Effect of heart rate on electrically induced repetitive ventricular responses in the digialized dog

After recovery from acetylstrophanthidin-induced ventricular tachycardia, a repetitive ventricular response (RVR) following a single diastolic stimulus could be elicited for 22 minutes. With atrial pacing at the maximum ventricular follow rate, RVR was obtainable for 70 minutes. A pause in pacing also evoked a ventricular ectopic beat; however, this persisted for only 27 minutes. The minimum heart rate required for RVR was always less than the minimum rate required for pause-induced ectopic beats. Extrasystoles following a pause during pacing, RVR in sinus rhythm, and RVR during rapid heart rates represent decreasing levels of ventricular automaticity corresponding to progressive dissipation of digitalis intoxication. The underlying mechanism for RVR probably is due to net loss of intracellular potassium, which can be induced both by digitalization and by rate acceleration.

Effects of acetylstrophanthidin on isolated veins of the dog

Acetylstrophanthidin, 1.0 to 10.0 µg/ml, caused contraction of strips of the lateral saphenous vein of the dog in an organ bath at 37°C. The superior mesenteric vein, which exhibited spontaneous activity in the organ bath, responded to acetylstrophanthidin with an almost immediate increase in tension and an increased frequency of contractions; this was followed by a sustained contraction which had the same time lag as the contraction of a saphenous vein strip from the same dog. In strips from both veins, the contraction was depressed by cooling and augmented by warming, an effect opposite to that seen with norepinephrine-induced contractions. A latent period of 10 to 60 minutes preceded the contraction of the saphenous vein; during this time, the vein showed increased reactivity to added norepinephrine, 5-hydroxytryptamine, acetylcholine, barium chloride, increased bath concentration of K + , and decreased bath concentration of Na + . The increased reactivity and contraction still occurred after catecholamine depletion of the vein by chronic lumbar sympathectomy and in umbilical veins, which are devoid of adrenergic nerves. In addition to its direct constrictor action on cutaneous and mesenteric veins, acetylstrophanthidin sensitized cutaneous veins to vasoactive agents and to changes in ionic environment. These actions did not depend on the presence of norepinephrine in the veins.

Fractionation of sodium effux in frog sartorius muscles by strophanthidin and removal of external sodium

The influence of strophanthidin, ouabain, and the removal of external sodium on the sodium efflux from frog sartorius muscle was measured. In freshly dissected muscles strophanthidin and ouabain in maximally effective concentrations reduced the efflux of sodium by about 50%. Of the sodium efflux which is strophanthidin-insensitive about 75% is inhibited after complete replacement of external sodium by lithium. In the absence of strophanthidin replacement of external sodium by lithium, calcium, or magnesium produces an initial rise in the sodium efflux, followed by a fall in the efflux as the exposure of the muscles to sodium-free media is continued. When the muscles are exposed for prolonged periods in sodium-free media, the fraction of internal sodium lost per minute is higher when returned to normal Ringer fluid than it was initially. The activation of sodium efflux by external sodium after long periods in sodium-free solutions is partly strophanthidin-sensitive and partly strophanthidin-insensitive. The internal sodium concentration is an important factor in these effects. The effects of temperature on the sodium efflux were also measured. Above 7 degrees C the Q(10) of both the strophanthidin-sensitive and strophanthidin-insensitive sodium efflux is about 2.0. Below 7 degrees C the strophanthidin-insensitive sodium efflux has a Q(10) of about 7.4.

Active length-tension relations compared in isometric, afterloaded and isotonic contractions of cat papillary muscle. Their dependence on inotropic state

Isometric, afterloaded and isotonic contractions of cat right ventricular papillary muscle were studied in vitro at 30°C or 37°C. The length to which muscle contracted under a given total load varied directly with initial muscle length. The discrepancies between contracted muscle lengths depended upon load and the inotropic state of the muscle. In control contractions at 12/min, the maximum discrepancy occurred at a load of 0.5 g or 1 g and averaged 0.47 ± 0.04 ( SD ) mm while after acetylstrophanthidin (0.75 µg/ml) the maximum discrepancy occurred between 3- and 5-g load and averaged 0.62 ± 0.07 mm ( P < 0.001). At low loads acetylstrophanthidin decreased the discrepancy at 0.5-g load from 0.41 ± 0.08 mm to 0.22 ± 0.05 mm ( P < 0.001). Acetylstrophanthidin increased developed tension at optimum muscle length by 71 ± 23% ( P < 0.001). Increasing frequency of contraction from 12/min to 60/min induced greater decrease in time to peak tension than occurred with acetylstrophanthidin (30% vs. 15%) but produced little change in either isotonic or isometric contracted length-tension relations. Since the differences between contracted length-tension relations were not influenced by abbreviation of active state induced by frequency changes, it is unlikely that the lesser abbreviation with acetylstrophanthidin was responsible for the changes it produced. Rather, they may be related to the intensity and duration of active state reflecting increased energy release in the contractile process.

