[Pyr1]-Apelin-13

Apelin is the endogenous ligand of APJ, which belongs to the superfamily of G protein-coupled receptors. In recent years, the apelin/APJ system has been detected in many tissues and emerges as a promising target for the treatment of various pathophysiological conditions. Pyr1-apelin-13 is the major isoform of apelin in human plasma; however its stability and proteolytic degradation pattern remain poorly understood. The aim of the present study was first to identify the cleavage sites of Pyr1-apelin-13 in mouse, rat and human plasma and rat cerebrospinal fluid, then to determine its stability to proteolytic degradation following intravenous administration in rats. Secondly, key residues were substituted by natural and unnatural amino acids in order to examine the impact on in vitro stability and degradation pattern. The kinetics of degradation revealed that the Leu5-Ser6 peptide bond of Pyr1-apelin-13 is the first cleavage observed in plasma, independently of the species.

The role of the endogenous apelin system in pregnancy is not well understood. Apelin's actions in pregnancy are further complicated by the expression of multiple forms of the peptide. Using radioimmunoassay (RIA) alone, we established the expression of apelin content in the chorionic villi of preeclamptic (PRE) and normal pregnant women (NORM) at 36-38 wk of gestation. Total apelin content was lower in PRE compared with NORM chorionic villi (49.7±3.4 vs. 72.3±9.8 fmol/mg protein; n=20-22) and was associated with a trend for lower preproapelin mRNA in the PRE. Further characterization of apelin isoforms by HPLC-RIA was conducted in pooled samples from each group. The expression patterns of apelin peptides in NORM and PRE villi revealed little or no apelin-36 or apelin-17. Pyroglutamate apelin-13 [(Pyr1)-apelin-13] was the predominant form of the peptide in NORM and PRE villi. Angiotensin-converting enzyme 2 (ACE2) activity was higher in PRE villi (572.

APJ was cloned as an orphan G protein-coupled receptor and shares a close identity with angiotensin II type 1 receptor (AT1R). Apelin is a peptide that has recently been identified as an endogenous ligand of the APJ. Apelin and APJ mRNA are expressed in peripheral tissue and the central nervous system. However, little is known about the effects of apelin in cardiovascular regulation. To examine the central and peripheral role of apelin, we injected the active fragment of apelin [(Pyr1)apelin-13] intracerebroventricularly (ICV, 5 and 20 nmol, n=6) or intravenously (IV, 20 and 50 nmol, n=4 or 5) in conscious rats. ICV injection of (Pyr1)apelin-13 dose-dependently increased mean arterial pressure (MAP) and heart rate (HR) (19+/-3 mm Hg and 162+/-26 bpm at 20 nmol). Pretreatment with ICV injection of the AT1R antagonist (CV-11974, 20 nmol) did not alter the apelin-induced increase in MAP and HR. IV injection of (Pyr1)apelin-13 also dose-dependently increased MAP and HR (13+/-2 mm Hg and 103+/-18 bpm at 50 nmol); however, the peripheral effects of apelin were relatively weak compared to its central effects.

Intracerebroventricular (i.c.v.) injections of apelins have been shown to modulate the central control of cardiovascular function, as well as the homeostasis of fluid and salt balance, and to some extent also body core temperature. Here, we investigated the effects of i.c.v. administration of [Pyr(1)]apelin13 (PyrAp13; 20nmol) dissolved in artificial cerebrospinal fluid (aCSF), as compared to aCSF alone, on fever and sickness behavior elicited in rats by intraperitoneal injection of bacterial lipopolysaccharide (LPS, 100 μg/kg). Injections of LPS induced a short phase of hypothermia followed by a biphasic fever, depression of motor activity, anorexia and adipsia. I.c.v. injections of PyrAp13 without systemic LPS application slightly augmented motor activity at statistically unaltered core temperature. In combination with LPS, central administration of PyrAp13 significantly reduced fever during the time period of 3-9h after injection, but did not significantly attenuate anorexia and adipsia, and had no effect on LPS-induced lethargy.