Under sympathetic activation, what is the combined effect on time to threshold and AP duration in pacemaker cells?

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Multiple Choice

Under sympathetic activation, what is the combined effect on time to threshold and AP duration in pacemaker cells?

Explanation:
Under sympathetic activation, beta-adrenergic stimulation raises cAMP and activates PKA in pacemaker cells. This speeds up diastolic depolarization by enhancing the funny current (If) and calcium currents that drive phase 4, so the threshold is reached more quickly. At the same time, PKA phosphorylation increases repolarizing potassium currents (notably IKs), speeding up repolarization and shortening the action potential duration. The net effect is a faster rate of firing with a shorter AP, which matches the option describing both a reduced time to threshold and a reduced AP duration.

Under sympathetic activation, beta-adrenergic stimulation raises cAMP and activates PKA in pacemaker cells. This speeds up diastolic depolarization by enhancing the funny current (If) and calcium currents that drive phase 4, so the threshold is reached more quickly. At the same time, PKA phosphorylation increases repolarizing potassium currents (notably IKs), speeding up repolarization and shortening the action potential duration. The net effect is a faster rate of firing with a shorter AP, which matches the option describing both a reduced time to threshold and a reduced AP duration.

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