Gonadotropin-Releasing Hormone Increases Plasma Catecholamines and Blood Pressure in Toads

Abstract

Synthetic gonadotropin-releasing hormone (GnRH) was injected intravenously into conscious, adult toads ( Bufo marinus) to elucidate the nervous and cardiovascular actions of the hormone. GnRH (0.001–1.0 nmol·kg^¹) produced dose-dependent increases in mean arterial blood pressure and pulse pressure, beginning within 3 min after injection. These pressor responses to GnRH were specific to the hormone since they could be inhibited reversibly by [D-pGlu¹, D-Phe², D-Trp^3,6]-GnRH. Arterial plasma concentrations of unconjugated catecholamines increased simultaneously with the rise in blood pressure following GnRH injection: the half-maximal pressor dose of GnRH (0.1 nmol·kg^–1) caused a 3-fold increase in plasma noradrenaline and a 20-fold increase in plasma adrenaline concentrations. Pretreatment of toads with an α-adrenergic antagonist, prazosin, and a β-adrenergic antagonist, propranolol, abolished the pressor responses to GnRH. We conclude that GnRH mobilizes catecholamines, which act through α- and β-adrenergic mechanisms to raise blood pressure. Thus, endogenous GnRH or GnRH-like peptides may coordinate the pituitary, nervous and cardiovascular mechanisms which prepare toads for seasonal reproductive activity.