Summary
The effects of phenylephrine on the shape of the contraction curve and on the cyclic adenosine 3′,5′-monophosphate (c-AMP) content were studied in electrically driven (frequency 0.2 Hz) cat papillary muscles. All experiments were done in the presence of 1 μM propranolol in order to minimize interference from β-adrenoceptors.
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1.
Phenylephrine increased the force of contraction in a concentration-dependent manner. Maximal effects (about 200% of control) occurred at 30 μM phenylephrine.
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2.
The positive inotropic effect (PIE) of phenylephrine was antagonized by phentolamine. Phentolamine, 5μM, produced a parallel shift of the concentration-response curve for the PIE of phenylephrine by about two log units to the right.
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3.
The PIE of 30 μM phenylephrine occurred without any detectable increase in the c-AMP levels of the preparations.
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4.
The PIE of 30μM phenylephrine developed about three times more slowly than the PIE of an equieffective concentration of isoprenaline.
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5.
The PIE of phenylephrine was accompanied by significant, concentration-dependent increases in both time to peak force and relaxation time.
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6.
It is concluded that the PIE of phenylephrine in the presence of propranolol is mediated mainly by a stimulation of α-adrenoceptors. It is unlikely to be related to an increase in c-AMP. With respect to time course and influence on the shape of the contraction curve it is qualitatively different from the effects of β-adrenoceptor stimulation. These data are taken to support the hypothesis that the mechanical effects of α-and β-adrenoceptor stimulating agents on the heart are produced by different mechanisms.
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Brückner, R., Hackbarth, I., Meinertz, T. et al. The positive inotropic effect of phenylephrine in the presence of propranolol. Increase in time to peak force and in relaxation time without increase in c-AMP. Naunyn-Schmiedeberg's Arch. Pharmacol. 303, 205–211 (1978). https://doi.org/10.1007/BF00498045
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DOI: https://doi.org/10.1007/BF00498045