Evidence for involvement of prostaglandins in central alpha adrenergic activity and LH release in sheep

Abstract

Circhoral pulsatile releases of immunoreactive luteinising hormone (LH) and prostaglandin F 2α (PGF 2α) occur synchronously into the jugular vein in ovariectomised sheep. Following a 4-hour control period, intra-carotid injections of phentolamine or intramuscular injections of phenoxybenzamine were given to ovariectomised sheep and the pulsatile release of LH and PGF 2α was monitored for a further 6 to 8 hours. Phenoxybenzamine caused a fall in LH and PGF 2α in jugular venous plasma. Phentolamine also reduced LH significantly but in this case a marked rise in PGF 2α as measured by radioimmunoassay (RIA) occurred after very high doses of phentolamine. Interpretation of the latter results was complicated by the fact that phentolamine at high dose levels interfered with the RIA of PGF 2α in plasma. Experiments were repeated in ovariectomised sheep with cannulae placed in the lateral ventricles of the brain for sampling cerebrospinal fluid (CSF). In contrast to the previously observed rise in jugular venous PGF 2α following high doses of phentolamine, a fall in CSF levels of immunoreactive PGF 2α occurred following intracarotid phentolamine or phenoxybenzamine in 3 out of 7 experiments, while no change was observed in the remaining 4 animals. Phentolamine did not reduce LH significantly in animals with intraventricular cannulae. The work provides support for the view that circhoral pulses of immunoreactive PGF 2α in sheep are neural in origin and may be related to sympathetic neurotransmission.