Supplemental dietary calcium fails to alter the acute effects of 1,2-dimethylhydrazine on O⁶-methylguanine, O⁶-alkylguanine-DNA alkyltransferase and cellular proliferation in the rat colon

Prior studies from our laboratory have demonstrated that K-ras G to A mutations were detectable in a high percentage of carcinomas which developed in the colons of animals treated with the known colonic procarcinogen, 1,2-dimethyl-hydrazine (DMH). Moreover, in this model, the incidence of these mutations was decreased by a supplemental dietary calcium regimen which concomitantly decreased the frequency of rats with multiple tumors as well as tumor size. In an attempt to clarify the possible mechanism(s) involved in this antimutagenic effect of supplemental calcium, two groups of Sprague-Dawley rats were fed semisynthetic diets containing either 0.87 or 1.80% cakium by weight for 3 weeks, s.c. injected with 100 mg/kg of DMH and killed prior to and at various time periods (16-144 h) after injection. The colons of animals were analyzed and compared with respect to O⁶-methylguanine content in DNA, O⁶-alkylguanine-DNA alkyltransferase levels as well as cellular proliferation, as assessed by immunohistochemical staining of colonic crypts by bromodeoxyundine. In certain experiments, these parameters were also analyzed in the proximal and distal colon before and at various times after administration of DMH. The results of these experiments demonstrated that supplemental dietary calcium was not found to influence significantly O⁶-methylguanine levels, alkyltransferase levels or cellular proliferation in the entire colon or in either colonic segment before or after the acute administration of DMH. DMH did, however, differentially alter all three of these biochemical parameters in the colonic segments (distal > proximal), possibly due to a greater degree of metabolic activation in the distal colon.