title: Calcium channel blocker inhibition of the calmodulin-dependent effects of thyroid hormone and milrinone on rabbit myocardial membrane Ca2+-ATPase activity.
abstract: The Ca2+-ATPase activity of rabbit myocardial membranes is stimulated in vitro by L-thyroxine and by milrinone, a bipyridine. These effects are concentration dependent and calmodulin requiring. The calcium channel blockers nifedipine and verapamil have been reported to have anti-calmodulin effects in other assay systems. In this study we have examined the effects of nifedipine and verapamil on rabbit myocardial membrane Ca2+-ATPase activity, in the absence (basal activity) and presence of exogenous L-thyroxine (T4), 10(-10) M, and milrinone, 10(-7) M. Basal enzyme activity was inhibited by a minimum of 10(-6) M nifedipine (IC50 of 3.4 X 10(-5) M) and 10(-5) M verapamil (IC50 of 1.5 X 10(-4) M). Both calcium antagonists inhibited enzyme stimulation by T4 and milrinone, with half-maximal inhibition of T4 and milrinone effects, respectively, at 2.9 X 10(-5) M and 9.0 X 10(-6) M nifedipine and 3.0 X 10(-5) M and 5.2 X 10(-5) M verapamil. The addition of exogenous purified calmodulin, 40 ng/micrograms membrane protein, in the presence of 10(-5) M nifedipine or verapamil restored T4-stimulated enzyme activity. Nifedipine and verapamil, each at a concentration of 10(-6) M, significantly inhibited binding of radioiodinated calmodulin to rabbit heart membranes in vitro. These studies provide evidence that nifedipine and verapamil have an anti-calmodulin effect in this myocardial enzyme system. Through interaction with calmodulin, the channel blockers inhibit thyroid hormone and milrinone stimulation of myocardial membrane Ca2+-ATPase.
authors.affiliation: Department of Medicine, State University of New York, Buffalo School of Medicine, Veterans Administration Medical Center.
authors.forename: P R
authors.initials: PR
authors.lastname: Warnick
authors.forename: F B
authors.initials: FB
authors.lastname: Davis
authors.forename: K M
authors.initials: KM
authors.lastname: Mylotte
authors.forename: P J
authors.initials: PJ
authors.lastname: Davis
authors.forename: M P
authors.initials: MP
authors.lastname: Dube
authors.forename: S D
authors.initials: SD
authors.lastname: Blas
mesh_terms: D000818:Animals; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002147:Calmodulin / Q000378:metabolism; D066298:In Vitro Techniques; D020105:Milrinone; D009206:Myocardium / Q000201:enzymology* / Q000378:metabolism; D009543:Nifedipine / Q000494:pharmacology*; D011728:Pyridones / Q000494:pharmacology*; D011817:Rabbits; D012508:Sarcolemma / Q000378:metabolism; D013449:Sulfonamides / Q000494:pharmacology; D013974:Thyroxine / Q000494:pharmacology*; D014700:Verapamil / Q000494:pharmacology*
chemical_list: D002147:Calmodulin; D011728:Pyridones; D013449:Sulfonamides; C017967:W 7; D014700:Verapamil; D000252:Calcium-Transporting ATPases; D009543:Nifedipine; D020105:Milrinone; D013974:Thyroxine