The inhibition of α1-adrenoceptor-mediated contractions of rabbit pulmonary artery by Ca2+-withdrawal, pertussis toxin and N-ethylmaleimide is dependent on agonist intrinsic efficacy

  • Sabine Liebau
  • Jens Hohlfeld
  • Ulrich Förstermann


Contractions were induced in rings of rabbit pulmonary artery with the preferential α1-adrenoceptor agonists, whereas St 587, clonidine and B-HT 920 were (parchloro-trifluoromethyl-phenylimino)[imidazolidine] and the preferential α2-adrenoceptor agonists, clonidine and B-HT 920 [6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-(4,5-d) azepine]. Phenylephrine and methoxamine acted as full agonists whereas St 587, clonidine and B-HT 920 were partial agonists (intrinsic activities 0.62, 0.38 and 0.42, respectively). Experiments with α1- and α2-adrenoceptor antagonists indicated that the receptors involved are of the α1 type only. Removal of extracellular Ca2+ inhibited maximal contractions to phenylephrine and methoxamine by 30% and 49%, respectively. The remaining contraction components of the full agonists were abolished by the “intracellular Ca2+ antagonist” TMB-8 [8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate]. Contractions to St 587, clonidine and B-HT 920 were virtually abolished in Ca2+-free medium. Pretreatment of the donor rabbits with pertussis toxin (2.5 μ/kg i. v., 5–6 days before sacrifice) attenuated the efficacies of the full agonists, phenylephrine and methoxamine by only 24% and 17%, respectively, whereas maximal contractions to the partial agonists, St 587, clonidine and B-HT 920, were inhibited by 46%, 61% and 75%, respectively. Also the sulfhydryl reagent, N-ethylmaleimide (10 μM), reduced contractile efficacies of phenylephrine and methoxamine to a lesser degree than those of St 587, clonidine and B-HT 920. When agonists were used at equieffective concentrations (i.e. EC30–40 for phenylephrine and methoxamine, EC70–80 for St 587 and EC99 for clonidine and B-HT 920) the degree of inhibition by removal of extracellular Ca2+, pertussis toxin and N-ethylmaleimide was similar for all agonists. These data suggest that a unitary al-receptor may stimulate contractions via two different mechanisms. At a low degree of receptor stimulation, contractions are mediated by a pertussis toxin- and N-ethylmaleimide-sensitive influx of external Ca2+. At a higher degree of receptor stimulation, an additional mechanism is activated which is insensitive to the two G protein inhibitors and mediated by Ca2+ mobilization from intracellular sites.

Key words

Phenylephrine Methoxamine St 587 Clonidine B-HT 920 Postsynaptic α1-adrenoceptor Pertussis toxin 


