LOW-DENSITY LIPOPROTEINS INDUCE PARASYMPATHETIC RESPONSIVENESS IN EMBRYONIC CHICK VENTRICULAR MYOCYTES IN PARALLEL WITH A COORDINATE INCREASE IN EXPRESSION OF GENES-CODING FOR THE M(2), MUSCARINIC RECEPTOR, G-ALPHA(I2),, AND THE ACETYLCHOLINE-SENSITIVE K+ CHANNEL

Growth of chick atrial cells in medium supplemented with lipoprotein-d epleted serum has been shown to result in an increase in total cell ch olesterol, and an increase in the negative chronotropic response to mu scarinic stimulation in parallel with an increase in levels of muscari nic receptors and the G-protein alpha-subunits alpha i and alpha o (Ha igh, L. S., Leatherman, G. F., O'Hara, D. S., Smith, T. W., and Galper , J. B. (1988) J. Biol. Chem. 263, 15608-15618). In this study we dete rmined whether growth of chick ventricular cells in medium supplemente d with lipoprotein depleted serum could alter levels of muscarinic rec eptors and G-protein alpha-subunits and induce a negative chronotropic response to muscarinic stimulation. We further determined whether lev els of mRNA coding for muscarinic receptors, G-proteins, and the acety lcholine sensitive K+ channel were coordinately regulated. Growth of e mbryonic chick ventricular cells from hearts 14 days in ovo in medium supplemented with lipoprotein depleted serum resulted in a 21 +/- 5% ( n = 3, +/- S.E.) increase in muscarinic receptor number as demonstrate d by [H-3]quinuclidinyl benzilate binding and a 4.7 +/- 1.0 (+/- S.E., n = 4)-fold increase in G alpha(i2) as demonstrated by Western blot a nalysis. These changes in receptor and G-protein were associated with a coordinate increase in levels of mRNA coding for the M(2) muscarinic receptor, G alpha(i2) and the acetylcholine sensitive K+ channel as d etermined by RNase protection. These increases were reversed by additi on of 30 mu M mevinolin, an inhibitor of HMG-CoA reductase activity. C arbamylcholine (0.1 an) had no effect on beat rate in ventricular cell s grown in medium supplemented with fetal calf serum. Cells grown in m edium supplemented with lipoprotein depleted serum demonstrated a 40 /- 8% (+/- S.E., n = 10, p < 0.0001) decrease in beat rate in response to 0.1 mM carbamylcholine which was reversed by the addition of 30 mu M mevinolin. These data suggest that, during growth in medium supplem ented with lipoprotein depleted serum, a component of the cholesterol biosynthetic pathway plays a role in the coordinate induction of mRNAs coding for receptors, G-proteins, and an effector (ion channel) that results in the induction of a parasympathetic response in the ventricu lar cell characteristic of the atrial phenotype.