Clodronate inhibits contraction and prevents the action of L-type calcium channel antagonists in vascular smooth muscle

Abstract

Clodronate (dichloromethylenebisphosphonate) decreased vasoconstriction of the isolated perfused rat tail artery mediated by norepinephrine and by Ca²⁺ in a K⁺-depolarizing solution. The norepinephrine contractile response was divided into two components by sequential manipulation of the composition of the perfusion fluid, where the first component is due to the release of Ca²⁺ from intracellular stores and the second to the influx of Ca²⁺ from extracellular fluid. Clodronate (20 μM) decreased only the first component of the response at a norepinephrine concentration of 50 nM, and both components of the response at a higher norepinephrine concentration (100 nM). The L-type Ca²⁺ channel blocking drugs, nicardipine (10 nM) and verapamil (1 μM), reduced the second component of the norepinephrine-mediated vasoconstriction, but in the presence of clodronate (20 μM) this blocking action was prevented. These results were confirmed by examining the interaction between clodronate and nicardipine on norepinephrine and K⁺-mediated lanthanum (La³⁺)-resistant unidirectional ⁴⁵Ca uptake. Nicardipine (1–10 nM) decreased the norepinephrine (100 nM) and K⁺-induced (60 mM) La³⁺-resistant unidirection ⁴⁵Ca uptake in a concentration-dependent manner, but in the presence of clodronate (20 μM) this concentration-dependent response was abolished. Thus, clodronate not only reduced agonist-induced Ca²⁺ release from intracellular stores and Ca²⁺ influx through L-type Ca²⁺ channels but also prevented L-type Ca²⁺ channel antagonists from exerting their effect. These results indicate clodronate has two sites of action during vascular smooth muscle contraction: the first on intracellular mobilization of Ca²⁺ and the second on L-type Ca²⁺ channels.