Functional evidence of inverse agonism in vascular smooth muscle

1 In the present study, depletion of internal Ca2+ stores sensitive to noradrenaline (1 mu M) in rat aorta, is the signal for the entry of extracellular Ca2+, not only to refill the stores but also, in our experimental conditions, to activate the contractile proteins. This induces an increase in the resting tone that constitutes, the first functional evidence of this Ca2+ entry. 2 The fact that methoxamine (100 mu M) reproduces the same processes as noradrenaline but clonidine (1 mu M) does not, indicates that alpha(1)-adrenoceptor activation is related to the increase in the resting tone observed after depletion of adrenoceptor-sensitive internal Ca2+-stores. 3 Benoxathian and WB 4101 (alpha(1A)- and alpha(1D)-adrenoceptor antagonists) selectively inhibit, in a concentration-dependent manner, this mechanical response observed in absence of the agonist, which suggests that these agents can act as inverse agonists and provide a functional model for studying this phenomenon. Since chloroethylclonidine (100 mu M) has no effect on this response, the participation of alpha(1B)-adrenoceptors can be ruled out. 4 Contractile responses to noradrenaline (1 mu M) in Ca2+-free medium were selectively blocked by chloroethylclonidine. This suggests that the response to noradrenaline in Ca2+-free medium mainly depends on the activation of the alpha(1B)-adrenoceptor subtype.