Neutral/negative α1-AR antagonists and calcium channel blockers at comparison in functional tests on guinea-pig smooth muscle and myocardium

Background: Constitutive (agonist-independent) activity is a prerogative of many G protein-coupled receptors (GPCRs) including α1-adrenoceptors (α1-ARs). Inhibition of such an activity at α1-AR subtypes by antagonists with negative efficacy is difficult to be adequately tested. Methods: In the present experimental approach, we compared the activity of three calcium channel blockers (nifedipine, diltiazem and verapamil) and of three potent benzodioxane-based α1-AR antagonists, differing for subtype selectivity and inverse agonist properties, in producing smooth muscle relaxation and negative inotropy under the same test conditions. We selected, as benzodioxane derivatives, (S)-WB4101, inverse agonist with slight α1A/α1B-α1D AR selectivity, and two previously developed analogues. Both of these are potent antagonists at α1D-AR, that is the α1- AR subtype suspected of the highest susceptibility to inverse agonists for its high degree of basal activity, but only one is inverse agonist. Results: We found that all the three benzodioxane-related α1-AR antagonists have significant intrinsic relaxant activity on non-vascular smooth muscle and moderate negative inotropic effect, while they do not relax aorta. Their potency is always lower than that of three calcium channel blockers. Conclusions: Intrinsic myorelaxant and negative inotropic activity of the three benzodioxane-based α1-AR antagonist is related neither to a particular profile of α1-AR subtype selectivity nor to whether or not being an inverse agonist, but it parallels the calcium antagonists effects indicating a direct interaction of the three α1-AR antagonists with L-type Ca2+ channels.