23. The plateau phase (phase 2) of an action potential observed in non-pacemaker

Question

23. The plateau phase (phase 2) of an action potential observed in non-pacemaker cardiac cells is a result of which of the following? a) a delayed efflux of Ca2+, which balances a slowly decreasing efflux of K. b) a delayed influx of Ca, which balances a slowly increasing efflux of K c) a delayed influx of Ca2, which balances a slowly increasing influx of d) a delayed influx of Na. & Ca". which balances a slowly increasing efflux of K' 24. A new diuretic is being studied in the human volunteers. Compared to placebo, the new drug increases urine volume, increases urinary Ca2, and decreases serum K. If this new drug ? has a similar mechanism of action to an established diuretic, it probably a) Blocks the Na /CI co-transporter in the distal convoluted tubule Blocks aldosterone receptors in the collecting duct a) b) c) d) Inhibits carbonic anhydrase in the proximal convoluted tubule Acts as an osmotic diuretic

Explanation / Answer

23) a delayed influx of calcium and sodium which balances a slowly increasing efflux of pottassium

During the plateau, the slow calcium channels open. These channels are keptopenrd for a longer period and cause influx of large number of calcium ions. Already the slow sodium channels are opened through which slow influx of sodium ions continues. The entry of both calcium and sodium ions into the muscle fibremakes inside of the cell positive. Thus, the positivity is maintained inside the muscle fibre producing prolonged depolarization, i.e, plateau

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