Summary
The effects of the putative α-adrenoceptor agonists phenylephrine, methoxamine and clonidine on force of contraction and on calcium-dependent action potentials were studied in guinea-pig papillary muscles.
Phenylephrine increased the force of contraction by stimulating α-adrenoceptors as well as β-adrenoceptors. It increased the amplitude and duration of slow action potentials, but this effect was exclusively due to stimulation of β-adrenoceptors. The positive inotropic effect mediated by α-adrenoceptors can presumably not be explained by an increase in calcium influx during the action potential via the slow inward current.
Methoxamine had no effect on the force of contraction at 10−5 and 10−4 mol/l, but at 10−4 mol/l it slightly decreased amplitude and duration of slow action potentials.
Clonidine produced a large increase in force of contraction and in amplitude and duration of slow action potentials. These effects were due to stimulation of H2-histamine receptors.
It is concluded that in guinea-pig papillary muscle the tested putative α-adrenoceptor agonists do not share a common α-adrenoceptor effect, but produce prominent effects which are mediated through either β-adrenoceptors (phenylcphrine), or H2-histamine-receptors (clonidine) or are non-specific (methoxamine) in nature.
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Sanchez-Chapula, J. Multiple effects of putative α-adrenoceptor agonists on the electrical and mechanical activity of guinea-pig papillary muscle. Naunyn-Schmiedeberg's Arch. Pharmacol. 316, 108–111 (1981). https://doi.org/10.1007/BF00505302
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DOI: https://doi.org/10.1007/BF00505302