We have investigated several effects of verapamil (5‐[(3,4‐dimethoxyphenethyl)methylamino]‐2‐(3,4‐dimethoxyphenyl)‐2‐isopro‐pylvaleronitrile) on platelet structure and function. At concentrations below a threshold of ∼4.2 ± 107 molecules · cell−1, verapamil binds to platelets according to a typical Langmuir adsorption isotherm (i.e., binding is saturable and noncooperative). By extrapolation, we calculate that saturation would occur at 6.8 ± 107 ± 1.9 ± 107 molecules platelet−1, with one bound verapamil molecule per two membrane phospholipids. Saturation is never achieved, however, because past the threshold surface concentration, the adsorption isotherm becomes discontinuous and further adsorption becomes a linear function of the concentration of drug in solution. We attribute this discontinuity to disorganization of the membrane bilayer which is stretched beyond cohesion by insertion of too many amphiphilic molecules of a length shorter than that of the phospholipid. The partitioning of verapamil between the bulk aqueous phase and the newly created lipid phase would then account for the linear portion of the adsorption isotherm. The discontinuity of binding is accompanied by discontinuities in the verapamil‐dependent swelling of platelets and the verapamil‐dependent inhibition of both ADP‐inducible binding of fibrinogen to, and aggregation of, platelets. In contrast, N‐methylverapamil (5‐[(3,4‐dimethoxyphenyl) methylamino]‐2‐(3,4‐dimethoxyphenyl)‐2‐isopropylvaleronitrile), a hydrophilic quaternary amine derivative of verapamil, neither swells platelets nor inhibits the ADP‐dependent processes that we investigated. From this we conclude that deprotonated verapamil is the operative species of the drug. Collectively, these data suggest that verapamil alters platelet structure and function by mechanisms involving disorganization of the platelet plasma membrane.
ASJC Scopus subject areas
- Pharmaceutical Science