Acta Pharmacologica Sinica (2010) 31: 19–26; doi: 10.1038/aps.2009.164; published online 16 Nov 2009

Aim: To quantitatively assess the effect of lowering external Ca²⁺ ([Ca²⁺]_o) on both endothelium-dependent and -independent relaxations in rabbit aorta. 

Methods: Isometric contractions and relaxations of isolated aortae were recorded.  When assessing the effect of reduced [Ca²⁺]_o on relaxations, the normal [Ca²⁺]_o solution was substituted with one of the reduced [Ca²⁺]_o solutions for one aorta, while a paired aorta was replenished with normal [Ca²⁺]_o solution. 

Results: The extent of acetylcholine (ACh)-induced relaxation, which is dependent on an intact endothelium, is time-dependent, and inversely related to [Ca²⁺]_o in a range of 0.02−2 mmol/L.  ACh-induced relaxations were not significantly altered by the magnitude of the precontraction induced by PGF_2α.  Nitroprusside-induced relaxations, which are independent of the endothelium, are also attenuated by reduced [Ca²⁺]_o.  Relaxant responses to ACh were significantly more susceptible to reduced [Ca²⁺]_o than nitroprusside-induced relaxations.  A maximally effective relaxing concentration of D600, an L-type Ca channel blocker methoxyverapamil, (10^-5 mol/L) attenuated ACh-induced relaxations, whereas nitroprusside-induced relaxations were unaffected by D600. 

Conclusion: Thus, endothelium-dependent relaxation is more dependent on [Ca²⁺]_o than endothelium-independent relaxation, and it seems likely that [Ca²⁺]_o plays an important role not only in contractile processes, but also in relaxant processes as well.

Keywords: calcium; relaxation; vascular smooth muscle; aorta; endothelium; nitroprusside; D600