Reflex cardiovascular effects of epicardial stimulation by acetylstrophanthidin in dogs

Application of 25 to 100 µg of acetylstrophanthidin to the epicardium of the left ventricle of anesthetized and unanesthetized dogs caused hypotension and bradycardia without signs of discomfort. The response developed after an average latency of 8 seconds and lasted up to 12 minutes. Cooling the cervical vagi to 8 to 10°C or prior application of 0.1% procaine hydrochloride to the epicardium of the heart blocked the response. The response was, therefore, a reflex; the sensory receptors were located in the surface layers of the left ventricle. Electrophysiological recordings from single and multifiber preparations of the right recurrent cardiac nerve showed that the receptors for this reflex were mechanoreceptors whose fibers belonged to the C group. The sinus bradycardia caused by digitalis glycosides may result partly from excitation of these receptors.

Ventricular arrhythmias related to antibiotic usage in dogs

In dogs, pretreatment with the macrolide antibiotic tylosin (5 milligrams per day per kilogram of body weight) increased the incidence of ventricular tachycardia and fibrillation during acute myocardial ischemia. Another group received a dose of acetyl strophanthidin which was nontoxic in controls, but which resulted in a ventricular arrhythmia in six of seven animals on antibiotic treatment. Enhancement of loss of potassium ion from the myocardium by the antibiotic was presumed to be related to the altered cardiac rhythm.

The nature of Wenckebach cycles

Various patterns of unstable second-degree AV block were observed in the dog following selective injection of acetylstrophanthidin into the AV node artery. Analysis of the changes in RR and PR intervals during slow transitions between 1:1 rhythm and 2:1 block showed that well-organized sequences of periodic arrhythmias occurred during these transitions, thus verifying a previous hypothesis made on the basis of model experiments. Wenckebach cycles of variable duration are seen near 1:1 rhythm, whereas cyclic patterns of partial 2:1 block (also called "reverse Wenckebach" cycles) appear near 2:1 block; these two modes are separated by 3:2 block characterized by alternation between long and short ventricular intervals. The close correspondence between experimental observations and the results of a simulation study with a relaxation-oscillator model of the AV junction raises the possibility that these periodic arrhythmias may be explained solely in terms of the properties of threshold and refractoriness of the AV junction.

After-effects of nerve impulses on signalling in the central nervous system of the leech

A long-lasting hyperpolarization has been observed in sensory neurones of the C.N.S. of the leech following natural stimulation applied to their receptive fields. The underlying mechanism of this hyperpolarization and its effects on signalling were investigated by intracellular recording.1. The amplitude and duration of the hyperpolarization depended on the number of action potentials. Whereas a brief burst of impulses had little effect, a prolonged sensory discharge was followed by a hyperpolarization of up to 30 mV that gradually declined over a period of several minutes.2. The hyperpolarization was abolished by ouabain, reversibly inhibited by strophanthidin or cooling, unaffected by Mg and accompanied by an increase in membrane resistance. These observations suggest that the hyperpolarization is the result of membrane current generated by an electrogenic pump.3. A hyperpolarization similar to that recorded in the cell body also occurred in the neuronal processes within the neuropile, where synaptic contacts are made. This led to significant changes in integrative activity, such as an increase in the amplitude of excitatory synaptic potentials, a reversal of inhibitory synaptic potentials or a conduction block in parts of the neurones. All of these effects could be mimicked by injecting hyperpolarizing currents into resting cells, or abolished by injecting a depolarizing current into a cell hyperpolarized by previous impulses.4. During neuronal hyperpolarization the sensitivity of the membrane potential to small increments of external K was enhanced by a factor of about three. This effect varied with the magnitude of the hyperpolarization produced by preceding impulses and could not be mimicked by applying K to a neurone hyperpolarized by the injection of current into its cell body.5. These findings are discussed in relation to possible effects that changes in the intracellular and extracellular concentrations of ions produced by activity might have on integration in the C.N.S.