This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
The action of digitalis on the frog's heart is very peculiar. At first it causes the pulsations to become slower and more powerful, then the contraction during systole becomes peristaltic, and the dilatation during diastole less and less complete, until finally the ventricle stands quite still, in such complete systolic contraction that its cavity is entirely obliterated. The auricles are sometimes distended with blood, sometimes only moderately dilated. According to Schmiedeberg this contraction is not tetanic, but is rather due to increased elasticity of the cardiac muscle which prevents its normal relaxation during diastole. When it is overcome by driving a nutrient fluid into the ventricle under pressure, or by partially paralysing the cardiac muscle by saponin, apomorphine, or hydrocyanic acid, the systolic stillstand is removed, and pulsation again commences.
1 Journal of Physiology, vol. iv. p. 165.
Fig. 213. - Pulse-wave, b before and a after injection of digitalis in a dog.
Digestive Organs. - Small doses of digitalin have a pleasant bitter taste but exercise no marked effect upon the digestive organs. Larger doses produce loss of appetite, nausea, and vomiting, with rumbling and pain in the abdomen, and sometimes diarrhoea. This occurs even when the drug is injected subcutaneously.
Urine. - All observers are agreed regarding the diuretic power of digitalis in cardiac disease, but most of them state that it has no such power in health. In my own experiments, however, in which I took the same quantity of food by weight and of fluid by measure during more than a hundred days, I found that, while small doses had little or no action, marked diuresis occurred when the drug was pushed so as to produce symptoms of poisoning. In these experiments also I found that while the diuresis continued the absolute quantity of solids excreted daily in the urine was increased, although their proportion to the urinary water was diminished. In cases of poisoning by digitalis, a marked diminution in the flow of urine frequently precedes a fatal issue; and on injecting digitalis into the veins of a dog, Mr. Power and I found that the secretion of urine became entirely arrested when the blood-pressure reached its maximum, and again commenced when the blood-pressure began to fall (p. 430). It is probably to the power of digitalis to arrest the action of the kidneys and thus stop its own excretion that its cumulative action is due (p. 42).
Effect of Temperature on the Action of Digitalis. - It has already been mentioned (p. 47) that digitalis has sometimes no action on the pulse in pneumonia. The inhibitory action of in the axilla in all the figures.
The unbroken line shows the pulse-rate, the dotted line shows the temperature
Fig. 214. - Shows the effect of rise of temperature alone. At the 195th minute both vagi were cut; the section was not followed immediately by any apparent effect. After eight minutes more, the pulse-rate rose slightly and then fell.
the vagi on the heart is lessened by heat, but their peripheral terminations, although weakened, are not completely paralysed. Dr. Cash and I have made some experiments which appear to show that a very high temperature has an action on the vagus centre in the medulla similar to its action on the ends of the nerve in the heart. It does not completely paralyse either the centre or the peripheral ends of the nerve, but it greatly weakens them. This weakening action is so great that it practically amounts to paralysis, for when the temperature rises above a certain point the pulse-rate suddenly rises just as it would do if both vagi were cut. This is shown in Fig. 214. When the pulse-rate has been thus quickened by heat, section of the vagi does not render it any quicker (Figs. 215 and 216).
Fig. 215. - Shows the effect of rise of temperature after injection of digitalis. At the 45th minute .75 c.c. (12 minims) tincture of digitalis were injected, and another similar injection was made at the 55th minute. At the 65th minute the heating was begun. After section of the vagi the pulse continued to rise, but not more rapidly than before.
Although the vagus centre is so much weakened by the action of the heat that it ceases to exercise any inhibitory action upon the heart, yet its functional activity is not completely destroyed even by very high temperatures, and irritation of an afferent nerve will still cause reflex slowing (p. 290) of the pulse, until immediately before the death of the animal from hyperpyrexia.
Fig. 216. - Shows the action of digitalis when given after the temperature has already risen. At the 30th minute the warming was begun; at the 100th minute .75 c.c. of tincture of digitalis was injected.
These experiments render it probable that the rapid rise in the pulse-rate, which a high temperature occasions, is chiefly of central origin, and is due to partial paralysis of the vagus centre, although diminished action of the peripheral ends of the vagus and increased action of the cardiac ganglia also aid in quickening the pulse.