We examined the inhibitory mechanism of BRL37344, a beta-adrenoceptor agonist that is considered to be specific to beta(3)-subtype, on muscarinic receptor-mediated contraction of the rat urinary bladder smooth muscle. BRL37344 produced apparently biphasic concentration-relaxation curves in the urinary bladder smooth muscle contracted with carbachol (0.6 microM). The first and second phases had estimated p D(2) (-logEC(50)) values of 7.80+/-0.34 and 4.62+/-0.18, respectively ( n=6). The first component of the BRL37344 concentration-response curve was not affected by propranolol (1 microM), whereas it was inhibited by higher concentrations of the drug (10 microM or 30 microM). The second component was completely resistant to propranolol. On the other hand, BRL37344 produced monophasic concentration-relaxation of 30 mM KCl-precontracted urinary bladder smooth muscle with a p D(2) value of 8.34+/-0.18 ( n=6). Pretreatment of the urinary bladder smooth muscles with BRL37344 (30, 100 and 300 microM) significantly ( P<0.05) shifted the concentration-response curves for carbachol-induced contractions. In radioligand binding experiments, BRL37344 concentration-dependently displaced the specific binding of [(3)H] N-methyl scopolamine to muscarinic receptors on rat urinary bladder smooth muscle membranes. Additionally, BRL37344 inhibited [(3)H] N-methyl scopolamine binding to cloned human muscarinic receptors (M(1)-M(5)) expressed in Chinese hamster ovary cells. These results suggest that BRL37344 attenuates muscarinic receptor-mediated contractions through prevention of the agonists binding to their receptors, in addition to stimulation of beta(3)-adrenoceptors, in rat urinary bladder.
MaxiK channel, the large-conductance Ca2+-sensitive K+ channel, facilitates a negative feedback mechanism to oppose excitation and contraction in various types of smooth muscles including urinary bladder smooth muscle (UBSM). In this study, we investigated how the contribution of MaxiK channel to the regulation of basal UBSM mechanical activity is altered in streptozotocin-induced diabetic rats. Although the urinary bladder preparations from both control and diabetic rats were almost quiescent in their basal mechanical activities, they generated spontaneous rhythmic contractions in response to a MaxiK channel blocker, iberiotoxin (IbTx). The effect of IbTx on the mechanical activity was significantly greater in diabetic rat than in control animal. Similarly, the basal mechanical activity was increased with apamin, an inhibitor for some types of small conductance Ca2+-sensitive K+ channels, and this effect was more pronounced for diabetic rat. However, in both control and diabetic animals, IbTx action was stronger than that of apamin. Diabetes also enhanced the responses to BayK 8644, an L-type Ca2+ channel agonist. The extent of this enhancement in diabetic bladder vs. control was, however, almost the same as that attained with IbTx. Expression levels for MaxiK channel as well as apamin-sensitive K+ channels and L-type Ca2+ channel were not altered by diabetes, when determined as their corresponding mRNA levels. These results indicate that diabetes can potentially increase the basal UBSM mechanical activity. However, in diabetic UBSM, the main negative-feedback system triggered by MaxiK channel is still preserved enough to counteract the possible enhancement of this smooth muscle mechanical activity.
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