Another well established visceral action of CRF and urocorti

Another well-established visceral action of CRF and urocortin administered peripherally is the long-lasting lowering of blood pressure observed in various species ranging from rodents to humans [22]. Existing evidence suggests that the hypotensive action of CRF and urocortin is mediated by the interaction with CRF2 receptors. Peripheral administration of urocortin and structurally homologous non-mammalian peptides, sauvagine and urotensin I that display a higher binding affinity to CRF2 than CRF1 IAA-94 [23], lowered blood pressure at a much greater potency than CRF in rats [22], [33]. Subsequent studies showed that intravenous (iv) injection of urocortin did not influence blood pressure in CRF2-mutant mice while a prominent depressor response was induced in wild type mice [3]. Lastly, CRF2 receptors are expressed in vascular tissues including the endothelium [12] and urocortin and CRF have been documented to produce vascular relaxation in vitro [12], [22]. However, direct pharmacological evidence of that activation of CRF2 receptor reduces arterial blood pressure using the recently characterized selective CRF2 endogenous agonist, Ucn II [24] and newly developed selective CRF2 receptor antagonists [26], [28] is still to be established. In the present study, we explored the dose-related influence of intravenous (iv) injection of hUcn II on basal arterial blood pressure in urethane-anesthetized rats. The CRF2 receptor mediated action of hUcn II was further characterized using the newly developed long acting and selective competitive CRF2 antagonist, astressin2-B [26]. Activation of medullary TRH pathways has been shown to have physiological relevance in driving the stimulation of autonomic outflow to viscera induced by acute cold restraint stress [1], [18]. Recent studies in CRF2 knockout mice indicate that CRF2 receptor activation helps in the recovery of stress as it relates to the endocrine and behavioral responses [2], [3], [5]. Therefore, we also investigated the influence of iv hUcn II on the hypertensive response to the stable, TRH analog, RX-77368 injected intracisternally (ic) [10], [20].
Materials and methods
Discussion The present study showed that systemic administration of hUcn II results in a potent hypotensive response in urethane-anesthetized rats. Urocortin II (3, 10, and 30 μg/kg) decreased dose-dependently baseline MAP by 25–57% 10 min after peptide injection. The effect is specific to hUcn II since there was no significant changes in MAP from baseline values after the iv injection of vehicle. Previous studies showed that the systemic administration of nonselective CRF1/CRF2 agonist, CRF, and more potently urocortin induced a sustained drop in basal blood pressure in conscious or anesthetized rats as well as in other species and in humans [6], [21], [25]. Collectively these data indicate that endogenous CRF agonists, CRF, urocortin and the newly characterized member of the CRF family, Ucn II [7], [13], [24] display hypotensive properties. The influence of Ucn II injected iv on the hypertensive response to central sympathetic activation was evaluated using the ic injection of the stable TRH analog, RX-77368. In vehicle pretreated rats, ic RX-77368 resulted in a marked elevation of MAP (+40±6 mm Hg from baseline), which was long lasting (over 60 min) consistent with our previous studies [10], [11]. Convergent reports established that the hypertensive response to injection of TRH or TRH analog into the cerebrospinal fluid involves brain TRH receptor mediated activation of sympathetic outflow and catecholamine release [4], [10], [20], [31]. hUcn II injected iv at 3, 10 or 30 μg/kg did not alter the net increase in MAP in response to RX-77368. However, due to the decrease in basal MAP induced by iv hUcn II at the highest dose, the rise in MAP induced by RX-77368 did not result in a hypertensive response in addition the magnitude of the net rise was attenuated by 25%. The action of hUcn II at 30 μg/kg was long lasting and potent since 80 min after an iv bolus injection followed 20 min later by ic RX-77368; MAP values were still decreased from baseline by −23.3±7.6 mm Hg while in the iv vehicle pretreated group, the hypertensive effect of RX-77368 was still maintained at +28.3±4.6 mm Hg.