Hormonal stimulation of arachidonate release from isolated perfused organs. Relationship to prostaglandin biosynthesis.

The lipids of isolated Krebs perfused rabbit kidneys and hearts were labelled with [14C]arachidonic acid. Subsequent hormonal stimulation (e.g. bradykinin, ATP) of the pre-labelled tissue resulted in dose-dependent release of [14C]prostaglandins; little or no release of the precursor [14C]arachidonic acid was observed. When fatty acid-free bovine serum albumin was added to the perfusion medium as a trap for fatty acids substantial release of [14C]arachidonic acid was detected following hormonal stimulation. The release of [14C]arachidonic acid was dose-dependent and greater than 3 fold that of [14C]prostaglandin release. Indomethacin by inhibiting the cyclo-oxygenase, completely inhibited release of [14C]prostaglandins and only slightly inhibited release of [14C]arachidonic acid. These results demonstrate that in both rabbit kidney and heart much more substrate is released by hormonal stimulation than is converted to prostaglandins. This suggests that either the deacylation reaction is not tightly coupled to the prostaglandin synthetase system or that there are two deacylation mechanisms, one which is coupled to prostaglandin synthesis while the other is non-specific. It has previously been shown that prostaglandin release due to hormones such as bradykinin is transient despite continued presence of the hormone (tachyphylaxis). By utilizing albumin to trap released fatty acid, it was found that hormone-stimulated release of arachidonic acid is also transient. This directly demonstrates that tachyphylaxis occurs at a step prior to the cyclo-oxygenase.