Sympathetic activation affects pacemaker activity by which change in phase 4 and Ca++ entry?

Sympathetic activation affects pacemaker activity by which change in phase 4 and Ca++ entry?

• A. Increases funny channels and opens Ca++ channels earlier, slope of phase 4
• B. Decreases funny channels and opens Ca++ channels later, slope of phase 4
• C. Increases threshold time but shortens AP duration
• D. Decreases phase 4 slope only

Answer: (A)

Sympathetic activation speeds up pacemaker automaticity by boosting the funny current (If) and increasing Ca2+ entry during diastolic depolarization. The rise in cAMP from beta-adrenergic stimulation enhances If, and also increases L-type Ca2+ channel activity, so the diastolic depolarization (phase 4) becomes steeper and Ca2+ enters earlier. This brings the pacemaker cell to threshold more quickly, raising heart rate.

Sympathetic activation speeds up pacemaker automaticity by boosting the funny current (If) and increasing Ca2+ entry during diastolic depolarization. The rise in cAMP from beta-adrenergic stimulation enhances If, and also increases L-type Ca2+ channel activity, so the diastolic depolarization (phase 4) becomes steeper and Ca2+ enters earlier. This brings the pacemaker cell to threshold more quickly, raising heart rate.