Mechanisms of calcitonin gene-related peptide-induced increases of pulmonary blood flow in fetal sheep

Role of Nitric Oxide in Mediating In Vivo Vascular Responses to Calcitonin Gene-Related Peptide in Essential and Peripheral Circulations in the Fetus

Background— The role of calcitonin gene-related peptide (CGRP) in cardiovascular regulation is gaining clinical and scientific interest. In the adult, in vivo studies have shown that CGRP-stimulated vasodilation in several vascular beds depends, at least in part, on nitric oxide (NO). However, whether CGRP acts as a vasodilator in the fetus in vivo and whether this effect is mediated via NO have been addressed only minimally. This study tested the hypothesis that CGRP has potent NO-dependent vasodilator actions in essential and peripheral vascular beds in the fetus in late gestation.

Methods and Results— Under anesthesia, 5 fetal sheep at 0.8 gestation were instrumented with vascular catheters and Transonic flow probes around an umbilical artery and a femoral artery. Five days later, fetuses received 2- and 5-μg doses of exogenous CGRP intra-arterially in randomized order. Doses were repeated during NO blockade with the NO clamp. This technique permits blockade of de novo synthesis of NO while compensating for tonic production of the gas, thereby maintaining basal cardiovascular function. CGRP resulted in potent and long-lasting NO-dependent dilation in the umbilical and femoral circulations, hypotension, and a positive cardiac chronotropic effect. During NO blockade, the femoral vasodilator response to CGRP was diminished. In contrast, in the umbilical vascular bed, the dilator response was not only prevented but reversed to vasoconstriction.

Conclusions— CGRP has potent NO-dependent vasodilator actions in fetal essential and peripheral vascular beds. CGRP-induced NO-dependent effects in the umbilical vascular bed may provide an important mechanism in the control and maintenance of umbilical blood flow during pregnancy.

Calcitonin gene-related peptide antagonism attenuates the haemodynamic and glycaemic responses to acute hypoxaemia in the late gestation sheep fetus

The fetal defence to acute hypoxaemia involves cardiovascular and metabolic responses, which include peripheral vasoconstriction and hyperglycaemia. Both these responses are mediated via neuroendocrine mechanisms, which require the stimulation of the sympathetic nervous system. In the adult, accumulating evidence supports a role for calcitonin gene-related peptide (CGRP) in the activation of sympathetic outflow. However, the role of CGRP in stimulated cardiovascular and metabolic functions before birth is completely unknown. This study tested the hypothesis that CGRP plays a role in the fetal cardiovascular and metabolic defence responses to acute hypoxaemia by affecting sympathetic outflow. Under anaesthesia, five sheep fetuses at 0.8 of gestation were surgically instrumented with catheters and a femoral arterial Transonic flow-probe. Five days later, fetuses were subjected to 0.5 h hypoxaemia during either i.v. saline or a selective CGRP antagonist in randomised order. Treatment started 30 min before hypoxaemia and ran continuously until the end of the challenge. Arterial samples were taken for blood gases, metabolic status and hormone analyses. CGRP antagonism did not alter basal arterial blood gas, metabolic, cardiovascular or endocrine status. During hypoxaemia, similar falls in Pa,O2 occurred in all fetuses. During saline infusion, hypoxaemia induced hypertension, bradycardia, femoral vasoconstriction, hyperglycaemia and an increase in haemoglobin, catecholamines and neuropeptide Y (NPY). In contrast, CGRP antagonism markedly diminished the femoral vasoconstrictor and glycaemic responses to hypoxaemia, and attenuated the increases in haemoglobin, catecholamines and NPY. Combined, these results strongly support the hypothesis that CGRP plays a role in the fetal cardiovascular and metabolic defence to hypoxaemia by affecting sympathetic outflow.

Calcitonin gene-related peptide contributes to the umbilical haemodynamic defence response to acute hypoxaemia

