Digitalis-like activity in human plasma. Purification, affinity, and mechanism

Area of Science:

• Biochemistry
• Cardiovascular Physiology
• Endocrinology

Background:

• Digitalis compounds are crucial for treating heart conditions by modulating the sodium pump.
• The existence of endogenous digitalis-like substances has been hypothesized but not fully characterized.
• Understanding endogenous regulators of the sodium pump is key to cardiovascular health.

Purpose of the Study:

• To isolate and characterize a digitalis-like factor from human plasma.
• To compare its mechanism of action with the known cardenolide, ouabain.
• To investigate its potential role in regulating sodium pump activity.

Main Methods:

• Purification of human plasma using dialysis, lyophilization, methanol extraction, and multi-stage reverse-phase chromatography.
• Assays for sodium pump activity, receptor binding, and Na,K-ATPase activity.
• Detailed kinetic, affinity, selectivity, and competition studies of the purified factor.

Main Results:

• A biologically active factor with digitalis-like properties was isolated from human plasma.
• The endogenous factor specifically inhibits the sodium pump by stabilizing the E2P form, analogous to ouabain.
• It exhibits a 8-20 fold greater apparent affinity for the Na,K-ATPase than known cardioactive steroids.

Conclusions:

• An endogenous digitalis-like factor exists in human circulation, distinct from exogenous cardenolides.
• This factor acts as a potent, specific inhibitor of the sodium pump (Na,K-ATPase).
• It may play a physiological role in regulating sodium pump activity and cardiac function.