The rat visceral yolk sac cultured in vitro was used as model system to study the mechanism of pinocytosis. Fluid-phase pinocytosis was completely inhibited at low temperature and on addition of metabolic inhibitors to the culture medium, was partially inhibited by cytochalasin B and colchicine and was also reduced to various extents by other factors such as EGTA and theophylline (Chapter 4). Polycations have been reported in the literature as stimulators of endocytosis. Here it was found that poly-L- ornithine, poly-L-lysine and DEAE-dextran stimulated tissue-association of
colloidal "^Au (Chapter 5) "by a mechanism which involved the formation of
a polycation-colloidal Au complex that bound to the outside of the visceral yolk sac (Chapter 6). Electron microscopical visualization of tissue which had been exposed to polycation-colloidal Au complexes did not reveal colloidal Au in association with the plasma membrane and potential explanations of this observation are discussed in (Chapter 7). DIVEMA is a synthetic polymer which has a wide range of biological activities although its mechanism of action is still obscure. Three different molecular weight distributions of DIVEMA and three DIVEMA derivatives had no stimulatory effect on pinocytosis in the rat visceral yolk sac; in some instances they were inhibitory (Chapter 8). None of the above substances were found to stimulate the rate of pinosome formation in the visceral yolk sac.
A method for radiolabelling PVP was evaluated (Chapter 9) and it was found possible to iodinate PVP with acceptable efficiency.
Excess radioiodide was removed by dialysis, but the resultant preparations of ^2^I-PVP showed spontaneous deiodination when stored at 4°C. The increase in radioiodide in preparations during a 6.5h culture period at 37°G was fairly small, so a technique was developed for quantitation of pinocytosis of these 12-^I-PVP preparations. Four samples of PVP with different molecular weight distributions were iodinated and preliminary experiments were carried out to investigate their rates of pinocytosis (Chapter 10). A correlation was observed between increasing molecular weight and a decreased rate of pinocytic uptake of PVP.
The study of the effects of polyamino acids, dextran derivatives and DIVEMAs on pinocytosis in the rat visceral yolk sac was paralleled by a study of the effect of these compounds in pinocytosis in the rat peritoneal macrophage (cultured in vitro) which was carried out by Dr. M.K. Pratten in this laboratory. The work of Dr. Pratten is referred to extensively in this thesis as the results described for the visceral yolk sac system are of wider interest when viewed comparatively.