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β 2 ‐Adrenergic receptor overexpression in the developing mouse heart: evidence for targeted modulation of ion channels
Author(s) -
An R.H.,
Heath B. M.,
Higgins J. P.,
Koch W. J.,
Lefkowitz R. J.,
Kass R. S.
Publication year - 1999
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1999.019aa.x
Subject(s) - endocrinology , medicine , genetically modified mouse , myocyte , transgene , patch clamp , ion channel , chemistry , receptor , voltage clamp , biology , electrophysiology , biochemistry , gene
1 We studied the effect of overexpression of the β 2 ‐adrenergic receptor (β 2 ‐AR) in the heart on ion channel currents in single cells isolated from hearts of fetal and neonatal transgenic and wild‐type mice. The β 2 ‐AR transgene construct was under the control of the murine α‐myosin heavy chain (α‐MHC) promoter, and ion channel activity was measured at distinct developmental stages using whole‐cell and perforated patch clamp techniques. 2 We found no change in L‐type Ca 2+ channel current ( I Ca ) density in early embryonic stages (E11‐13) of β 2 ‐AR transgenic positive (TG+) mice, but significant increases in I Ca density in intermediate (E14‐16, 152 %) and late (E17‐19, 173.7 %) fetal and neonatal (1 day post partum, 161 %) TG+ compared with transgenic negative (TG‐) mice. This increase in I Ca was accompanied by a negative shift in the peak of the current‐voltage relationship in TG+ mice. 3 Transient (< 3 min) or prolonged (16‐24 h) exposure of TG‐ neonatal stage myocytes to 8‐Br‐cAMP (300 μM) increased I Ca density and caused a shift in the current‐voltage relationship to a similar extent to that seen in TG+ mice. In TG+ myocytes, 8‐Br‐cAMP had no effect. Exposure of TG+ cells to Rp‐cAMPS reversed both the shift in voltage dependence and reduced the peak current density observed in these myocytes. We concluded from these results that the L‐type Ca 2+ channel is maximally modulated by cAMP‐dependent protein kinase (PKA) in TG+ mice and that the α‐MHC promoter is functional in the ventricle as early as embryonic day 14. 4 In contrast, we found that slow delayed rectifier K + channels were not changed significantly at any of the developmental stages studied by the overexpression of β 2 ‐ARs compared with TG‐ mice. The sensitivity of murine slow delayed rectifier K + channels to cAMP was tested by both transient and prolonged exposure to 8‐Br‐cAMP (300 μM), which increased the slow delayed rectifier K + channel current ( I K,s ) density to a similar extent in both TG‐ and TG+ neonatal myocytes. In addition, we found that there was no difference in the concentration dependence of the response of I Ca and I K,s to 8‐Br‐cAMP. 5 Thus, overexpression of the β 2 ‐AR in the heart results in distinct modulation of I Ca , but not I K,s , and this is not due to differences in the 8‐Br‐cAMP sensitivity of the two channels. Instead, these results are consistent with both compartmentalization of β 2 ‐AR‐controlled cAMP and distinct localization of L‐type Ca 2+ and slow delayed rectifier K + channels. This cAMP is targeted preferentially to the L‐type Ca 2+ channel and is not accessible to the slow delayed rectifier K + channel.