Final answer:
Inhibition of the Na-K-ATPase would disrupt the electrochemical gradient across a mammalian cell's plasma membrane, leading to ionic imbalance, altered membrane potential, and disrupted cellular processes.
Step-by-step explanation:
If you were to treat a normal mammalian cell with a substrate that inhibits the Na-K-ATPase, the immediate effect upon the cell would be a disruption in the electrochemical gradient across the plasma membrane. The Na-K-ATPase, also known as the sodium-potassium pump, is responsible for pumping three sodium ions out of the cell and two potassium ions into the cell, against their respective concentration gradients, using the energy from ATP hydrolysis. Inhibition of the pump would lead to an accumulation of sodium ions inside the cell and a loss of potassium ions, which affects the cell's resting membrane potential and could disrupt cell volume, as well as various cellular processes that rely on the gradient as a driving force, such as secondary active transport.
This disturbance would also affect the net negative charge of the interior of the cell, which is essential for functions such as nerve impulse transmission. Over time, the loss of this ionic balance could lead to cellular dysfunction and could potentially be lethal if the pump's activity is not restored.