EFFECTS OF LOW-DENSITY LIPOPROTEINS AND MEVINOLIN ON SYMPATHETIC RESPONSIVENESS IN CULTURED CHICK ATRIAL CELLS - REGULATION OF BETA-ADRENERGIC RECEPTORS AND ALPHA-S

Google Scholar
  • 25 June 1989
    • journal article
    • research article
    • Vol. 264  (18) , 10779-10786
Abstract
We have previously demonstrated that cultures of myocytes from embryonic chick atria grown in media supplemented with fetal calf serum from which lipoproteins have been removed demonstrate a nearly 10-fold increase in sensitivity of beating to the muscarinic cholinergic agonist carbamylcholine compared with cells grown with control medium. This increased response to carbamylcholine was associated with a 1.4-fold increase in total cell cholesterol, a 2-fold increase in number of muscarinic receptors which bind agonist with high affinity, and a 2-fold increase in the levels of the .alpha. subunits of Go and Gi (Haigh, L. S., Leatherman, G. G., O''Hara, D. S. Smith, T. W., and Galper, J. B. (1988) J. Biol. Chem. 263, 15608-15618). In the studies reported here, we determined the responsiveness of cells grown in lipoprotein-depleted serum (LPDS) to .beta.-adrenergic stimulation. Isoproterenol stimulated a contractile response of 58% measured as an increase in amplitude of contraction with a half-maximal effect at 3 .times. 10-7 M for cells grown in fetal serum, but had no significant effect on amplitude of contraction on cells grown in LPDS. In cells grown in media supplemented with fetal calf serum, isoproterenol (1 .times. 10-3 M) stimulated adenylate cyclase activity 100% over basal with and EC50 of 7 .times. 10-6 M compared with an increase of 32% in cells grown in media supplemented with LPDS. .beta.-Adrenergic receptor number as measured by the binding of 125I-pindolol decreased from 24 .+-. 3 (.+-.S.E., n = 6) fmol/mg protein in cells grown under control conditions to 12 .+-. 2 (n = 6) fmol/mg protein in media supplemented with LPDS. The level of .alpha.s as measured both by ADP-ribosylation with cholera toxin in the presence of 32P-NAD and by immunoblotting with specific antibody to .alpha.s decreased by 3-fold in cells grown in media supplemented with LPDS compared with control. All of these effects of growth of cells in LPDS were reversed by incubating cells witH LPDS plus 30 .mu.M mevinolin, an inhibitor of endogenous cholesterol synthesis. These studies indicate that growth of cells in media supplemented with LPDS results in a coordinated decrease in the levels of .beta.-adrenergic receptors and .alpha.s. Taken together with our previous studies these data support the hypothesis that the receptors and guanine nucleotide-binding proteins which mediate sympathetic and parasympathetic responsiveness in the heart are reciprocally regulated.