Functional uncoupling of the inhibitory alpha 1-adrenergic response from a G-protein in innervated cultured cardiac cells by K+ depolarization.
Under normal physiological conditions, the adult rat heart exhibits an alpha 1-adrenergic mediated decrease in rate. The negative chromotropic effect of alpha 1-stimulation in the adult depends upon maturation of sympathetic innervation and the presence of a pertussis toxin (PT)-sensitive guanine nucleotide binding (G) protein. We have previously used a cell culture model of neonatal myocardial cells to demonstrate more directly that sympathetic innervation is an important feature of the mature response. After alpha 1-adrenergic stimulation, neonatal rat ventricular myocytes cultured with sympathetic ganglion cells [nerve-muscle (NM) co-cultures] respond predominantly by a decrease in rate, whereas pure muscle cultures show an exclusive increase in rate. Since it has been reported that the inhibitory alpha 1-response in intact tissue is lost upon depolarization, the present study was designed to determine whether the negative chronotropic response could be reversed by potassium (K+) depolarization. We also investigated whether there might be an associated reduction in the PT-sensitive G protein linked to the negative chronotropic response. Thus, the effect of high K+ depolarization on both the alpha 1-adrenergic chronotropic response and the level of the PT-sensitive G protein was examined in NM co-cultures. Extracellular high K+ acutely and reversibly converted the phenylephrine-mediated chronotropic response from negative to positive. The positive chronotropic response in high K+ was alpha 1-mediated and not secondary to catecholamine release from adrenergic neurons. Loss of the inhibitory response in high K+ was not associated with a change in the level of the PT-sensitive G protein. Thus, the presence of a PT-sensitive G protein is necessary, but not sufficient to permit the expression of the mature alpha 1-adrenergic negative chronotropic response in innervated cardiac cells in culture.
Han
H M
HM
Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York, NY 10032.
Bilezikian
J P
JP
Robinson
R B
RB
D000818:Animals; D002478:Cells, Cultured; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006321:Heart / Q000294:innervation; D006339:Heart Rate / Q000187:drug effects; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D037342:Pertussis Toxin; D010656:Phenylephrine / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology*; D051381:Rats; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects* / Q000378:metabolism; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D011942:Receptors, Adrenergic, alpha; D010566:Virulence Factors, Bordetella; D010656:Phenylephrine; D037342:Pertussis Toxin; D019204:GTP-Binding Proteins; D011188:Potassium