THE EFFECT OF VERAPAMIL ON THE Ca²⁺‐TRANSPORTING AND Ca²⁺‐ATPase ACTIVITY OF ISOLATED CARDIAC SARCOLEMMAL PREPARATIONS

Abstract

The effect of (±)‐, (+)‐ and (‐)‐verapamil on the Ca2+‐binding, Ca2+‐transporting activity, and Ca2+‐dependent adenosine triphosphatase (ATPase) activity of isolated cardiac sarcolemmal preparations was studied. Enzymatic treatment was used to establish the nature of the sites facilitating [14C]‐(±)‐verapamil binding. (±)‐Verapamil 1 μm inhibited the passive binding of 45Ca2+. The (+)‐ and (‐)‐isomers were equiactive. (±)‐Verapamil 1 μm inhibited the ATP‐dependent transport of 45Ca2+ and the associated activation of the Ca2+‐sensitive ATPase. The activity resided in the (‐)‐isomer. Lineweaver‐Burk plots for the initial rates of ATP‐dependent transport showed that the inhibition induced by the (‐)‐isomer was accompanied by a reduced Km and Vmax. Enzymatic removal of N‐acetyl neuraminic acid and galactose residues increased [14C]‐(±)‐verapamil binding; removal of N‐acetylglucosamine and treatment with phospholipase C and trypsin decreased the binding. These results have been interpreted to mean that (‐)‐verapamil interferes with the ATP‐dependent Ca2+‐transporting properties of the sarcolemma, and that this effect is accompanied by an altered activity of the intrinsic Ca2+‐sensitive ATPase. N‐acetylneuraminic acid and galactose residues do not provide binding sites for verapamil at the cell surface.

Cat ventricular muscle treated with D600: effects on calcium and potassium currents.

“…In addition, these enzymically dissociated cells lack an intact cell coat (Isenberg & Kl6ckner, 1980) and it is quite possible that the cell coat constitutes a barrier to the penetration of D600 into the cell (cf. Mas-Oliva & Nayler, 1980).…”

Self-association of Verapamil in Aqueous Electrolyte Solution

“…Most of these effects have been attributed to a selective inhibition of slow inward transport of calcium across cell membranes (3,4). Since the drug acts at the level of the cell membrane (5,6), its surface and self-association properties may have a strong influence on its biological activity. Verapamil has been shown to be surface active (7,8) and there is evidence for self-association from its ability to solubilize aromatic compounds in aqueous solution (9).…”

Verapamil Hydrochloride: Pharmacological Properties and Role in Cardiovascular Therapeutics

“…Verapamil does not modify calcium uptake, binding, or exchange by myocardial micro some^.^^^^ It inhibits the ATP-dependent calcium-transporting properties of the sarcolemma and intrinsic calciumsensitive ATPase; this activity resides in the ( -) isomer. 39 Verapamil probably has to pass through the cell membrane to exert its effect^,^^^^^ binding to the inner surface of the ~lasmalemma.~~ Only one molecule of verapamil is required to interact with the receptor…”