Origins of heart rate variability: autonomic blockade of large magnitude, transient bradycardia in conscious rabbits.
BACKGROUND
Heart period sequences are composed of recurrent, similarly shaped, physiologically meaningful, transient heart period structures. One such structure is a large magnitude transient bradycardia (LMTB), which resembles published reports of responses to alerting stimuli. Alerting responses are sensitive to alpha1-adrenoceptor and cholinergic receptor blockade.


OBJECTIVE
To determine whether LMTBs are related to alerting responses by assessing the response of rabbit LMTBs to autonomic inhibitors.


METHODS
Using a Latin square, crossover design, six conscious, unsedated rabbits received placebo, intramuscular glycopyrrolate 0.05 mg/kg and intravenous prazosin 0.15 mg/kg, and six rabbits received placebo, intravenous metoprolol 0.15 mg/kg and intravenous ICI-118,551 0.1 mg/kg. Heart period and blood pressure were continuously recorded, digitized and analyzed off-line.


RESULTS
LMTBs were suppressed by 93% and 95% by alpha1-adrenergic and muscarinic blockade, respectively, but not by beta1-adrenergic or beta2-adrenergic blockade. The drugs altered none of the morphological parameters of shape, magnitude or duration, or any of the hypotensive features.


CONCLUSIONS
LMTBs are similar to alerting responses; they are both expressed as transient hypotensive and bradycardic events that are markedly reduced by muscarinic and alpha1-adrenoceptor blockade but not by beta-adrenoceptor blockade. The presence of LMTBs may indicate ongoing, subtle changes in the laboratory environment.
Roach
D
D
Cardiovascular Research Group, University of Calgary, Calgary, Alberta, Canada.
Thakore
E
E
Sheldon
R
R
D058668:Adrenergic alpha-1 Receptor Antagonists; D000317:Adrenergic alpha-Antagonists / Q000008:administration & dosage*; D058672:Adrenergic beta-2 Receptor Antagonists; D000319:Adrenergic beta-Antagonists / Q000008:administration & dosage*; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects / Q000502:physiology; D001919:Bradycardia / Q000188:drug therapy / Q000503:physiopathology*; D002940:Circadian Rhythm / Q000187:drug effects / Q000502:physiology; D003242:Conscience; D018592:Cross-Over Studies; D004562:Electrocardiography / Q000187:drug effects; D005260:Female; D006024:Glycopyrrolate / Q000008:administration & dosage; D006339:Heart Rate / Q000187:drug effects / Q000502:physiology; D007273:Injections, Intramuscular; D007275:Injections, Intravenous; D008790:Metoprolol / Q000008:administration & dosage; D018727:Muscarinic Antagonists / Q000008:administration & dosage*; D011224:Prazosin / Q000008:administration & dosage; D011817:Rabbits; D014630:Vagus Nerve / Q000187:drug effects*
D058668:Adrenergic alpha-1 Receptor Antagonists; D000317:Adrenergic alpha-Antagonists; D058672:Adrenergic beta-2 Receptor Antagonists; D000319:Adrenergic beta-Antagonists; D018727:Muscarinic Antagonists; D008790:Metoprolol; D006024:Glycopyrrolate; D011224:Prazosin