Rationale: Cardiotonic steroids (CTS), used to treat heart failure for over 200 years, inhibit the sodium-potassium pump (Na/K/ATPase), and increase cardiac contractility by inhibiting efflux of Na through the pump (“digitalis hypothesis”). It is now known that inhibition of the Na/K/ATPase in patients with heart failure increases mortality, and all major beneficial treatments increase Na/K/ATPase activity. Endogenous circulating CTS such as ouabain have been thought to inhibit the Na/K/ATPase, although sporadic studies have shown that ouabain stimulates the pump. The current study investigated this phenomenon.
Objective: To examine if effects of reported endogenous ouabain concentrations on the Na/K/ATPase might be stimulatory or inhibitory.
Methods & Results: Cardiac myocytes were isolated from male New Zealand White rabbits, placed in a Tyrodes’ solution, and whole-cell patch clamped. They were exposed to ouabain for 30 seconds, followed by exposure to a potassium-free solution, with the difference in current giving the Na/K/ATPase pump current in response to ouabain. Compared to the control current of 0.51 ± 0.05 pA/pF, the ouabain-induced current was significantly (P<0.05) increased to 0.69 ± 0.09 pA/pF at 5 nM. This shows that ouabain stimulates the Na/K/ATPase at nanomolar concentrations, even with the brief exposure not expected to achieve steady-state binding. It suggests circulating CTS might stimulate the pump rather than inhibit it.
Cell viability assays carried out on the breast cancer cell line MCF7, whose Na/K/ATPase structure is extremely similar to that of cardiac myocytes, showed significantly elevated viability above control values after 24 hours’ treatment with subnanomolar concentrations of ouabain; maximum viability was 116 ± 10% at 0.28 nM (P˂0.05). This suggests a biological effect of circulating CTS might exist, possibly due to stimulation of Na/K/ATPase.
Conclusion: Although circulating cardiotonic steroids have been thought for many years to inhibit the pump, they are most likely stimulate it.