Effects of prostaglandin E1 and isoproterenol on cyclic AMP content of human fibroblasts modified by time and cell density in subculture

In confluent cultures of “young” (< 30 generations) human fibroblasts, maximally effective concentrations of prostaglandin E₁ (5.6 μM) and isoproterenol (2 μM) increased cyclic AMP content several hundred-fold and approximately 30-fold, respectively. On the first day after initiation of cultures at either low (approx. 3 · 10⁵ cells) or high (approx. s · 10⁶ cells) cell density the magnitude of the isoproterenol effect was similar to that in confluent cultures. It increased during the next few days, reaching a maximum around day 2–3, and then declined. On any day during the period of subculture, the magnitude of the isoproterenol effect was inversely related to cell density. Alterations in response to prostaglandin E₁ as a function of time in subculture or cell density were less dramatic. The effects of prostaglandin E₁ were, however, smaller at some point during the first few days of subculture than after day 7, and when effects of prostaglandin E₁ were minimal, those of isoproterenol were maximal and approached those of prostaglandin E₁. On any day of subculture, cells in cultures of higher density tended to accumulate more cyclic AMP in response to prostaglandin E₁ than did those in low density cultures. The effects of prostaglandin E₁ and isoproterenol on cyclic AMP content were qualitatively similar in “young” and in “old” (> 60 generations in culture) human fibroblasts although the changes associated with duration of subculture and cell density tended to be less marked with “old” cells. In the “young” fibroblasts responsiveness to isoproterenol and prostaglandin E₁ appears to correlate with cell morphology and with the fractional rate of growth in subcultures. It is suggested that the capacities of the fibroblasts to respond to these two agents may be altered independently during growth of human fibroblasts.