Effect of calcium-antagonist and calmodulin-antagonist drugs on calmodulin-dependent contractions of chemically skinned vascular smooth muscle from rabbit renal arteries

Summary

• 1. Renal arteries from rabbits were chemically skinned by incubation with Triton X-100, and subsequently stored in buffered glycerol.

• 2. In the presence of Mg-ATP, of EGTA-buffered calcium, and of calmodulin, miniature strips of the skinned arteries developed tension the strength of which was approx. 15–20% of that of viable renal arteries.

• 3. Tension development was dependent on the concentration of both calcium and calmodulin.

• 4. The effect of eight vasodilator drugs, the majority of them being “calmodulin antagonists” or “calcium antagonists”, on the skinned arteries was assessed. In concentrations up to 10⁻³ M, verapamil, D-600, and hydralazine proved to be ineffective, and the same was found with the dihydropyridine derivatives, nifedipine and felodipine, at 0.6×10⁻³ M and 0.8×10⁻⁴ M, respectively, i.e. at saturation in a 9∶1 contracting buffer/ethanol mixture (v/v).

• 5. In a concentration-dependent manner, trifluoperazine, W-7, and fendiline relaxed Ca-calmodulin-induced tension or prevented tension development when given prior to the activation by Ca-calmodulin. However, considerably higher concentrations of the drugs were necessary for half-maximal relaxation than the reported concentrations for half-maximal saturation of hydrophobic binding sites at the calmodulin molecule.

• 6. These findings suggest that at therapeutic blood levels, the vasodilator properties of calcium antagonists and other direct vasodilators cannot be explained by interference with the binding of myosin light chain kinase to calmodulin.