Baroreflex mechanisms buffering alpha-adrenergic agonists in conscious dogs.
To determine the relative importance of the mechanisms utilized by the arterial baroreflex in buffering the pressor and vasoconstrictor responses to alpha-adrenergic receptor agonists, we studied responses to norepinephrine and phenylephrine in conscious dogs. The dogs were studied 2-8 wk after instrumentation with aortic catheters and aortic electromagnetic flow probes to measure arterial pressure and cardiac output. Total peripheral resistance was calculated on-line by a digital computer. The dogs were studied after beta-adrenergic receptor blockade (propranolol 1.0 mg/kg) to eliminate the complicating inotropic effects of the agonists studied. Norepinephrine (0.2 microgram/kg bolus) increased mean arterial pressure by 30 +/- 3 mmHg, total peripheral resistance by 51 +/- 4 mmHg . l-1 . min-1, and decreased heart rate by 26 +/- 3 beats/min. After arterial baroreceptor denervation, norepinephrine increased mean arterial pressure by 69 +/- 8 mmHg, total peripheral resistance by 48 +/- 6 mmHg . l-1 . min-1, and heart rate did not change. After ganglionic blockade (hexamethonium 40 mg/kg), norepinephrine increased mean arterial pressure by 76 +/- 3 mmHg, total peripheral resistance by 47 +/- 4 mmHg X l-1 X min-1, and heart rate did not change. Only after elimination of the buffering by heart rate by use of cholinergic receptor blockade (atropine 0.1 mg/kg) or ventricular pacing could buffering of the vasoconstrictor responses to alpha-adrenergic receptor agonists be demonstrated. Thus in conscious dogs the primary mechanism for buffering increases in arterial pressure induced by alpha-adrenergic receptor agonists is compensatory changes in heart rate and cardiac output with little buffering of total peripheral resistance.
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D001794:Blood Pressure; D002302:Cardiac Output; D003714:Denervation; D004285:Dogs; D001330:Electronic Data Processing; D006339:Heart Rate / Q000187:drug effects; D018738:Hexamethonium; D006584:Hexamethonium Compounds / Q000494:pharmacology; D009638:Norepinephrine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011311:Pressoreceptors / Q000502:physiology*; D011433:Propranolol / Q000494:pharmacology; D012016:Reference Values; D012018:Reflex / Q000502:physiology*; D014655:Vascular Resistance; D014661:Vasoconstriction / Q000187:drug effects
D000316:Adrenergic alpha-Agonists; D006584:Hexamethonium Compounds; D010656:Phenylephrine; D018738:Hexamethonium; D011433:Propranolol; D009638:Norepinephrine