Since the experiments of Series A, which were intended as preliminary tests, have shown that 0.3 gram of caffein per kilo when introduced into the peritoneal cavity is rapidly absorbed and is fatal, much smaller doses were employed in subsequent trials with the drug. This is shown in series B, which may be divided into two groups. Group I, consisting of rabbits 69, 70, 92, 93, and 309, which received 0.2 gram of caffein per kilo, and Group II, Nos. 307 and 308, into which 0.15 gram of caffein per kilo was injected. Three rabbits of Group I (Nos. 92, 93, 309) died from the effects of caffein; rabbit 309 twenty minutes after injection, and rabbits Nos. 92 and 93, twenty hours and two and one-half hours, respectively, after the administration of caffein. In both of these rabbits symptoms appeared within five minutes after the injections were made. Rabbits 69 and 70, it will be noticed, survived the same amount of caffein in proportion to body weight as was given to the other members of this group. Increased peristalsis and the distribution of the dose may account for the greater resistance of rabbit No. 69. The case of rabbit No. 70 is evidently one of exceptional resistance to caffein, since both the rabbits of Group II died from the effects of a much smaller dose, namely, 0.15 gram of caffein per kilo. Moreover, macroscopical examination at the autopsy of Nos. 307 and 308 failed to show any lesions which might tend to lessen the resistance to caffein.
That a dose of 0.15 gram per kilo is therefore in all probability the minimum fatal dose for the rabbit when injected into the peritoneal cavity appears from the results of the experiments in series C, in which smaller doses, 0.125 gram of caffein per kilo caused mild symptoms only, while 0.1 gram per kilo rarely induced any symptoms. It may be remarked that the rabbits of series C were fed carrots while rabbits Nos. 307 and 308 received oats. Their resistance to caffein may be different, but, as was pointed out in the earlier part of this investigation, diet does not seem to influence the toxicity of the single dose of caffein. Doses of 150 and of 100 to 125 mg per kilo, when injected into the peritoneal cavity, may be considered, respectively, as the minimum fatal and minimum toxic doses for the gray rabbit. Analysis of the experiments in series D shows much greater resistance to caffein than in the other rabbits which received it intraperitoneally. Thus, after the administration of 0.2 gram per kilo to each of five rabbits, no effect was observed in two cases (Nos. 310, 311), while in two others (Nos. 74, 75) symptoms developed, but they survived. Only one rabbit, No. 312, died from the effects of this dose; the autopsy showed the presence of degeneration of the liver and petechial hemorrhages on the cortex of the kidneys in the case of No. 310, which was probably the cause of death rather than the caffein.
Two decigrams of caffein can not be considered, therefore, the fatal dose for rabbits. This is further corroborated by the results obtained in experiments with larger doses. Rabbit 78, which received 257 mg per kilo, failed to show any symptoms. The same amount in proportion to body weight in No. 323 caused mild symptoms only, while the rapid death of rabbit No. 317 after the same dose of caffein may be explained by the lesion found at autopsy, thus affording additional evidence that disease may decrease the resistance to caffein. It will be observed that all the members of this series were young rabbits and, as will be shown later, young animals of other species are likewise more resistant to caffein than adult animals. Similar results were obtained by von Anrep, who observed that atropin is less toxic in young than in full-grown animals.
Observations were also made on the diuretic effect of caffein when injected into the peritoneal cavity. The results shown in the following table indicates the stimulating effect on renal secretion whether the diet consisted of oats or of carrots. The urine of some rabbits contained moderate amounts of sugar after from 0.2 to 0.15 gram of caffein per kilo was given; albumen was observed in one case, but in none of the others. In rabbit No. 311 albumin was found before the injection of caffein, but none in the urine which was collected three hours after caffein was injected.
Effect of caffein on renal secretion.
| No. | Weight. | Caffein per kilo. | Urine. | Time. | Diet. |
|---|---|---|---|---|---|
| Grams. | Gram. | cc. | |||
| 307 | 1,320 | 0.150 | 100 | 4.5 hours | Oats. |
| 308 | 1,305 | .150 | 180 | do. | Do. |
| 295 | 1,205 | .100 | 100 | 2 hours 20 minutes | Carrots. |
| 293 | 1,605 | .100 | 90 | 3 hours | Do. |
| 292 | 1,595 | .100 | 105 | 4 hours | Do. |
Note.—The amount of urine secreted in three hours by control rabbits, on a carrot diet, varied between 35 and 50 cc, the average weight of the animals being a little above 1,600 grams. The secretion of urine on an oat diet was much less for an equal period of time.
Table 3.—Intraperitoneal injections.
| SERIES A. | |||||
|---|---|---|---|---|---|
| No. | Weight. | Caffein per kilo | Time of appearance of symptoms in | Duration of life. | Remarks. |
| Grams. | Gram. | ||||
| 71 | 1,659 | 0.3 | 85 minutes | 24 hours | Gray. |
| 61 | 2,143 | .3 | At the end of injection | Black. | |
| 72 | 1,402 | .3 | 8 minutes | 1.25 hour | Gray and white. |
| SERIES B, GROUP I. | |||||
| 70 | 1,487 | 0.2 | 10 days | Gray and white. | |
| 93 | 1,492 | .2 | 5 minutes | 2.5 hours | Maltese; given second dose after 3 days, died 2.5 hours later. |
| 69 | 1,492 | .2 | About 5 minutes | Survived | White. |
| 92 | 1,388 | .2 | 5 minutes | 24 hours | Yellow. |
| 309 | 1,500 | .2 | 20 minutes | Belgian; oats. | |
| SERIES B, GROUP II. | |||||
| 308 | 1,350 | 0.15 | 3 hours and 40 minutes | About 2.5 days | Belgian; oats. |
| 307 | 1,320 | .15 | 1 hour 24 minutes | 24 hours | Do. |
| SERIES C. | |||||
| 223 | 1,165 | 0.125 | 2 hours | Survived | Belgian; carrots. |
| 293 | 1,605 | .1 | do. | Do. | |
| 295 | 1,205 | .1 | do. | Do. | |
| 292 | 1,595 | .1 | do. | Do. | |
| 298 | 1,205 | .125 | 1.5 hours | do. | Do. |
| SERIES D. | |||||
| 310 | 880 | 0.2 | 2 days1 | About 2.5 days1 | Belgian; oats. |
| 311 | 650 | .2 | Survived | Do. | |
| 312 | 740 | .2 | 40 minutes | 4.5 hours | Maltese; oats. |
| 78 | 659 | .257 | 15 minutes | Survived | Yellow and white; oats. |
| 75 | 842 | .2 | do. | do. | Gray and white. |
| 74 | 692 | .2 | do. | do. | |
| 317 | 635 | .252 | 1 hour 25 minutes | About 2 hours | Belgian; oats. |
| 323 | 820 | .25 | 1 hour 15 minutes | Survived | White; oats. |