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Cultured heart cells as a model for β-adrenoceptors in a heart pacemaker

Chronotropic spare β-adrenoceptors and spare adenylyl cyclase for (−)-isoprenaline but not for (−)-dichloroisoprenaline in rat cardiocytes

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Summary

Spontaneously beating rat cardiocytes were cultured. The beating frequency and cellular cyclic AMP was measured. Adenylyl cyclase and β-adrenoceptors (labeled with 3H(−)-propranolol) were assayed in a cardiocyte homogenate. Work with spontaneously beating right atria of reserpine-pretreated rats and with chicken cardiocytes is also presented.

  1. 1.

    (−)-Isoprenaline (ISO) and (−)-dichloroisoprenaline (DCI) increased the beating frequency of rat cardiocytes with EC50's of 0.4 nmol/l and 15 nmol/l, respectively; the intrinsic activity of DCI was 0.8. DCI antagonized surmountably the effects of ISO; an equilibrium dissociation constant K P of 20 nmol/l was estimated. On rat atria the EC50 of DCI was 20 nmol/l, the intrinsic activity 0.7 and the K P 15 nmol/l. The quantitative agreement of chronotropic DCI data in cardiocytes and atria shows that cardiocytes are a convenient model for the study of β-adrenoceptors in a heart pacemaker.

  2. 2.

    DCI and low concentrations (<1 nmol/l) of ISO did only increase cellular cyclic AMP of rat cardiocytes in the presence of 1 mmol/l 3-isobutyl-1-methyl-xanthine (MIX). The accumulation of cellular cyclic AMP induced by ISO (with or without MIX) or DCI was not linear with time. The EC50's were 62 nmol/l for ISO and 30 nmol/l for DCI; the intrinsic activity of DCI was 0.1. Stimulation of the adenylyl cyclase of a cardiocyte homogenate by ISO was linear with time; the EC50 for ISO was 79 nmol/l.

  3. 3.

    The positive chronotropic effects and effects on cellular cyclic AMP of ISO and DCI were antagonized surmountably by (−)-bupranolol. Equilibrium dissociation constants K B of 0.2–0.3 nmol/l were estimated for (−)-bupranolol. These K B's agree with K B values from myocardial tissues, suggesting that the effects of ISO and DCI on rat cardiocytes are mediated through the same β-adrenoceptors.

  4. 4.

    3H-(−)-Propranolol bound in saturable manner and stereo-selectively to rat cardiocyte β-adrenoceptors (equilibrium dissociation constant K D=4 nmol/l). (−)-Bupranolol, ISO and DCI competed with 3H-(−)-propranolol for β-adrenoceptors with K D's of 0.7 nmol/l, 107 nmol/l and 26 nmol/l, respectively.

  5. 5.

    Concentrations of ISO that cause near maximal chronotropic effects in rat cardiocytes are considerably below those required for a comparable degree of β-adrenoceptor saturation (spare receptors) and adenylyl cyclase stimulation (spare cyclase). Contrarily, when DCI occupies β-adrenoceptors, chronotropic effects and cyclic AMP effects appear closely associated. An average of 135 β-adrenoceptor-DCI complexes are needed to cause the same chronotropic effect as one β-adrenoceptor-ISO complex.

  6. 6.

    The beating frequency of chicken cardiocytes was only marginally and inconstantly increased by ISO. However, ISO increased cellular cyclic AMP in chicken cardiocytes; the effect was antagonized by 10 nmol/l (−)-bupranolol and partially surmounted by ISO. The data are consistent with the existence of β-adrenoceptors in chicken cardiocytes. Both β-adrenoceptors and adenylyl cyclase appear poorly coupled to ionic channels responsible for pacemaker activity in chicken cardiocytes.

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  1. Lehrstuhl für Klinische Physiologie, Physiologisches Institut der Universität, Universitätsstrasse 1, D-4000, Düsseldorf, Germany

    Alberto J. Kaumann

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Kaumann, A.J. Cultured heart cells as a model for β-adrenoceptors in a heart pacemaker. Naunyn-Schmiedeberg's Arch. Pharmacol. 320, 119–129 (1982). https://doi.org/10.1007/BF00506312

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  • DOI: https://doi.org/10.1007/BF00506312

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