Summary
The chronotropic and inotropic effects of isoproterenol, adrenaline and phenylephrine were examined in isolated right and left rabbit atria.
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1.
Spontaneous right atrial rate (RAR) and left atrial contractile force (LACF) (electrically stimulated at 2 Hz) were monitored during separate concentration-response curves to all 3 agonists. The order of potency for both parameters was isoproterenol > adrenaline > phenylephrine.
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2.
At maximum agonist effect, adrenaline produced the greatest LACF increase, followed in order by isoproterenol and phenylephrine.
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3.
When phenylephrine (6.0×10−5 M) was added to the left atria maximally stimulated with isoproterenol, an additional 32% increase in LACF was recorded, but an additive effect was not noted when phenylephrine was added to atria maximally stimulated with adrenaline. The LACF achieved by a combination of isoproterenol and phenylephrine approximated the maximum LACF induced by adrenaline alone.
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4.
At maximum agonist effect, isoproterenol and adrenaline produced a greater RAR increase than phenylephrine. Further, phenylephrine did not increase the maximum RAR effect of isoproterenol or adrenaline.
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5.
Adrenergically mediated chronotropic increases appear to be mediated by a beta adrenoceptor and the maximum response to agonists of full intrinsic activity can not be further increased by agonist combinations. However inotropic increases appear to be mediated by 2 adrenoceptor types which may be simultaneously stimulated to produce additive effects. The two adrenoceptors mediating inotropic change are probably alpha and beta.
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Deceased on November 1980. Submitted in final form by Dr. C. M. Mokler of the same department.
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Holl, J.E. Selective additive effect of phenylephrine to the inotropic action of isoproterenol on rabbit left atria. Naunyn-Schmiedeberg's Arch. Pharmacol. 318, 336–339 (1982). https://doi.org/10.1007/BF00501174
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DOI: https://doi.org/10.1007/BF00501174