Human cardiac beta1- or beta2-adrenergic receptor stimulation and the negative chronotropic effect of low-dose pirenzepine.
OBJECTIVE
The M1-muscarinic receptor antagonist pirenzepine in low doses (<1 mg intravenously) decreases heart rate. We investigated whether these effects of pirenzepine differ in volunteers with activated cardiac beta1-adrenergic receptors versus activated cardiac beta2-adrenergic receptors.


METHODS
In 17 male volunteers (25 +/- 1 years) we studied effects of pirenzepine infusion (0.5 mg intravenous bolus followed by continuous infusion of 0.15 microg/kg/min) on heart rate and heart rate-corrected duration of electromechanical systole (QS2c, as a measure of inotropism) that had been stimulated by activation of cardiac beta1-adrenergic receptors (bicycle exercise in the supine position for 60 minutes at 25 W) or cardiac beta2-adrenergic receptors (continuous intravenous infusion of 100 ng/kg/min terbutaline).


RESULTS
Bicycle exercise and terbutaline infusion significantly increased heart rate and shortened QS2c. When pirenzepine was infused 20 minutes after the beginning of the exercise or terbutaline infusion, heart rate decreased in both settings by approximately the same extent (approximately -10 to -14 beats/min), although exercise and terbutaline infusion continued; however, QS2c was not affected. Pirenzepine (0.05 to 1 mg intravenous bolus)-induced decrease in heart rate was abolished after 6 days of transdermal scopolamine treatment of volunteers.


CONCLUSIONS
Low-dose pirenzepine decreased heart rate by muscarinic receptor stimulation, because this was blocked by scopolamine. Moreover, low-dose pirenzepine did not differentiate between cardiac beta1- or beta2-adrenergic receptor stimulation; however, low-dose pirenzepine did not affect cardiac contractility as measured by QS2c. Low-dose pirenzepine therefore exerted a unique pattern of action in the human heart: it decreased heart rate (basal and beta1- and/or beta2-adrenergic receptor-stimulated) without affecting contractility.
Jakubetz
J
J
Institute of Pharmacology and Toxicology and the Department of Anesthesiology, Martin-Luther-University of Halle-Wittenberg, Halle/Saale, Germany.
Schmuck
S
S
Wochatz
G
G
Ruhland
B
B
Poller
U
U
Radke
J
J
Brodde
O E
OE
D000318:Adrenergic beta-Agonists; D000328:Adult; D000704:Analysis of Variance; D018592:Cross-Over Studies; D015444:Exercise / Q000502:physiology; D006321:Heart / Q000187:drug effects*; D006329:Heart Conduction System / Q000187:drug effects; D006339:Heart Rate / Q000187:drug effects; D006801:Humans; D007262:Infusions, Intravenous; D008297:Male; D018727:Muscarinic Antagonists / Q000008:administration & dosage / Q000494:pharmacology*; D009200:Myocardial Contraction / Q000187:drug effects; D010890:Pirenzepine / Q000008:administration & dosage / Q000494:pharmacology*; D018342:Receptors, Adrenergic, beta-1 / Q000187:drug effects*; D018343:Receptors, Adrenergic, beta-2 / Q000187:drug effects*; D012016:Reference Values; D016037:Single-Blind Method; D013726:Terbutaline; D013997:Time Factors
D000318:Adrenergic beta-Agonists; D018727:Muscarinic Antagonists; D018342:Receptors, Adrenergic, beta-1; D018343:Receptors, Adrenergic, beta-2; D010890:Pirenzepine; D013726:Terbutaline