Naunyn-Schmiedeberg's Archives of Pharmacology

, Volume 306, Issue 2, pp 119–125 | Cite as

Agonistic and antagonistic effects of various α-adrenergic agonists in human platelets

  • Peter Lasch
  • Karl H. Jakobs


In human platelets, the effects of various α-adrenergic agonists were studied on platelet aggregation and adenylate cyclase activity. Out of many phenylethylamine derivatives tested, only some catecholamines were able to act as α-adrenergic agonists inducing platelet aggregation and inhibition of adenylate cyclase with the order of potency: adrenaline > noradrenaline>α-methylnoradrenaline. Other phenylethylamine and imidazoline derivatives, which act as potent α-adrenergic agonists in various systems, neither induced primary aggregation nor adenylate cyclase inhibition, when tested at concentrations up to 1 mM. Since binding studies indicated high affinities of these agents to the platelet α-adrenergic receptor, their effects on adrenaline-induced aggregation and adenylate cyclase inhibition were studied.

Both types of α-adrenergic agonists tested, phenylethylamine and imidazoline derivatives, prevented adrenaline-induced aggregation and adenylate cyclase inhibition. The imidazolines, xylometazoline, oxymetazoline, naphazoline, clonidine and tetryzolin, were the most effective antagonists with similar potencies as observed with the typical α-adrenergic antagonists, phentolamine and yohimbine. Phenylethylamine derivatives such as phenylephrine, methoxamine, synephrine and norfenefrine, similarly antagonized the adrenaline-induced responses but higher concentrations were required. The potencies of these phenylethylamine derivatives were similar to those of the classical α-adrenergic antagonists, phenoxybenzamine and azapetine. The results indicate that the platelet α-adrenergic receptor, which has many similarities with the α2-adrenergic receptor with regard to affinities of various α-adrenergic agonists, completely differs from that found in other tissues, inasmuch as only some catecholamines acted as agonists whereas other phenylethylamine derivatives and imidazoline derivatives acted as antagonists.

Key words

Platelets α-Adrenergic receptors Platelet aggregation Adenylate cyclase inhibition 


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Copyright information

© Springer-Verlag 1979

Authors and Affiliations

  • Peter Lasch
    • 1
  • Karl H. Jakobs
    • 1
  1. 1.Pharmakologisches InstitutUniversität HeidelbergHeidelbergGermany

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