Plasma orexin-A-like immunoreactivity levels and renal function in patients in a geriatric ward

Orexin-A is a neuropeptide mainly produced by hypothalamic neurons with functions in the central nervous system such as regulation of the sleep-wake cycle. Recent studies suggest that orexin-A also plays major roles in peripheral tissues. Although a few studies have reported a role for the kidney in the dynamics of orexin-A, little is known about the association between plasma orexin-A-like immunoreactivity (orexin-A-LI) levels and renal function. We evaluated this association, and also explored other clinical characteristics associated with plasma orexin-A-LI levels. In this cross-sectional study, we included 70 consecutive patients aged ≥65 years admitted to the geriatric ward of Nagoya University Hospital from December 2017 to January 2018. Patients taking suvorexant (an orexin receptor antagonist) were excluded. On hospital days 2-4, fasting blood was collected in the morning. We evaluated associations between plasma orexin-A-LI levels and renal function and other clinical characteristics. Renal function was evaluated in two ways: the estimated glomerular filtration rate (eGFR) using serum creatinine, and estimated creatinine clearance (eCrCl) using the Cockroft-Gault formula. Pearson's correlation coefficient revealed that plasma orexin-A-LI levels were negatively correlated with the eGFR (r = -0.351, p = 0.003) and eCrCl (r = -0.342, p = 0.004). There were no significant associations between plasma orexin-A-LI levels and the primary diagnosis, body mass index, duration of fasting, or other clinical characteristics. In conclusion, plasma orexin-A-LI levels were negatively correlated with renal function in patients in a geriatric ward. Renal function may affect the study design and data interpretation in studies of plasma orexin-A-LI.