Despite clinical advances in obstetric practice, undiagnosed fetal hypoxaemia still contributes to a high incidence of perinatal morbidity. The fetal defence to hypoxaemia involves a redistribution of blood flow away from peripheral circulations towards essential vascular beds, such as the umbilical, cerebral, myocardial and adrenal circulations. In marked contrast to other essential vascular beds, the mechanisms mediating maintained perfusion of the umbilical circulation during hypoxaemia remain unknown. This study determined the role of calcitonin gene-related peptide (CGRP) in the maintenance of umbilical blood flow during basal and hypoxaemic conditions. Under anaesthesia, five sheep fetuses were instrumented with catheters and a Transonic probe around an umbilical artery, inside the fetal abdomen, at 0.8 of gestation. Five days later, fetuses were subjected to 0.5 h hypoxaemia during either i.v. saline or a selective CGRP antagonist in randomised order. Treatment started 30 min before hypoxaemia and ran continuously until the end of the challenge. The CGRP antagonist did not alter basal blood gas or cardiovascular status in the fetus. A similar fall in Pa,O2 occurred in fetuses during either saline (21 +/- 0.8 to 9 +/- 0.9 mmHg) or antagonist treatment (20 +/- 0.9 to 9 +/- 1.2 mmHg). Hypoxaemia during saline led to significant increases in arterial blood pressure, umbilical blood flow and umbilical vascular conductance. In marked contrast, hypoxaemia during CGRP antagonist treatment led to pronounced falls in both umbilical blood flow and umbilical vascular conductance without affecting the magnitude of the hypertensive response. In conclusion, CGRP plays an important role in the umbilical haemodynamic defence response to hypoxaemia in the late gestation fetus.

Involvement of calcitonin gene-related peptide in control of human fetoplacental vascular tone

Calcitonin gene-related peptide (CGRP), one of the most potent endogenous vasodilators known, has been implicated in vascular adaptations and placental functions during pregnancy. The present study was designed to examine the existence of CGRP-A receptor components, the calcitonin receptor-like receptor (CRLR) and receptor activity-modifying protein 1 (RAMP1), in the human placenta and the vasoactivity of CGRP in the fetoplacental circulation. Immunofluorescent staining of the human placenta in term labor using polyclonal anti-CRLR and RAMP1 antibodies revealed that labeling specifically concentrated in the vascular endothelium and the underlying smooth muscle cells in the umbilical artery/vein, chorionic artery/vein, and stem villous vessels as well as in the trophoblast layer of the placental villi. In vitro isometric force measurement showed that CGRP dose dependently relaxes the umbilical artery/vein, chorionic artery/vein, and stem villous vessels. Furthermore, CGRP-induced relaxation of placental vessels are inhibited by a CGRP receptor antagonist (CGRP8–37), ATP-sensitive potassium (KATP) channel blocker (glybenclamide), and cAMP-dependent protein kinase A inhibitor (Rp-cAMPS) and partially inhibited by a nitric oxide inhibitor ( Nω-nitro-l-arginine methyl ester). We propose that CGRP may play a role in the control of human fetoplacental vascular tone, and the vascular dilations in response to CGRP may involve activation of KATP channels, cAMP, and a nitric oxide pathway.

Adrenomedullin in mammalian embryogenesis

Here are summarized data supporting that adrenomedullin (AM) is a multifunctional factor involved in the complex regulatory mechanisms of mammalian development. During rodent embryogenesis, AM is first expressed in the heart, followed by a broader but also defined spatio-temporal pattern of expression in vascular, neural, and skeletal-forming tissues as well as in the main embryonic internal organs. AM pattern of expression is suggestive of its involvement in the control of embryonic invasion, proliferation, and differentiation processes, probably through autocrine or paracrine modes of action. AM levels in fetoplacental tissues, uterus, maternal and umbilical plasma are highly increased during normal gestation. These findings in addition to other physiological and gene targeting studies support the importance of AM as a vasorelaxant factor implicated in the regulation of maternal vascular adaptation to pregnancy, as well as of fetal and fetoplacental circulations. AM is also present in amniotic fluid and milk, which is suggestive of additional functions in the maturation and immunological protection of the fetus. Altered expression of AM has been found in some gestational pathologies, although it is not yet clear whether this corresponds to causative or compensatory mechanisms. Future studies in regard to the distribution and expression levels of the molecules known to function as AM receptors, together with data on the action of complement factor H (an AM binding protein), may help to better define the roles of AM during embryonic development.

Nitric oxide-mediated vasodilatory effect of atrial natriuretic peptide in forearm vessels of healthy humans

1. The aim of the present study was to determine whether the vasorelaxant effect of atrial natriuretic peptide (ANP) is, in part, endothelium dependent in humans. 2. We used veno-occlusive plethysmography to measure forearm blood flow (FBF) during intra-arterial infusions of ANP (4, 8, 16, 32 pmol/min per dL forearm tissue volume) before and after the inhibition of nitric oxide (NO) synthesis by N(G)-monomethyl-L-arginine (L-NMMA; 100 micromol) in seven normal healthy subjects. 3. Atrial natriuretic peptide caused a dose-dependent increase in FBF both before and after L-NMMA and significantly reduced the plasma concentration of angiotensin (Ang) II. Administration of L-NMMA significantly diminished the increase in FBF in response to ANP infusion (P < 0.05). 4. These results suggest that the forearm vasodilative response to ANP is modulated, in part, by an endothelium-derived NO-mediated mechanism associated with a decrease in AngII caused by ANP.