A study of some metabolic requirements for repair of sublethal irradiation damage.

Abstract

The effects of x radiations on survival of biological systems have been studied with the technic of single-cell tissue culture as described by Puck (1, 2). The survival curves obtained are usually sigmoid in shape. When survival fractions are plotted on a logarithmic scale against the graded doses of x radiations on a linear scale, the curve begins with a small but finite and negative slope, followed by a region of exponential response (Fig. 1). Such a response means that a given increment of dose is more effective in the high-dose region (exponential response) than in the low-dose or threshold region. This in turn means that damage must be accumulated before a cell can be lethally affected and, therefore, that cells surviving exponentially have experienced saturation of sublethal damage. This concept led to the first study and demonstration of recovery of mammalian cells from sublethal irradiation damage by Elkind and Sutton (3, 4), who used Chinese hamster cells. They found that when a given dose of x radiations was delivered in two fractions and the time between fractions varied, survival after the second dose followed an oscillating pattern characterized by an initial survival increase of variable magnitude and a subsequent decrease, followed by a second rise. A plot of the two-dose response on semilogarithmic coordinates of survival against time between two doses showed the following temporal relationship. At a period of one to three hours there was an increase in survival followed by a minimum at four to six hours, which in turn was followed by a further increase between seven and ten hours. These maxima and minima have been shown in work with other mammalian systems in vitro (5, 6) and in vivo (7, 8). The rise in survival to the first maximum was interpreted as repair of sublethal damage. A typical recovery curve from this laboratory is shown in Figure 2. Since the shapes of the recovery curves under discussion were the result of incubation between two doses in complete medium or in the animal, it seemed desirable to obtain information regarding the nutritional requirements for repair of irradiation damage. In a study of possible inhibitors of repair by HeLa cells, Berry (9) used 2,4-dinitrophenol, which is known to uncouple oxidative phosphorylation. He found no inhibition of repair when this energy-producing pathway was inhibited. Since little other information could be elicited from the literature, the following study was undertaken. Material and Methods Tissue Culture: Log phase asynchronous Chinese hamster cells (V79), grown on 9 cm Petri dishes for twenty hours, were exposed uncovered to two doses of x radiations (50 kVp, 20 mA, h.v.1. 0.1 mm A1, dose rate 2,169 R/min.). Before exposure to the first dose of 500 R, the glass-attached cells were rinsed with balanced salt and glucose solution and the solution removed.