Effect of vitamin A acid on cyclic nucleotides of cultured keratinocytes

Primary cultures of guinea pig ear keratinocytes were treated with vitamin A acid at plating, or at 14 h or 14 days after plating. The intracellular content of the cyclic nucleotides cAMP and cGMP was determined by radioimmunoassay at intervals during a period of 50 h after treatment. When added at plating, vitamin A acid produced a wave of DNA synthesis and increase in DNA which was at maximum between 30 and 40 h after plating, and coincided with decreased cAMP levels. This may represent a subpopulation of keratinocytes in S phase. Treatment with vitamin A acid at 14 h or 14 days after plating resulted in an immediate but temporary fall in cAMP and cGMP, and a wave of thymidine uptake but no increase in DNA per dish. Thus, a single treatment with vitamin A acid is mitogenic only when applied at plating. At other times, it can cause changes in cyclic nucleotide content without any observable cell proliferation.