Question

Glycosides such as ardenolides from foxglove plants Digitalis purpurea and bufadienolides from the venom of the cane toad Bufo marinus, turn off the Na+/K+ pump. What will this do to RMP? Will cells be harder or easier to stimulate? Why? How does this figure into Goldman’s equation?

Answer 1

Na+-K+ pump transports 3 Na+ out of the cell and 2 K+ into the cell.So it maintains negative charge on the inside.When this pump is turned off; the RMP will be less negative.So the cell will be easier to excite.Also there will be more Na+ on the inside; so Na+-Ca2+ exchanger which transports Ca2+ out and Na+ in also stops functioning.So increased Ca2+ concentration on the inside will lead to stronger contraction.

Goldmann equation is used to calculate RMP by using the permeability of the membrane for different ions and their relative concentrations on the outside and inside.In this case; the concentration of sodium inside increases and concentration of potassium inside decreases.So denominator increases and numeratot decreases.So the absolute value of membrane potential decreases ;ie.it becomes less negative.