Dysregulation of the Na/K-ATPase (NKA) in the kidney, cardiovascular system, and peripheral nervous system is believed to contribute to pathogenesis of diabetes mellitus (DM) and its complications. Recently we demonstrated that, in addition to endogenous ouabain (EO), mammalian tissues contain another NKA inhibitor, a bufadienolide marinobufagenin (MBG). In vitro MBG, a natriuretic and a vasoconstrictor, acts as a selective inhibitor of alpha-1 NKA, the main isoform of the sodium pump in renal tubules and vascular smooth muscle. To determine whether digitalis-like NKA inhibitors are linked to NKA dysregulation in DM, we studied changes in renal excretion and plasma levels of MBG and EO and the activity of erythrocyte NKA in male Wistar rats with type 1 DM and type 2 DM. Rats with type 1 DM were studied four weeks following a single intraperitoneal injection of 65 mg/kg streptozotocin (STZ)(n = 12), and rats with type 2 DM were studied 10 weeks after intraperitoneal injection of STZ during the neonatal period (n = 12). Renal excretion and plasma levels of EO did not change in rats with both types of DM as compared to that in the control groups. Renal excretion (57.5 +/- 9.4 pmol/kg/ 3 hours vs. 12.6 +/- 2.1 pmol/kg/ 3 hours; P less than 0.01) and plasma levels (2.23 +/- 0.82 nmol/L vs. 0.29 +/- 0.07 nmol/L; P less than 0.01) of MBG increased, and NKA activity in erythrocytes was inhibited by 50% in rats with type 1 DM as compared to controls. In rats with type 2 DM, plasma levels (1.48 +/- 0.09 nmol/L vs. 0.46 +/- 0.02 nmol/L; P less than 0.01) and renal excretion (21.3 +/- 3.2 pmol/kg/ 3 hours vs. 13.1 +/- 2.1 pmol/kg/ 3 hours) of MBG also became elevated, but less than in the animals with type 1 DM. Accordingly, activity of NKA in erythrocytes from rats with type 2 DM was inhibited by 35%. In vitro treatment of erythrocytes from rats with type 1 and type 2 DM with anti-MBG antibody reversed the DM induced inhibition of the NKA. These results suggest that digitalis-like factors are involved in the pathogenesis of DM and that MBG, rather than EO, is responsible for DM-induced NKA inhibition.