Water immersion (WI) is followed by hypoxemia and hemodilution, and induces alterations in renal hemodynamics (increase in tubular sodium load). These facts justified the performance of studies which aimed to assess the influence of WI on erythropoietin (EPO) secretion. Serum EPO and atrial natriuretic peptide (ANP) concentrations as well as plasma renin activity (PRA) were estimated in 18 patients with essential hypertension (EH) and in 9 healthy subjects (HS): before, after 2 h of WI and 2 h after discontinued WI. WI was followed by a significant increase in plasma volume and decrease in PRA, which were of similar magnitude in both examined groups. Patients with EH showed significantly higher basal levels of serum EPO (66.7+/-11.4 mU/ml in EH vs. 20.0+/-3.4 mU/ml in HS) and ANP (110.6+/-15.4 pg/ml in EH vs. 75.6+/-8.2 pg/ml in HS). WI was followed by a significant increase in both EPO (by 34.0+/-8.9 mU/ml in EH and 17.0+/-5.4 mU/ml in HS) and ANP (by 106.9+/-19.2 pg/ml in EH and 149.4+/-16.9 pg/ml in HS). Only in EH, a significant correlation was found between serum EPO level and mean arterial pressure post WI and natriuresis during WI, respectively.
Conclusions: (1) water immersion induced increase in plasma EPO both in healthy subjects and patients with essential hypertension. (2) Patients with EH are characterized by elevated basal plasma levels of ANP and EPO. (3) Participation of the renin-angiotensin system and ANP in the regulation of EPO secretion could not be proven both in normotensive subjects and hypertensive patients.