Comparison of the Effects of Belatacept and Anticalcineurins on Endothelial Function in Renal Transplant Patients - <BELAFENDO>

Inclusion Criteria: For the Belatacept group: \- Patients who have undergone a graft biopsy due to impaired renal function finding criteria for chronic toxicity of anticalcineurins leading to the introduction of Belatacept. For the anticalcineurin group: * Renal transplant patients treated with anticalcineurins for more than a year. * Stable renal function (defined by a creatinine level in µmol/l stable for 3 months (variation +/-20%) For both groups: * Date of kidney transplant greater than 1 year * Age between 18 and 75 years inclusive * Patient having received clear information from one of the investigators, having read and understood the information letter and signed the consent form * Women : * of childbearing age (defined by the CTFG as a fertile woman, after menarche and until menopause, except in cases of permanent sterility (including hysterectomy, bilateral salpingectomy or bilateral oophorectomy) * using effective contraception according to the WHO (combined hormonal contraception (containing estrogens and progestins), progestin-only contraception, intrauterine device (IUD), male or female condoms) for at least 4 weeks before inclusion and during the study And, * presenting a negative urine pregnancy test at inclusion; * menopausal: menopause according to the CTFG is defined as the absence of periods for 12 months without any other medical cause. An elevated follicle-stimulating hormone (FSH) level in the postmenopausal interval can be used to confirm a postmenopausal state in women who are not using hormonal contraception or hormone replacement therapy. However, in the absence of 12 months of amenorrhea, a single FSH measurement is insufficient. * Patient benefiting from a social protection scheme Exclusion Criteria: * Stage 5 chronic renal failure (defined by a CKD-EPI GFR\<15 ml/min/1.73m²) * Dialysis patient * History of myocardial infarction or stroke less than 6 months old * Systolic heart failure requiring hospitalization in the 6 months preceding inclusion or known heart failure with an LVEF \<30% * BMI\>35 kg/m² * Severe hepatic insufficiency (Child-Pugh class C) * Contraindication to NATISPRAY 0.30 mg/dose, solution for oral spray (and in particular hypersensitivity to nitrates in accordance with the SPC (Summary of Product Characteristics) of NATISPRAY) * Person deprived of liberty by an administrative or judicial decision or person placed under judicial protection, or guardianship or curatorship * Previous or current treatment with Belatacept * Severe high blood pressure (DBP ≥ 110 mm Hg and/or SBP ≥ 180 mm Hg) * Presence or history of functional or ligated or thrombosed bilateral arteriovenous fistula, preventing explorations * Pregnant, breastfeeding woman, or absence of proven effective contraception * Excessive alcohol consumption (no more than 10 drinks per week) * Active smoking with a daily consumption of more than 21 mg of nicotine per day or taking nicotine substitutes with a dose greater than 21 mg/24 hours * Drug addiction or suspected illicit drug use * Patient participating or having participated in the 4 weeks preceding inclusion in a clinical trial