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Effects of acetylcholine, propranolol, and verapamil on sinus node refractoriness of the rabbit
1Department of Medicine, University Hospital, Vancouver, Canada.
Abstract:
At a critical premature interval, atrial premature beats encounter sinus node refractoriness and are blocked on entering and fail to reset the sinus node, resulting in interpolation of the premature beat. The transition from reset to interpolated response has been used to define the effective refractory period of the sinus node (SNERP). In an in vitro preparation of rabbit sinus node, we evaluated the effects of acetylcholine, propranolol, and verapamil on SNERP. Results obtained in the control state were compared with those obtained during superfusion with drugs, all of which prolonged refractoriness: acetylcholine from 233 +/- 41 (SD) to 325 +/- 88 ms; propranolol from 215 +/- 60 to 241 +/- 67 ms; and verapamil from 192 +/- 69 to 254 +/- 79 ms (p less than 0.005 with all drugs). The site of block of premature beats was mapped between sinus node and crista terminalis with an intracellular microelectrode. All three drugs resulted in block of premature beats at sites farther from the primary pacemaker site. Thus, acetylcholine, propranolol, and verapamil prolong sinus node refractoriness.
Insights
Acetylcholine, propranolol, and verapamil prolong sinus node refractoriness by increasing the effective refractory period (SNERP). These drugs also shifted the site of block for premature beats farther from the sinus node pacemaker.
Area of Science:
- Cardiovascular Physiology
- Electrophysiology
Background:
- The effective refractory period of the sinus node (SNERP) is crucial for understanding cardiac rhythm regulation.
- Atrial premature beats can be blocked or reset the sinus node, providing a method to measure SNERP.
Purpose of the Study:
- To investigate the effects of acetylcholine, propranolol, and verapamil on sinus node refractoriness.
- To determine if these drugs alter the site of block for premature atrial beats.
Main Methods:
- Utilized an in vitro rabbit sinus node preparation.
- Measured SNERP under control conditions and during superfusion with acetylcholine, propranolol, and verapamil.
- Mapped the site of block of premature beats using an intracellular microelectrode.
Main Results:
- All tested drugs (acetylcholine, propranolol, verapamil) significantly prolonged SNERP (p < 0.005).
- Acetylcholine increased SNERP from 233 ms to 325 ms.
- Propranolol and verapamil also prolonged SNERP, with verapamil showing a notable increase from 192 ms to 254 ms.
- Drug superfusion shifted the site of premature beat block farther from the sinus node's primary pacemaker site.
Conclusions:
- Acetylcholine, propranolol, and verapamil effectively prolong sinus node refractoriness.
- These drugs alter the electrophysiological properties of the sinus node, influencing impulse conduction and block.
- The findings contribute to understanding the mechanisms underlying cardiac rhythm control and the effects of common pharmacological agents.