An Analysis of Dna Loss and Synthesis in the Rat Adrenal Medulla Nuclei Upon Cold Stimulation

Abstract

The peculiar changes previously observed in DNA content of rat adrenal medulla cell nuclei upon intermittent cold exposure (15 hr at -4-4°C followed by 9 hr at room temperature) have been further studied with the aid of Feulgen histophotometry and H3-thymidine radioautography. The amount of DNA decreases progressively with increasing length of cold exposure until 300 hr ( -32 %). Later a rapid change takes place, whereby DNA content per nucleus returns to values which are slightly, but consistently lower than normal. At termination of a period of cumulative exposure to cold, an analysis of a whole-day experimental cycle shows that the DNA decrease is due to loss of DNA during cold exposure and that DNA synthesis occurs upon return to room temperature. The balance between these two processes can be divided into three stages: (a) loss of DNA up to 300 hr of cumulative cold exposure; (b) marked increase in DNA by 350 hr; (c) oscillation around zero or slightly negative at 400 hr and beyond. These variations are due to: (1) the extension of DNA synthesis into the period of cold exposure as clearly demonstrated by radioautography (stage b), and (2) a later still greater DNA loss (stage c) which partly offsets the increased synthesis. A complex pattern of adaptation of the adrenal medulla cells, as regards DNA content, to the repetitive cold stimulus is thus demonstrated.

Iii. Decrease of Labeled Dna in Cells of the Adrenal Medulla After Intermittent Exposure to Cold

“…Our present observations and those previously reported (12,13,17) have shown that DNA can be lost and resynthesized ; unless some of the basic concepts of molecular genetics are entirely wrong, loss of genetic DNA should lead to loss of function and to an inability of the cells to replace DNA since the templates would have been lost . Our findings can be adequately explained on the assumption that metabolic DNA is involved and that it functions as previously suggested (22,23) .…”

“…Using Feulgen photometry, biochemical techniques, and interference-microscopy, Viola-Magni (12) found a loss of 40% to 45% after 20 days of intermittent cold-treatment, about 2 % per day. The observations by radioautography show that synthesis of DNA starts almost immediately after transfer of a rat from a temperature of 4°C to one of 22°C (17), and the net daily loss of 2 % during intermittent cold treatment can be regarded as the result of a loss of 6 .9% during the 15 hr of cold-treatment followed by a resynthesis of 4 .9% during 9 hr at room temperature . The daily loss of 2 % of the DNA during intermittent cold-treatment can thus be regarded as due to a negative balance of losses and gains (17) .…”

“…The decrease takes place during the 15 hr of exposure to +4°C and is partially compensated for by synthesis that takes place during the 9 hr at room temperature (17) . The marked loss of label (80%) observed in the animals which were exposed again for 300 hr to low temperature after the injection of the precursor, as compared with the less marked loss of label (30%) observed in rats kept at room temperature after injection, can be attributed to the influence of the ambient temperature on the turnover of DNA .…”

“…This difference can be accounted for by the fact that synthesis of DNA (as shown by thymidine 'H incorporation) takes place during the cold-exposure periods in Wistar rats, whereas it is confined only to the periods at room temperature in Italico rats .' If the intermittent cold treatment is prolonged for more than 300 hr, synthesis of DNA takes place during the cold also in Italico rats, thus leading to an almost null balance, over a period of a few days (17) .…”

Ii. Differences in Adrenal Medulla Nuclear Dna Content Among Rats of Different Strains Following Intermittent Exposure to Cold

“…It has also been shown (3) that the loss of DNA is followed by an incorporation of thymidine 3H in the nuclei, indicating 4 52 synthesis of DNA, when the animals are transferred to room temperature . These two facts strongly suggest that at the end of a given period of cumulative exposure to cold the decrease of the DNA content per nucleus is due to the balance between the loss of DNA occurring during cold exposure and the DNA synthesis which takes place upon return to room temperature (4) .…”