Norepinephrine raises resting potential OR lowers threshold potential @ SA node?

[Eugenie98]

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Does norepinephrine released by the sympathetic nervous system in the SA node either:

1) raise the slope of the resting potential depolarization so that the threshold potential is reached more quickly, thus inducing the spike phase (aka Vmax) of the action potential to occur more rapidly OR

2) lower the threshold potential without changing the slope of the resting potential depolarization, thus inducing the spike phase (aka Vmax) of the action potential to occur more rapidly

Or is it a combination of the two?

The following books give differing answers:

1) http://books.google.com/books?id=CQ...&hl=en&sa=X&oi=book_result&resnum=7&ct=result

2) http://books.google.com/books?id=XM...=ACfU3U0BKmfmpAKW2eMZMHt3rOKcfR5SIw#PPT275,M1

 

[nfg05]

http://www.cvphysiology.com/Arrhythmias/A005.htm

That page says:

To increase heart rate, the autonomic nervous system increases sympathetic outflow to the SA node, with concurrent inhibition of vagal tone. Inhibition of vagal tone is necessary for the sympathetic nerves to increase heart rate because vagal influences inhibit the action of sympathetic nerve activity. Sympathetic activation, which releases norepinephrine (NE), increases pacemaker rate by decreasing gK+ and increasing slow inward gCa++ and gNa+; the pacemaker current (If) is enhanced. These changes increase the slope of phase 4 so that the pacemaker potential more rapidly reaches the threshold for action potential generation.

That would suggest option 1, but I would hope you know that this will probably not come up in an MCAT question and definitely would be explained in a passage if it were to be asked about.