Actinomycin D effect on lag in vitamin D-mediated calcium absorption in the chick

Abstract

The time lag has been determined between the administration of vitamin D3 and the enhancement of Ca absorption across the intestinal mucosa of the intact rachitic chick. With doses of 20,000, 100, or 10 international units (IU) the lag in response was 5-8, 10-15, or 15-25 hr. respectively, with both oral and intraperitoneal doses. When 500 IU of H3-vitamin D3 was injected intraperitoneally, a high level of radioactivity was found within 1 hr. and maintained for 24 hr. in all tissues examined except the adrenals. It was proposed that a hypothesis for the delay in biological response to vitamin D was due to the necessity for vitamin D to induce the synthesis of the appropriate enzyme systems or the alteration of membrane structure necessary for Ca absorption. This was tested with actinomycin D. If actinomycin D was given at any time up to 5 hr. after 100 IU of vitamin D treatment, no stimulation of Ca absorption occurred. Also it was found that actinomycin D treatment for 36-48 hr. could effectively convert a vitamin D-treated chick to a vitamin D-deficient chick. This suggests a turnover time of 24-36 hr. for the biochemical machinery associated with vitamin D-mediated Ca absorption.