The search for a G protein coupled receptors (GPCR) specific for angiotensin III led us to isolate the murine homolog of the human orphan receptor, APJ, that shares 31% sequence identity with the angiotensin II receptor type 1 (AT1). At the end of 1998, the sequence of the endogenous ligand of the human APJ receptor was isolated and named apelin. We pharmacologically characterized this receptor. We then demonstrated that apelin and its receptor are co-expressed with arginine vasopressin (AVP) in magnocellular vasopressinergic neurons from the supraoptic and paraventricular hypothalamic nuclei. We showed that the central injection of apelin in lactating rats decreased the phasic electrical activity of AVP neurons and the secretion of AVP into the bloodstream, increasing aqueous diuresis. Apelin has also a diuretic effect via a direct renal action by increasing renal blood flow and by counteracting in collecting ducts, the antidiuretic effect of AVP occurring via vasoppressin receptors type 2 (V2).
Apelin is thus a natural inhibitor of the anti-diuretic effects of AVP. Finally, in rats and humans, we found that apelin and AVP are conversely regulated by osmotic stimuli, thereby both participating to the maintenance of body fluid homeostasis. Apelin and its receptor are also present in the heart, kidney and blood vessels and the systemic injection of apelin decreases blood pressure, improves cardiac contractility and reduces cardiac loading. Apelin may therefore play a crucial role in maintaining water and electrolyte balance and cardiovascular function.
Since the half-life of apelin in the blood circulation is in the minute range, we therefore aimed to develop metabolically stable apelin-17 analogs. We generated P92 by classic chemical substitutions and LIT01-196 by original addition of a fluorocarbon chain to the N terminus of K17F. Both analogs were much more stable in plasma (half-life >24 h for LIT01-196) than K17F (4.6min). Analogs displayed a subnanomolar affinity for the apelin receptor and behaved as full agonists with regard to cAMP production, ERK phosphorylation, and apelin receptor internalization. Ex vivo, these compounds induced vasorelaxation of rat aorta and glomerular arterioles, respectively, precontracted with norepinephrine and angiotensin II, and increased cardiac contractility. In vivo, after intracerebroventricular administration in water-deprived mice, P92 and LIT01-196 were 6 and 160 times, respectively, more efficient at inhibiting systemic vasopressin release than K17F. Administered intravenously (nmol/kg range) in normotensive rats, these analogs potently increased urine output and induced a profound and sustained decrease in arterial blood pressure. In conclusion, these new compounds, which favor diuresis and improve cardiac contractility while reducing vascular resistances, represent promising candidates for the treatment of heart failure and water retention/ hyponatremic disorders.
Directrice du laboratoire "Neuropeptides centraux et régulations hydrique et cardiovasculaire"
Inserm U1050, CIRB, Collège de France, Paris
Invité par Xavier Houard.