maximal twitch tension (Tmax), rates of development of tension and relaxation, time to peak tension and rate of activation.
We studied the effects of amrinone on contractility and aftercontractions of ouabain-superfused ventricular muscle from neonatal and 3-month-old dogs. In 3-month ventricular muscle, amrinone, 5.3 X ...10(-4) M, alone, increased active tension by 150%. When amrinone was superfused over ouabain-treated ventricular muscle, the increment in contraction was greatest for those muscles in which ouabain had induced a small positive inotropic effect and diminished as the positive inotropic effect of ouabain increased. Amrinone alone did not induce aftercontractions but increased the amplitude of those induced by ouabain by 92 +/- 6%. In neonatal ventricular muscle, amrinone alone was negatively inotropic and it decreased the increment in active tension induced by ouabain.
We investigated the mechanism by which lytic granules extracted from cytotoxic T lymphocytes (CTL) damage guinea pig ventricular myocytes in order to determine whether their actions can be related to ...the overall immunological rejection of the transplanted heart. Granule-induced myocyte morphological changes and final destruction were preceded by shortening of action potential duration (APD) and reductions of the resting potential and the action potential amplitude. APD shortening was probably caused by a granule-induced increase in outward current (most likely non-specific). Ryanodine, which blocks Ca2+ release from the sarcoplasmic reticulum, did not interfere with the morphological and electrophysiological effects of lytic granules. Fura-2 imaging indicated that Ca2+i initially increased about 2-fold from 90.0 +/- 11.5 nM, while cell length decreased less than 5% from a mean value of 99.0 +/- 9.0 microns. A further increase in Ca2+i (greater than 10 fold) was associated with progressive contracture and destruction, suggesting that the structural damage inflicted by lytic granules is caused by Ca2+i overload. The results indicate that the cytocidal action of CTL-derived lytic granules may be involved in immunologically induced damage, even to the extent of rejection of the transplanted heart.
It has been postulated that one of the mechanisms of hypotension associated with cirrhosis is an attenuated responsiveness to catecholamines despite the increased activity of the sympathetic nervous ...system and the elevated plasma concentrations of the sympathetic neurotransmitter, norepinephrine. This abnormality was studied in a dog model of portal hypertension and cirrhosis. Twelve weeks after bile duct ligation (n = 16), intrasplenic pressure rose significantly from 6.3 +/- 0.4 to 14.6 +/- 1.6 mmHg (p < 0.05), mean arterial pressure had fallen from 106 +/- 4 to 83 +/- 8 mmHg (p < 0.01), cardiac output had risen from 3.1 +/- 0.2 to 3.8 +/- 0.8 l/min (p < 0.05) and plasma norepinephrine concentrations rose from 0.22 +/- 0.12 to 1.17 +/- 0.52 nmol/l (p < 0.05). In 7 sham-operated dogs, the changes in these 4 variables over the same period were non-significant. In vivo pressor responsiveness was tested by studying the effects of intravenous and intra-arterial infusions of norepinephrine and the non-selective beta-adrenoceptor agonist, isoproterenol. In vitro responsiveness was tested by measuring the effects of isoproterenol on the isometric twitch of isolated ventricular strips and the effects of norepinephrine on femoral, mesenteric and renal arterial rings. There was no significant change in the in vivo responses of chronic bile-duct-ligated dogs at 12 weeks compared to the preoperative assessment, or to sham-operated dogs at 12 weeks. Furthermore, there was no significant difference between the in vitro responses of ventricular strips to isoproterenol or arterial rings to norepinephrine prepared from chronic bile-duct-ligated and sham-operated dogs.
Vascular instability as defined by systemic hypotension and unresponsiveness to endogenous or exogenous vasoactive substances is a feature of both patients and experimental animals with obstructive ...jaundice. In this study, we have attempted to dissect the possible mechanisms for these abnormalities using both in vivo and in vitro methods. In vivo cumulative pressor responses (Rmax) to intravenous and intraarterial infusions of norepinephrine and angiotensin II and to intravenous infusion of angiotensin I were studied pre- and postoperatively in chronic bile duct-ligated dogs and compared to sham-operated dogs. Preoperatively, the pressor responses to the cumulative infusion of six doses of vasoactive substances in the pre-sham operated and pre-chronic bile duct-ligated dogs were not significantly different. Postoperatively, in the sham-operated dogs, there was no significant change in systemic blood pressure at 1 and 3 weeks, and only in isolated instances were significantly different pressor responses found compared to the preoperative result. In chronic bile duct-ligated dogs, the mean systemic blood pressure fell significantly from 117.2 +/- 3.1 to 107.2 +/- 3.0 mm Hg (p less than 0.01) at 1 week and remained significantly lower at 3 weeks 109.7 +/- 2.6 mm Hg (p less than 0.05). The Rmax to intravenous, but not intraarterial norepinephrine, was significantly decreased. In contrast, the Rmax to both intravenous and intraarterial angiotensin II infusions were significantly depressed at both 1 and 3 weeks. Similarly, the response to intravenous angiotensin I was significantly depressed. Cardiac output rose moderately in two sham-operated dogs from an average of 3.1 to 3.5 liters per min by 3 weeks associated with a decrease of 14.8% in peripheral vascular resistance.
