TBR test 1 biological sciences q 98

[2010premed]

With the addition of TEA (tetraethylammonium) to cardiac cell preparations, the action potential plot exhibits an increase in the plateau phase intensity (mV) and an absence of the repolarization phases (phases 3 and 4). Based on this observation, which ion must be affected by TEA?

• K+
• B. CA+2

Answer ; K+

In the passage, it states, “This plateau is due to increases in Ca+2 channel influx through slow Ca+2 channels. The final stages of repolarization in cardiac muscle are dependent on Ca+2 channel closure and increases in K+ channel conductance (phases 3 and 4).

Therefore, how would to eliminate Ca+2?

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[Lily20]

I was wondering the same thing! Did you manage to figure it out?

 

[AwayFromReality]

I don't have access to that test but I'm assuming the graph they showed you in the passage is the same as any other action potential graph which means that mV is on the y-axis. I think what they are saying in this question is that the plateau itself is not affected but rather where the plateau on the graph is occurring is affected. In this case we are seeing an increase in plateau phase intensity (mV) which means the plateau is occurring farther up on the y-axis and nothing else about the plateau is changing. This would imply that the Ca2+ channels are not affected. The passage also states that only Ca2+ and K+ channels are responsible for repolarization. Since there is no repolarization now and it looks like Ca2+ channels are probably not affected (as evidenced by us still seeing a plateau) the better answer would be K+ channels

 

[erythrocyte666]

Firstly, coincidentally, TEA is used as a neurotoxin for studying effects of voltage-gated K channels in neurons.
Secondly, repolarization always means K efflux, whatever excitable cell you look at - because 1) that's the only + charge inside the cell still having a concentration gradient and 2) the Na/K ATPase needs extracellular K ions to later pump out the Na ions
Thirdly, the plateau is increased because it is caused by a balance between K efflux and Ca influx; if K efflux is decreased due to TEA, the imbalanced higher Ca influx will raise the plateau.
If Ca was inhibited, you wouldn't even see a plateau; it would just be a generic action potential spike.