Carbachol-induced sustained tonic contraction of rat detrusor muscle

Objective To investigate the underlying contractile mechanism of the sustai ned tonic contraction (SuTC) induced by repetitive carbachol application in rat detrusor muscles. Materials and methods Longitudinal muscle strips with no mucosa were obtain ed from the anterior wall of the urinary bladder in 12-week-old Sprague-Daw ley rats. Carbachol (5 mu mol/L) was applied repetitively to induce SuTC. T he carbachol-induced SuTC was assessed in the presence of various Ca2+-chan nel blockers and drugs affecting intracellular Ca2+ concentration. Results The first application of carbachol elicited a large phasic contract ion followed by a tonic contraction (TC); the carbachol-induced contraction was completely reversed by washing out the solution. However, the initial phasic contraction was not reproduced after a second or further application of carbachol. There was consistently only a SuTC with no phasic contractio n. The amplitude of the SuTC was 85% of the TC induced by the first carbach ol application. The application of atropine (1 mu mol/L) to the bath comple tely blocked SuTC. The carbachol-induced SuTC was insensitive to nicardipin e (5 mu mol/L) and extracellular polyvalent cations (1 mmol/L, La3+, Co2+, Cd2+, Ni2+). Moreover, a similar SuTC was induced even after the complete e limination of extracellular Ca2+ by adding 2 mmol/L EGTA to the Ca2+-free T yrode solution. To exclude intracellular Ca2+ sources related to the sarcop lasmic reticulum (SR), the effects of SR Ca2+ pump inhibitors, cyclopiazoni c acid (CPA, 10 mu mol/L) and thapsigargin (0.5 mu mol/L) were tested. The carbachol-induced SuTC was insensitive to pretreatment with CPA and/or thap sigargin. To deplete the ryanodine-sensitive Ca2+ pool, muscle strips were repetitively stimulated with caffeine (10 mmol/L) in the presence of 10 mu mol/L ryanodine, which did not affect the carbachol-induced SuTC. Conclusions Although the characteristics of the carbachol-induced SuTC have not been defined, these results show that a significant proportion of the carbachol-induced contraction in rats is contributed by the SuTC, which is present even in the complete absence of external Ca2+. The SuTC was not aff ected by limiting the contributions of internal Ca2+ sources. This suggests that the SuTC in rat bladders is unrelated to known Ca2+ mobilization mech anisms.