Effects of severe hemorrhage on plasma ANP and glomerular ANP receptors

Atrial natriuretic peptide (ANP) plays an important role in blood volume and electrolyte homeostasis in normovolemia and in hypervolemic states. The currently available information on the effects of hypovolemia on plasma ANP is contradictory. Moreover, possible regulation of ANP receptors during severe hemorrhagic hypovolemia has not been investigated. This study evaluated the effects of severe hemorrhage on plasma ANP and on the regulation of glomerular ANP receptor subtypes in anesthetized rats. Constant rate bleeding of 50% of total blood volume within 2 h induced a reproducible shock state characterized by marked decreases in blood pressure, heart rate, and hematocrit and an increase in plasma renin activity and aldosterone. Hemorrhaged rats exhibited a gradual significant increase in plasma ANP from 39.3 ± 2.9 to 114.7 ± 20.0 pmol/l 1 h after the bleeding (P < 0.001 from the initial value and P < 0.02 from the final value of sham-shock rats). Hemorrhage induced a significant decrease in total glomerular ANP binding sites (172 ± 25 vs. 363 ± 39 fmol/mg protein in hemorrhaged and sham-shock rats, respectively, P < 0.05). This decrease was mainly due to a significant decrease in ANP(c) receptors (132 ± 22 vs. 312 ± 40 fmol/mg protein in hemorrhaged and sham-shock rats, respectively, P < 0.05). Hemorrhage did not change glomerular ANP(A) receptor density. No significant differences in the affinity of the glomerular receptor subtypes for ANP were detected. Our data indicate that plasma ANP increases after prolonged severe hemorrhage. It is suggested that downregulation of renal ANP(c) receptors leads to reduced clearance of ANP and contributes to elevation of its plasma level after severe hemorrhage.