“The inhibitory effect of flavoxate was also completely antagonized by combined treatment with any two antagonists for adenosine A1 (8-cyclopentyl-1,3-dipropylxanthine), dopamine D2 (sulpiride) or adrenergic alpha 2 ( yohimbine ) receptors, although each antagonist alone did not significantly block the flavoxate-induced inhibition of cAMP formation.”