Total LD and isozyme variations during in vitro fibroblast aging with a discussion of its theoretical implication

Dermal fibroblast cultures were obtained from 18 patients. Total lactic dehydrogenase (LD) and LD isozymes were determined after each population doubling (PD). The lactic dehydrogenase was assayed by the method of Wacker, Ulmer and Vallee. The isozymes were analyzed by gel electrophoresis. Some investigators have reported conflicting results as to the quantity of LD produced during in vitro aging. These researchers sampled a small number of PDs. We found that the total LD varied with each PD. A distinct isozyme pattern has been described for adult, fetal and neoplastic fibroblasts. We have found all three patterns occurring during the in vitro aging of our normal diploid fibroblasts. In spite of our asynchronous population of cells the LD and its isozymes continually fluctuate. An asynchronous population of cells would be expected to produce a dampening of the total LD. In order to explain this finding we speculate a synchrony is established in a "gated" entrance from subcycle Gq into the S stage or from G1 and G0 into the S stage. In order to evaluate, accurately, changes occurring during in vitro aging it is imperative to assay each PD.