Positive chronotropic responses produced by α‐adrenoreceptors in the pithed rat

L. ‐H TUNG, M. J. RAND, O. H. DRUMMER, W. J. LOUIS

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Abstract

Phenylephrine (1–100 γg/kg, intravenously) produced dose‐dependent increases in heart rate and blood pressure in the pithed rat. The positive chronotropic response to phenylephrine (10 μg/kg) was reduced in a dose‐dependent manner by propranolol (0.01–0.3 mg/kg), but higher doses of propranolol (up to 3 mg/kg) did not reduce the response by more than about 50%. The residual response was virtually abolished by phentolamine (0.3 mg/kg) or prazosin (3 μg/kg). Labetalol (3 mg/kg) which has both α‐ and β‐blocking activity, also abolished the positive chronotropic response. The pressor response to phenylephrine (1–30 μg/kg) was enhanced by propranolol (1 mg/kg) and abolished by phentolamine (1 mg/kg) and prazosin (30 μg/kg). Labetalol (3 mg/kg) reduced the response to phenylephrine by 73%. Propranolol (0.3 mg/kg) completely blocked the chronotropic and vasodepressor effects of isoprenaline (0.1 μg/kg). It is concluded that phenylephrine acts on both α1‐ and β1‐adrenoreceptors to produce an increase in heart rate, on α1‐adrenoreceptors to produce vasoconstriction and on β2‐adrenoreceptors to produce vasodilation. This latter effect is usually masked by the predominant vasoconstrictor action.

Original languageEnglish
Pages (from-to)217-223
Number of pages7
JournalJournal of Autonomic Pharmacology
Volume2
Issue number4
DOIs
Publication statusPublished - Dec 1982
Externally publishedYes

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