Potentiation of sympathetic neurotransmission in bovine isolated irides by isoprostanes

Author： Opere Catherine Awe S. Harris Lydia LeDay Angela Ohia Sunny

ISSN： 1071-5762

Source： Free Radical Research, Vol.35, Iss.3, 2001-01, pp. : 257-264

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Abstract

Isoprostanes (IsoP) are formed by free radical catalyzed peroxidation of arachidonic acid independent of the cyclooxygenase enzyme. In the present study, we examined the effect of IsoP on norepinephrine (NE) release from the bovine isolated iris. Furthermore, we studied the role of IsoP's in hydrogen peroxide (H 2 O 2 )-induced enhancement of NE release from this tissue. Isolated bovine irides were prepared for studies of [ 3 H]NE release using the superfusion method. Release of [ 3 H]NE was induced via electrical field stimulation. Both 8-iso-prostaglandin E 2 (E 2 -IsoP) and 8-iso-prostaglandin F 2 (F 2 -IsoP) produced a concentration-related enhancement of field-stimulated [ 3 H]NE release from isolated bovine irides, an effect that was mimicked by the thromboxane (Tx) receptor agonist, U46619 and by H 2 O 2 . The Tx-receptor antagonist, SQ 29548 inhibited responses to E 2 -IsoP (10μM) with an IC 50 of 370±50 nM. SQ 29548 (10μM) also blocked the enhancement of electrically-evoked [ 3 H]NE release induced by U46619 (10μM) but not that caused by H 2 O 2 (300μM). The Tx synthetase inhibitor, carboxyheptylimidazole (10μM) prevented the stimulatory effect of E 2 -IsoP on evoked [ 3 H]NE release without affecting responses induced by H 2 O 2 . We conclude that IsoP's can enhance sympathetic neurotransmission in the bovine isolated iris, an effect that can be blocked by a Tx-receptor antagonist. Furthermore, endogenously produced Tx's mediate the stimulatory effect of IsoP's on NE release. However, endogenously generated IsoP's or Tx's are not involved in H 2 O 2 -induced potentiation of sympathetic neurotransmission.