Water immersion (WI) is followed by hypoxemia and haemodilution, and induces alterations in renal haemodynamics (increase of tubular sodium load). These facts justified the performance of studies which aimed to assess the influence of WI on EPO secretion. Serum EPO and atrial natriuretic peptide (ANP) concentration and plasma renin activity (PRA) were estimated in 18 patients with essential hypertension (EH) and in 9 healthy subjects (HS) before, after two hours of WI and two hours after discontinued WI. WI was followed by a significant increase of plasma volume and decrease of 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 significant increase of 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 MAP and post-WI natriuresis respectively.
Conclusions: 1. Patients with EH are characterized by elevated basal serum levels of ANP and EPO. 2. Participation of the renin-angiotensin system and ANP in the regulation of EPO secretion could not be proven both in normotensive and hypertensive patients. 3. Involvement of EPO in the pathogenesis of EH seems to be likely.