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What is the meaning of calcium conductance in ion channels. I encountered this in the following text:

It was established that the µ and δ opioid receptors open potassium channels, which results in reduction of calcium conductance (Simon, 2005).

Also why should opening a potassium channel reduce calcium conductance. How are they related ?

Reference:

Simon, E. J. (2005). Opiates: Neurobiology. In J. H. Lowinson, P. Ruiz, R. B. Millman & J. G. Langrod (Eds.), Substance abuse : a comprehensive textbook (4th ed., pp. xxiv, 1421 p.). Philadelphia: Lippincott Williams & Wilkins.

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This might be a mistake in the text you read or the referenced book. Opioid receptors can regulate calcium channels as well as potassium channels. –  Mad Scientist Sep 3 '13 at 14:53
    
@MadScientist you're right that opioid receptors regulate both calcium and potassium channels but the snippet is about mu and delta receptor types. The text also says: The activation of κ receptors was found to reduce calcium conductance by closing calcium channels. It was recently found that all three types of opioid receptors can act by both mechanisms; i.e., they can open potassium channels or close calcium channels –  Ehsan Abd Sep 3 '13 at 19:43
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1 Answer

Conductance is the inverse of resistance, and measures how much of a given substance flows throught a channel. In this context, it means how many calcium ions enter the cell in a period of time.

There are at least two ways potassium channels may prevent the calcium to enter in the cell.

1) Potassium intake by ion channels decrease the membrane potential, restoring it to its rest state. Since many calcium channels are voltage-dependent, a reduction of the membrane potential would close them, effectively decreasing the calcium conductance.

2) Potassium channels may be coupled to different signaling pathways (i.e. G proteins), wich may affect indirectly other calcium channels.

In the case of opioid receptors it seems to be the first mechanism. With potassium channels open, the neuron is less likely to be activated, because it needs higher stimulation to reach the action potential. Calcium channels open in neurons mainly during action potential events (althought they may exist other calcium channels that open in other conditions, i. e in response to hormones or neuromodulators).

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