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. Berridge MJ (1984) Inositol trisphosphate and diacylglycerol as second messengers. Biochem J 220:345–360Google Scholar
  2. Birnbaumer L, Codina J, Mattera R, Yatani A,Scherer N, Toro MJ, Brown AM (1987) Signal transduction by G proteins. Kidney Internat 32 (Suppl. 23):514–537Google Scholar
  3. Brand MD, Felber SM (1984) The intracellular calcium antagonist TMB-8 (8-(NN-diethylamino)octyl-3,4,5-trimethoxybenzoate) inhibits mitochondrial ATP production in rat thymocytes. Biochem J 224:1027–1030Google Scholar
  4. Brown AM, Birnbaumer L (1988) Direct G protein gating of ion channels. Am J Physiol 254: H401-H410Google Scholar
  5. Chiou CY, Malagodi MH (1975) Studies on the mechanism of action of a new Ca2+ antagonist, 8-(N,N-diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride in smooth and skeletal muscles. Br J Pharmacol 53:279–285Google Scholar
  6. Docherty JR (1987) No evidence for the involvement of more than one subtype of αl-adrenoceptor in contractions of the rabbit pulmonary artery (Abstract). Blood Vessels 24:204Google Scholar
  7. Docherty JR, Starke K (1981) Postsynaptic α-adrenoceptor subtypes in rabbit blood vessels and rat anococcyguns muscle studied in vitro. J Cardiovasc Pharmacol 3:854–866Google Scholar
  8. Exton JH (1987) Calcium signalling in cells — molecular mechanisms. Kidney Internal 32 (Suppl. 23):S68-S76Google Scholar
  9. Gilman AG (1986) Receptor-regulated G proteins. Trends Neurosci 9:460–463Google Scholar
  10. Godfraind T, Miller RC, Socrates Lima J (1982) Selective α1- and α2-adrenoceptor agonist-induced contractions and 45Ca fluxes in the rat isolated aorta. Br J Pharmacol 77:597–604Google Scholar
  11. Haeusler G, de Peyer JE, Yajima M, Schultz G (1986) Vascular smooth muscle: availability of calcium through α-adrenoceptor stimulation. J Cardiovasc Pharmacol 8 (Suppl 8):S107-S116Google Scholar
  12. Haeusler G, de Peyer JE, Schulz G (1987) Vascular effects of α1-and α2-adrenoceptor agonists in vitro and in hypertensive rats. J Cardiovasc Pharmacol 10 (Suppl 4):S15-S18Google Scholar
  13. Harden TK, Scheer AG, Smith MM (1982) Differential modification of the interaction of cardiac muscarinic cholinergic and beta-adrenergic receptors with a guanine nucleotide binding component(s). Mol Pharmacol 21:570–580Google Scholar
  14. Holck MI, Jones CH, Haeusler G (1983) Differential interactions of clonidine and methoxamine with the postsynaptic α-adrenoceptor of rabbit main pulmonary artery. J Cardiovasc Pharmacol 5:240–248Google Scholar
  15. Jakobs KH, Lasch P, Minuth M, Aktories K, Schultz G (1982) Uncoupling of α-adrenoceptor-mediated inhibition of human platelet adenylate cyclase by N-ethylmaleimide. J Biol Chem 257:2829–2833Google Scholar
  16. Jakobs KH, Aktories K, Minuth M, Schultz G (1985) Inhibition of adenylate cyclase. In: Cooper DMT, Seamon KB (eds) Advances in cyclic nucleotide and protein phosphorylation research, vol 19. Raven Press, New York, pp 137–150Google Scholar
  17. Legan E, Chernow B, Parrillo J, Roth BL (1985) Activation of phosphatidylinositol turnover in rat aorta by α1-adrenergic receptor stimulation. Eur J Pharmacol 110:389–390Google Scholar
  18. Liebau S, Hohlfeld J, Förstermann U (1988) The postsynaptic α1-adrenoceptor of rabbit pulmonary artery may be coupled to two signal transduction systems that can be distinguished with pertussis toxin, N-ethylmaleimide and low extracellular Ca2+. Naunyn-Schmiedeberg's Arch Pharmacol (Suppl) 338:R63Google Scholar
  19. MacKay D (1978) How should values of pA2 and affinity constants for pharmacological competitive antagonists be estimated? J Pharm Pharmacol 30:312–313Google Scholar
  20. van Meel JCA, De Jonge A, Kalkman HO, Wilffert B, Timmermans PB, van Zwieten PA (1981) Organic and inorganic calcium antagonists reduce vasoconstriction in vivo mediated by postsynaptic α2-adrenoceptors. Naunyn-Schmiedeberg's Arch Pharmacol 316:288–289Google Scholar
  21. van Meel JCA, Wilffert B, De Zoeten K, Timmermans PBMWM, van Zwieten PA (1982) The inhibitory effect of newer calcium antagonists (nimodipine and PY 108–068) on vasoconstriction in vivo mediated by postsynaptic α2-adrenoceptors. Arch Int Pharmacodyn Ther 260:206–217Google Scholar
  22. van Rossum (1963) Cumulative dose-response curves. Arch Int Pharmacodyn Ther 143:299–330Google Scholar
  23. Snedecor GW, Cochran WG (1967) One way classifications-analysis of variance. In: Statistical methods, 6th edn. Iowa State University Press, Ames (Iowa), pp 258–298Google Scholar
  24. Taylor CW, Merritt JE (1986) Receptor coupling to polyphosphoinositide turnover: a parallel with the adenylate cyclase system. Trends Pharmacol Sci 7:238–242Google Scholar
  25. Timmermans PBMWM, van Zwieten PA (1980a) Postsynaptic α1-and α2-adrenoceptors in the circulatory system of the pithed rat: selective stimulation of the α2-type by B-HT 933. Eur J Pharmacol 63:199–202Google Scholar
  26. Timmermans PBMWM, van Zwieten PA (1980b) Vasoconstriction mediated by postsynaptic α2-adrenoceptor stimulation. Naunyn-Schmiedeberg's Arch Pharmacol 313:17–20Google Scholar
  27. Ui M (1984) Islet-activating protein, pertussis toxin: a probe for functions of the inhibitory guanine nucleotide regulatory component of adenylate cyclase. Trends Pharmacol Sci 5: 277–279Google Scholar
  28. Yamamoto H, van Breemen C (1985) Inositol-1,4,5-trisphosphate releases calcium from skinned cultured smooth muscle cells. Biochem Biophys Res Commun 130:270–274Google Scholar

Copyright information

© Springer-Verlag 1989

Authors and Affiliations

  • Sabine Liebau
    • 1
  • Jens Hohlfeld
    • 1
  • Ulrich Förstermann
    • 1
  1. 1.Department of Clinical PharmacologyHannover Medical SchoolHannover 61Federal Republic of Germany

Personalised recommendations