Potentiation of sympathetic neurotransmission in bovine isolated irides byisoprostanes

Isoprostanes (IsoP) are formed by free radical catalyzed peroxidation of ar achidonic acid independent of the cyclooxygenase enzyme. In the present stu dy, we examined the effect of IsoP on norepinephrine (NE) release from the bovine isolated iris. Furthermore, we studied the role of IsoP's in hydroge n peroxide (H2O2)-induced enhancement of NE release from this tissue. Isola ted bovine irides were prepared for studies of [H-3]NE release using the su perfusion method. Release of [H-3]NE was induced via electrical field stimu lation. Both 8-iso-prostaglandin E-2 (E-2-IsoP) and 8-iso-prostaglandin F-2 alpha, (F-2-IsoP) produced a concentration-related enhancement of field-st imulated [3H]NE release from isolated bovine irides, an effect that was mim icked by the thromboxane (Tx) receptor agonist, U46619 and by H2O2. The Tx- receptor antagonist, SQ 29548 inhibited responses to E-2-IsoP (10 muM) with an IC50 of 370 +/- 50 nM. SQ 29548 (10 muM) also blocked the enhancement o f electrically-evoked [H-3]NE release induced by U46619 (10 muM) but not th at caused by H2O2 (300 muM). The Tx synthetase inhibitor, carboxyheptylimid azole (10 muM) prevented the stimulatory effect of E-2-IsoP on evoked [H-3] NE release without affecting responses induced by H2O2. We conclude that Is oP's can enhance sympathetic neurotransmission in the bovine isolated iris, an effect that can be blocked by a Tx-receptor antagonist. Furthermore, en dogenously produced Tx's mediate the stimulatory effect of IsoP's on NE rel ease. However, endogenously generated IsoP's or Tx's are not involved in H2 O2-induced potentiation of sympathetic neurotransmission.