Mammalian species differ in their myocardial responsiveness to cardiac glycosides; whereas glycosides induce a marked positive inotropic effect in species such as dog, rabbit and guinea-pig, the rat ...myocardium is virtually insensitive. We investigated the physiological basis for this phenomenon by testing the hypothesis that the inter-species variations in the response of the myocardium to cardiac glycosides results, at least in part, from species-related differences in the "thyroid status". In the present study we focused on the toxic effects of the glycosides, and studied ouabain-induced delayed afterdepolarizations (DAD): (1) in guinea-pigs, rats and mice, which encompass a wide range of thyroid statuses, as indicated by their O2 consumption and thyroid hormone levels; (2) in guinea-pigs and rats in which the thyroid status was decreased by propylthiouracil treatment or increased by thyroxine administration (in the former species only). DAD were readily induced in guinea-pigs after 40 to 60 min superfusion with 10(-6) M ouabain and 5.4 mM Ca2+. In rats, DAD were induced only when the Ca2+ concentration was raised to 8.1 mM, but were absent in mice even after 2 h of superfusion with ouabain and 8.1 mM Ca2+. In guinea-pigs and rats (at cycle length = 500 ms), DAD amplitude was (means +/- S.E.): 2.8 +/- 0.7 mV and 1.1 +/- 0.4 mV, respectively. The slope of the DAD ascending limb (dV/dt) in guinea-pigs was 47.6 +/- 8.6 mV/s and in rats was 8.1 +/- 3.4 mV/s. In both species DAD characteristics were altered by the thyroid status. In eu-, hyper- and hypothyroid guinea-pigs, DAD amplitude and dV/dt (cycle length = 500 ms) were as follows: 2.8 +/- 0.7 mV and 47.6 +/- 8.6 mV/s; 1.2 +/- 0.4* mV and 12.6 +/- 3.9* mV/s; 7.5 +/- 0.6* mV and 204.0 +/- 18.4* mV/s, respectively (*, P less than 0.005, compared to euthyroid guinea-pigs). The occurrence of triggered beats was also dependent on the thyroid status. They occur more frequently in hypothyroidism and less frequently in hyperthyroidism. Hypothyroidism in rats augmented ouabain toxicity as reflected by an increase in DAD amplitude and dV/dt by 109% and 105%, respectively (P less than 0.05, as compared to euthyroid rats). In conclusion, we suggest that species-related differences in the thyroid status may contribute to the inter-species (as well as for the intra-species) variations in the myocardial responsiveness to cardiac glycosides.
Following some preliminary reports, mammalian fibres from rabbit and rat have recently been successfully studied in detail by means of the voltage clamp. The early transient or sodium conductance ...system was found to be similar to that in frog and squid axons. However, the delayed conductance or potassium currents were found to be negligible. Only after chemical and osmotic manipulations, which were said to expose channels buried under the myelin, did Chiu and Ritchie find delayed currents in rabbit fibres. If confirmed, this would mean that the membrane conductance system of mammalian fibres is so different from that of invertebrate and amphibian axon models as to make the data base gathered from amphibian myelinated fibres (frog and toad) and invertebrate giant axons (squid and myxicola) irrelevant to human nd other mammalian fibres. However, we show here that it is possible to find in the normal nodal membrane of rat myelinated fibres potassium currents that flow through channels which are similar in many respects to those found in the frog node of squid axons.
We studied the relationship between immunologic rejection and changes in contractility of isolated perfused papillary muscle, using heterotopically transplanted rat hearts. Rejection assessed by ...mononuclear cell infiltration was associated with depressed twitch amplitude and lower rates of tension development and relaxation. The relationship between maximum developed tension and Ca2+o was attenuated in muscle from the rejecting allografted heart, as compared with muscle of normal or isografted hearts. To determine the effects of rejection on ventricular electrophysiologic properties, we recorded transmembrane action potentials in isolated ventricular myocardium. We found that in rejecting allografted hearts the resting potential, action potential amplitude, and maximum upstroke velocity of phase zero were significantly reduced compared with normal and isografted hearts. The attenuation in the mechanical and electrophysiologic properties was largely prevented by treating the transplanted rat with anti-lymphocyte-globulin on days 0, 1, and 2 after transplantation. In summary, the present study demonstrates that immunologic rejection of the heterotopically allotransplanted rat heart is associated with marked attenuation of both mechanical and electrophysiologic properties of the ventricular myocardium.
The effects of monovalent internal cations Cs+, Li+ and Na+ on potassium channel conductance in the frog node of Ranvier were studied by means of the voltage clamp. As previously reported, when ...10-80% of the internal K+ was replaced by one of the above cations, the steady-state current-voltage relationship was significantly modified. The main effect was a voltage-dependent attenuation of the currents. We demonstrate that the current attenuation is associated with a change in the channel gating kinetics. For small depolarizations the kinetics can be described by the usual potassium conductance activation time constant, tau n. However, under certain experimental conditions (e.g. substitution of the intracellular K+ with 10% Cs+), during larger depolarizations, stepping the membrane potential to values above 40-60 mV, the conductance develops with two time constants: tau n and a new, slower time constant that, in contrast to tau n, grows with membrane potential. These results can be explained by assuming that the cations may occupy two different sites in the channel; when the first site is occupied the channel is blocked, while occupation of the second site results in slowing of the gating kinetics in the affected channels.
