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β-Adrenoceptors have been demonstrated in the urinary bladders of many animals including the guinea pig. However, there is little information on the subtypes involved in the antispasmodic activity of β-adrenoceptor activation in the guinea-pig detrusor. The present study uses the non-selective β-agonist isoproterenol, the antagonist nadolol, the β₂-selective agonists salbutamol and terbutaline, the antagonist ICI 118551, and the β₁-selective antagonist metoprolol, to demonstrate functionally the subtypes existing in the guinea-pig detrusor. Isoproterenol dose-dependently reduces the myogenic activity in the guinea-pig detrusor induced by mild depolarization with 20 mM potassium in the tissue bath. At the supramaximal concentration of 30 µM, isoproterenol achieves 73 ± 2% of the reference maximal response. This activity of isoproterenol is reduced to 9 ± 5, 24 ± 6 and 54 ± 1 % in the total blockade of β, β₁ and β₂ with nadolol, metoprolol and ICI 118551, respectively. Consistently, salbutamol and terbutaline at the same concentration produce only 35 ± 1 and 38 ± 4% of the response, respectively. Thus, both β₁- and β₂-adrenoceptors are present in the detrusor of the guinea-pig urinary bladder. Although activation of either subtype results in antispasmodic action, the larger portion of the antispasmodic activity appears to be associated with the activation of the β₁-subtype.

The effects of K⁺ channel openers and blockers on smooth muscles of vascular and nonvascular origin from guinea pigs have been investigated. Cromakalim, pinacidil, nicorandil and minoxidil sulfate all abolished the spontaneous myogenic activity of the guinea pig portal vein and the KCl-evoked activity of detrusor strips with the same rank order of potency. Whereas both apamin and charybdotoxin stimulated myogenic activity of the detrusor strips, they produced insignificant effects on spontaneously active portal vein strips and failed to antagonize the mechanoinhibitory effects of cromakalim in the two tissues. Glibenclamide, on the other hand, only stimulated the myogenic activity of portal vein strips but antagonized the mechanoinhibitory effects of cromakalim, pinacidil, nicorandil and minoxidil sulfate in both tissues. Rubidium, at millimolar concentrations, stimulated the myogenic activity, and antagonized the actions of cromakalim in both tissues. The data indicate that there are definite functional dissimilarities as exhibited by the differential response of the two tissues to K⁺ channel modulators. These findings may be exploited in the design of new drugs with tissue selectivity.

The mechanoinhibitory effect of estradiol on the myogenic activity of guinea pig urinary bladder detrusor muscles was studied. In detrusor muscles tonically contracted with 80 mmol/l KCl, 17β-estradiol and the nonestrogenic isomer 17α-estradiol at 30 µmol/l reduced the contraction by 64 ± 3 and 59 ± 1 %, respectively. In detrusor muscles maintained in Ca²⁺-free media and depolarized with 80 mmol/l KCl, the contractile response of muscles to the reintroduction of Ca²⁺ was inhibited in a dose-dependent manner by 17β-estradiol, suggesting that 17β-estradiol blocked entry of extracellular Ca²⁺ into bladder smooth muscle cells and reduced the rise of intracellular Ca²⁺ required for contraction. In detrusor muscles mildly depolarized with 15 mmol/l KC1, 17β-estradiol reduced the myogenic activity with an IC₅₀ of 6.8 ± 1.3 µmol/l. The higher activity of 17β-estradiol in this latter test indicated that estradiol could also possess some K⁺ channel opening activity. Glibenclamide at 1 µmol/l did not affect the relaxant activity of 17β-estradiol in detrusor muscles stimulated with 15 mmol/l; however, this activity was diminished in a dose-dependent manner by iberiotoxin. Collectively, these results have demonstrated that in addition to the Ca²⁺ antagonist activity, 17β-estradiol possesses K⁺ channel opening activity in guinea pig urinary bladder smooth muscles, activating probably not the adenosine triphosphate sensitive, but the Ca²⁺-dependent large-conductance K⁺ channels.

A subset of antiandrogen compounds, the N-aryl-3,3,3-trifluoro-2-hydroxy-2-methylpropanamides 1, were found to activate ATP sensitive potassium channels (KATP) and represent a new class of potassium channel openers (PCOs). A structure-activity relationship was carried out on the western region of this series with the goal of obtaining an activator of the ATP sensitive potassium channel suitable for use in the treatment of urge urinary incontinence. In particular three large 4-(N-aryl) substituents, the (N-phenyl-N-methylamino)sulfonyl, benzoyl, and 4-pyridylsulfonyl moieties, yielded non-antiandrogen, KATP potassium channel openers (39, 41, and 64, respectively) that are bladder selective in an in vivo rat model that simultaneously measures bladder contractions, heart rate, and blood pressure. Substitutions of the aryl rings of 41 and 64 gave several derivatives that also display selectivity in the in vivo rat model; however, none appear to offer a substantial advantage over 41 and 64. The PCO activity of 41 and 64 resides in the (S)-(-) enantiomers. ZD6169, 41(S), has been selected into development for the treatment of urge urinary incontinence.