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Microsomal preparations of normal bovine iris-ciliary body generate prostacyclin-like but not thromboxane-A2-activity
Abstract:
Indomethacin-treated bovine iris-ciliary body microsomes (IBIM) have been studied for their ability to convert PG endoperoxides into either thromboxane-A2 (TxA2)-like or prostacyclin (PGI2)-like activity. The biological activity of the ocular tissue microsomes were compared with either indomethacin-treated human platelet microsomes (for TxA2-like activity) or rabbit aorta microsomes (for PGI2-like activity) under appropriate incubation conditions. No evidence could be found for the formation of TxA2-like activity from PG endoperoxides by the IBIM. In contrast, when the IBIM were incubated with PGH2 for 1 min at 22 degrees C without cofactors, PGI2-like activity was produced, causing profound relaxation of the isolated dog coronary artery preparation without contracting the rabbit aorta and inhibiting arachidonic acid-induced platelet aggregation. Equivalent quantities of boiled IBIM failed to alter the biological activity of PGH2 under identical conditions. Tranylcypromine (500 microgram/ml) completely abolished the appearance of PGI2-like activity. Furthermore, the PGI2-like activity found was stable for 10 min at 22 degrees C at pH 8.5 but completely lost under similar conditions at pH 5.5. It is concluded that microsomal preparations of normal bovine iris-ciliary body can synthesize PGI2-like activity in substantial amounts but not TxA2-like activity.
Insights
Bovine iris-ciliary body microsomes synthesize prostacyclin (PGI2)-like compounds, not thromboxane-A2 (TxA2)-like compounds. This ocular tissue produces significant PGI2-like activity from prostaglandin endoperoxides.
Area of Science:
- Biochemistry
- Ocular Pharmacology
- Prostanoid Synthesis
Background:
- The iris-ciliary body is a key ocular tissue with potential roles in regulating intraocular pressure and inflammation.
- Prostaglandins, including prostacyclin (PGI2) and thromboxane-A2 (TxA2), are potent lipid mediators involved in various physiological and pathological processes.
Purpose of the Study:
- To investigate the capacity of indomethacin-treated bovine iris-ciliary body microsomes (IBIM) to metabolize prostaglandin endoperoxides.
- To determine if IBIM can generate thromboxane-A2 (TxA2)-like or prostacyclin (PGI2)-like activity.
- To compare the prostanoid-forming potential of IBIM with established microsomal preparations.
Main Methods:
- Incubation of indomethacin-treated IBIM with prostaglandin H2 (PGH2).
- Assay of biological activity using dog coronary artery relaxation and rabbit aorta contraction.
- Assessment of platelet aggregation inhibition and stability of generated activity under varying pH and temperature conditions.
- Inhibition studies using tranylcypromine.
Main Results:
- IBIM did not produce any detectable TxA2-like activity from PG endoperoxides.
- IBIM generated significant PGI2-like activity upon incubation with PGH2.
- The produced PGI2-like activity caused relaxation of dog coronary arteries and was inhibited by tranylcypromine.
- The activity was stable at pH 8.5 but lost at pH 5.5, consistent with PGI2 characteristics.
Conclusions:
- Bovine iris-ciliary body microsomes possess the enzymatic machinery to synthesize PGI2-like activity.
- These ocular microsomes do not appear to synthesize TxA2-like activity.
- The findings suggest a potential role for PGI2 in ocular physiology originating from the iris-ciliary body.