Dose-response with whole animals

The more difficult problem with whole animals concerns events which occur over long periods of time. The LD50 value must always be accompanied with an indication of the time over which the animals were observed before the experiment is terminated. If not, every treatment would be considered lethal since every animal dies eventually, or no chemical would be considered lethal since both control and treated animals would die eventually. Thus, observation periods of 24 hours or two weeks are often chosen as end points. When dealing with carcinogenesis, however, the time of the study is considered the life time of the animal which in the case of mice or rats may extend to two years or more. Furthermore, since control animals may display spontaneous tumors and the tumor incidence in both treated and control animals may be small, the total nuaiber of animals in the experiment often plays a key role in determining the accuracy of the results. The responses discussed here are classified as quantal since each animal provides only one piece of data. The animal either dies or it does not it develops tumors or it does not. The same observation cannot be repeated in the same animal and the effect of a higher dose in that animal cannot be investigated. 

The bone becomes depleted of calcium salts when the urine is acidic over a relatively long period. This was shown by Goto (17) who fed rabbits large doses of hydrochloric acid. He then showed that urinary calcium loss occurred in concert with a marked reduction in mass of the skeletal system, and also that the total non-fat dry weight of bone decreased,implying a loss of bone matrix. A dose-dependent, dietary acid induced loss of labelled calcium from rat bone has been reported by Thorn and his coworkers (18). They demonstrated that in response to graded doses of ascorbic acid, cells in tissue culture, and bones in whole animals fed such doses were depleted of the labelled calcium. 

A wide diversity of dose-response (incidence) relationships has been observed among the neoplasms induced experimentally by chemicals (Zeise et al., 1987), radiation (UNSCEAR, 1977) or both. Although neoplasms of virtually every type have been induced in one experiment ra- another, not all types of neoplasms are observed in animals of any one species or strain. Under some conditions, moreover, the incidence of certain neoplasms has actually been observed to decrease with increasing dose of whole-body irradiation (see Figure 3.1). 

Because in vitro studies are generally less complex than whole-animal studies, elucidation of an ingredient s metabolic pathway and toxicity characteristics may be facilitated. Different doses of the solubilized ingredient can be incubated with either confluent or preconfluent cells cultured in 10-cm dishes or 96-wells plates to establish uptake and toxicity levels of both the ingredient and its metabolic byproducts. Time-course and dose-response experiments should be conducted to check the growth characteristics of the cells and the toxicity of the ingredient. The production of metabolites can also be followed upon adding... 

---