Endogenous Activation of Prostaglandin Synthesis in Rabbit Colonic Explants Cultured in vitro

Abstract

Rabbit colonic biopsy specimens cultured in vitro showed an optimal release of prostaglandin E₂ (PGE₂) after 6–7 h of incubation. The amplitude but not the PGE₂ production profile was dependent on stimuli applied—that is, A23187 > phorbol 12-myristate 13-acetate > control. This study was undertaken to determine the nature of this time-dependent optimum. The following hypotheses were tested: increased substrate availability, de novo synthesis of prostaglandin synthetase (PS), and translocation of PS or increased activity of PS. Arachidonic acid, either continuously present or given as a pulse, increased the overall amplitude equally but did not change the production profile. Cycloheximide, 0.2 μM, which inhibited 50% of the protein synthesis, increased the PGE₂ production by 80% at 7 h. The cytoskeletal disrupting agent cytochalasin B had no effect. Homogenates of specimens cultured 6 h showed an increased PGE₂ production. This was partly explained by a large increase in the PGE₂ production capacity of the particulate matter obtained after 45 min of centrifugation at 100,000 g. It is concluded that the PGE₂ production peak is due to an activation of preformed PS and that the PS activity is under the control of a peptide inhibitor, dependent on protein synthesis.

Key Words:

• Arachidonic acid
• A23187
• cycloheximide
• cyclooxygenase
• cytochalasin B
• organ culture
• phorbol 12-myristate 13-acetate
• prostaglandin E₂