ENDOGENOUS MARINOBUFAGENIN-LIKE IMMUNOREACTIVE FACTOR AND NA+,K+ ATPASE INHIBITION DURING VOLUNTARY HYPOVENTILATION

In previous studies investigators found that conditioned hypoventilatory breathing potentiated a sodium-sensitive form of hypertension in dogs that was not mediated by sympathetic nervous system arousal. Our study investigated effects of 30 minutes of voluntary hypoventilation, maintained by a respiratory gas monitor and feedback procedure, in 16 normotensive humans of both sexes on (1) plasma concentrations of endogenous digitalis-like factors (ouabain-like and marinobufagenin-like immunoreactivity), (2) activity of erytherocyte Na+,K+-ATPase, (3) inhibitory activity of plasma Na+,K+-ATPase, and (4) blood pressure. Increased end tidal Pco(2) (41+/-0.78 mm Hg versus 37.6+/-1.03 mm Hg) was associated with (1) an increase in plasma marinobufagenin-like immunoreactivity (1.23+/-0.47 versus 4.96+/-1.19 nmol/L), (2) an inhibition of Na+,K+-ATPase in red blood cells (3.68+/-0.22 versus 2.15+/-0.25 mmol P-i . mL(-1). h(-1); P<.01), (3) increase in plasma Na+,K+-ATPase inhibitory activity (34.9+/-4.0% versus 48.8+/-2.1%, P<.02), and (4) increases in systolic (112.4+/-2.6 versus 107.6+/-1.8 mm Hg) and diastolic (73.5+/-2.1 versus 68.8+/-2.1 mm Hg) blood pressures. Plasma levels of ouabain-like immunoreactivity did not increase significantly. Incubation of erythrocytes obtained during hypoventilation with antidigoxin antibody restored the Na+,K+-ATPase activity (3.99+/-0.34 mmol P-i . mL(-1). h(-1)). Cessation of hypoventilation was associated with decreases in diastolic blood pressure (70.5+/-2.2 mm Hg) and restoration of Na+,K+-ATPase activity in erythrocytes (2.99+/-0.43 mmol P-i . mL(-1). h(-1)). On the basis of organic extraction and thin-layer chromatography followed by separation with the use of reverse-phase high-performance liquid chromatography, the material coeluting with marinobufagenin was separated from human urine. This material cross-reacted with anti-marinobufagenin antibody. These results demonstrate the presence of a bufadienolide-like Na+,K+-ATPase inhibitor in human plasma and support the view that breathing pattern may participate in blood pressure control via release of a rapidly acting circulating Na+,K+-ATPase inhibitor.