Summary
On the isolated rabbit papillary muscle experiments were carried out to determine whether the positive inotropic effects mediated by α- and by β-adrenoceptors are brought about by different mechanisms or not.—For this reason the influence of temperature and the effect of the calcium antagonist D600 on the responses to phenylephrine and to isoprenaline were compared.
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1.
The maximal inotropic effects of phenylephrine, isoprenaline and calcium were not affected by raising the temperature of the organ bath from 37° to 42°C whereas the basal developed tension of the muscle was significantly decreased.
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2.
The dose-response curve for phenylephrine was markedly shifted to the right by raising the temperature (ΔpD 2=0.89), while that for isoprenaline was also shifted to the right, but to a lesser extent (ΔpD 2=0.23).
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3.
In the presence of 1.5 × 10−8M pindolol the shift of the dose-response curve for phenylephrine induced by elevation of temperature was more prominent (ΔpD 2=1.91), whereas phentolamine (3 × 10−6M) inhibited the temperatureinduced shift.
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4.
The positive inotropic effect of phenylephrine—mediated by α-adrenoceptors under blockade of β-adrenoceptors by 1.5 × 10−8M pindolol—was markedly depressed by D600 (10−7 and 3 × 10−7M): the dose-response curve was shifted to the right and the maximal response was depressed.
On the other hand, the positive inotropic effect of isoprenaline—mediated by β-adrenoceptors—was affected to a lesser extent by D600 and the maximal response was not changed.
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5.
These results indicate that the stimulation of α-adrenoceptors in the rabbit papillary muscle induces a positive inotropic response through a biochemical process different from that caused via β-adrenoceptors, i.e., stimulation of α-adrenoceptors may increase the intracellular calcium level mainly by changing the transmembrane calcium flux.
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This work was supported by the Deutsche Forschungsgemeinschaft.
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Endoh, M., Wagner, J. & Schümann, H.J. Influence of temperature on the positive inotropic effects mediated by α- and β-adrenoceptors in the isolated rabbit papillary muscle. Naunyn-Schmiedeberg's Arch. Pharmacol. 287, 61–72 (1975). https://doi.org/10.1007/BF00632638
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DOI: https://doi.org/10.1007/BF00632638