title: Phosphorylation of ventricular sarcolemmal membranes does not alter binding properties of nitrendipine.
abstract: Isoproterenol increased contractility in isolated cat papillary muscles 2-fold with an EC50 of 6.3 X 10(-8) M. Nifedipine (3 X 10(-7) M) reduced contractility in control muscles by 43%; however, inotropic state was restored by isoproterenol with a comparable EC50 of 5 X 10(-8) M. To test the hypothesis that this effect might result from cAMP-dependent phosphorylation of a Ca2+ channel-associated protein, [3H]nitrendipine binding was used to probe the high-affinity 1,4-dihydropyridine site in either phosphorylated or dephosphorylated sarcolemmal vesicles. Kd and Bmax values for binding to phosphorylated sarcolemmal vesicles (0.14 +/- 0.027 nM and 479 +/- 62 fmol/mg protein, respectively) were not significantly different from control values P greater than 0.4). Similarly, dephosphorylation of sarcolemmal vesicles did not alter binding parameters. These data demonstrate that phosphorylation of sarcolemmal vesicles neither alters the binding affinity for [3H]nitrendipine nor promotes an interconversion of dihydropyridine-binding sites from high to low affinity or vice versa. While phosphorylation may regulate the slow Ca2+ channel, this is not reflected as changes in [3H]nitrendipine-binding parameters determined in vitro. Furthermore, the cyclic AMP-dependent phosphorylation state of sarcolemmal proteins does not appear to account for wide variations (more than 100-fold) between Kd values from binding studies and IC50 values determined in pharmacological investigations.
authors.forename: J S
authors.initials: JS
authors.lastname: Hayes
authors.forename: N
authors.initials: N
authors.lastname: Bowling
authors.forename: B G
authors.initials: BG
authors.lastname: Conery
authors.forename: R F
authors.initials: RF
authors.lastname: Kauffman
mesh_terms: D000408:Alamethicin / Q000494:pharmacology; D000818:Animals; D002415:Cats; D004095:Dihydropyridines; D004305:Dose-Response Relationship, Drug; D005260:Female; D006352:Heart Ventricles / Q000378:metabolism; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D009543:Nifedipine / Q000031:analogs & derivatives* / Q000378:metabolism; D009568:Nitrendipine; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism; D011725:Pyridines / Q000378:metabolism; D012508:Sarcolemma / Q000378:metabolism*; D013997:Time Factors
chemical_list: D004095:Dihydropyridines; D011725:Pyridines; D000408:Alamethicin; C038806:1,4-dihydropyridine; D009568:Nitrendipine; D011494:Protein Kinases; D009543:Nifedipine; D007545:Isoproterenol