The possible involvement of the phospholipid phase of membranes in mediating the effects of verapamil on Ca2+ transport.
The effect of verapamil in a model system of A23187-induced Ca2+-uptake into liposomes was studied. This was done in order to separate the effects of verapamil on the lipid phase of membranes from its effects on membraneous proteins. In the absence of A23187, the liposomes exhibited a very low Ca2+ permeability, which did not change with addition of verapamil. Creation of a valinomycin-induced negative inside membrane potential combined with increased membrane permeability to Ca2+ (A23187), increased Ca2+-entry fivefold and more. Addition of verapamil under these conditions led to a further increase in Ca2+ entry. The negative inside polarization of the liposomes' membrane (as estimated from [3H]TPP+ uptake) was not affected by verapamil. [3H] Verapamil bound specifically to native synaptic plasma membranes with a Kd = 87.4 nM +/- 21.5 (SD) and Bmax = 2.19 pmol/mg protein +/- 0.92 (SD). Specific binding to the liposomes could not be demonstrated. High nonspecific binding of up to about 20% of the total verapamil in the external solution was observed (3.8 pmoles [3H]verapamil/mg phospholipid when 30 nM verapamil was used and 50 nmoles/mg phospholipid when 200 microM [3H] verapamil was used). The high nonspecific binding of verapamil to the liposomes had no detectable effect on the fluidity of their membrane, as seen in fluorescence-anisotropy studies with the fluorescent probe DPH.
D001693:Biological Transport, Active / Q000187:drug effects; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D007700:Kinetics; D008081:Liposomes; D008563:Membrane Lipids / Q000502:physiology*; D008954:Models, Biological; D010743:Phospholipids / Q000502:physiology*; D013816:Thermodynamics; D014634:Valinomycin / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology*
D008081:Liposomes; D008563:Membrane Lipids; D010743:Phospholipids; D014634:Valinomycin; D000001:Calcimycin; D014700:Verapamil; D002118:Calcium