Calcium ethylenediaminetetraacetate (CaEDTA) toxicity: Time- and dose-response studies on intestinal DNA synthesis in the rat

Abstract

The effect of CaEDTA on intestinal DNA synthesis was studied by measuring the incorporation of [ methyl- 3H ]thymidine. An ip injection of thymidine (0.25 μCi/g body weight) was given 1 hr before sacrifice. DNA was extracted from a 10% (w/v) homogenate of mucosal scrapings by a modification of the Schmidt-Thannhauser procedure. Radioactivity in the DNA fraction was assayed by liquid scintillation counting. Dose-response studies showed that a constant iv infusion of 3.0 mmoles/kg/24 hr for 24 hr was required to produce a significant inhibition of DNA synthesis. Time-response studies revealed that 18 hr of CaEDTA infusion at a dosage of 6 mmoles/kg/24 hr were required for significant inhibition of DNA synthesis; the degree of inhibition occurring arabinoside (250 mg/kg) injection. Although folic acid (250 mg/kg) did not protect, the infusion of ZnEDTA, CoEDTA, or ZnCaEDTA afforded complete protection against inhibition of DNA synthesis. Even when the rats were moribund following CaEDTA administration, the intestine was capable of recovering its ability to synthesize DNA. This recovery was significantly enhanced by supplementation with zinc, but not by cobalt or manganese. When parenteral alimentation was employed, the survival of the rats was markedly improved. It is proposed that zinc serves as a low-molecular weight nutrient essential to initiation of DNA synthesis and that the mechanism of inhibition of DNA synthesis during the administration of CaEDTA may be the blocking of normal zinc uptake into the cell. Since the intestine is rapidly regenerating at a time when the rat is moribund it is suggested that inhibition of DNA synthesis in the intestine is not the direct cause of death.