Poster Presentation Australian Society for Medical Research Annual Scientific Meeting 2016

Low concentrations of ouabain stimulate the sodium potassium pump – implications for circulating endogenous cardiotonic steroids? (#137)

Yeon Jae Kim 1 2 , Elisha Hamilton 1 2 , William Hannam 1 , Chia-Chi Liu 1 2 , Rachel Teh 1 , Helge H Rasmussen 1 2 , Alvaro Garcia 1 3
  1. North Shore Heart Research Group, Kolling Institute, St Leonards, NSW, Australia
  2. Sydney Medical School, University of Sydney, Sydney, NSW, Australia
  3. School of Chemistry, University of Sydney, Sydney, NSW, Australia

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.