6?%
Rieees of the upper urinary tract of rabbits and guinea-pigs were bathed in Tyrode solution at 37OC and gassed and 5% CO,, ~h~c-rh~c ~n~actions were induced by 40 mmol/l KC1in rabbit ureter preparations 0,40,and $0 mmol/l KC1 in guinea-pig ureters. Cromakalim (1 10e7 - 1. 10m5 mol/l) antagonized the activity of the ureters ~n~n~ation~~e~de~dy, ~bi~ion of the mean height and of e frequency of tions being almost complete at I * 1W mol/l. The results with cromakalim were compared to those of pinacidil and nicorandil, In addition, derivatives of ~rorn~~ bearing a methyl group in the ii-position of the p~o~~-2-on ring were included: (-)-(3R, 4S, ~“R)-~~~~3,4-~y~~2,2~e~yl~(2’-ox~S’-methyl-l’-p~o~~yl)-2H-benzo[b]pyran-3-ol (S 0121) inhibited the frequency of contraction of ureters similar to eromalcalim but showed only moderate hyperpolarising activity in smooth muscle cells (3.8 f 1.3 mV; n = 5). nts of human ureters and renal pelvis from surgical preparations were depolarised with 80 mmol/l K~l~~t~ Tyrode solution. ~rorn~~ and S 0121 were added curatively from to 10m7 mol/l and the resulting tension was recorded. The relaxing effect of S 0121 was more pronounced compared to cromak&lim. In concl~ion, the data indicate that Kf-channel-openers are potent inhibitors of the KC1 induced phasic-rhythmic activity in isolated prorations of the urinary tract. The observed dependency of the i~bition on the KCl-eoncentration up to 80 mmol/l leads to assume not only an opening of potassium channels but also other relaxing mechanisms, Regarding the structural derivative S 0121 h~~ol~sa~on seems to be of minor importance for its observed relaxation. l
1o.tu.o!J.s 1
Clinically, diabetes mellitus causes alterations in urinary bladder function. Although several ~ves~gators have studied the effects of diabetes on isolated strips of urinary bladder smooth muscle, there is no general agreement on whether diabetes induces any alterations in the contractile response to agonists. In part, the controversy is related to the increased bladder mass which occurs fo~o~g the induction of diabetes. Attempts to normalize the contractile responses may have contributed to the diverse results. In addition, it is unclear whether appropriate length-tension conditions were used for maximal tension development. Studies were done to further examine the effects of diabetes on bladder function. In particular, we examined whether the bladder hypertrophy associated with streptozotoci diabetes in the rat is responsible for any changes in bladder action. This was done by also examinin g bladder function in sucrosed~g rats, a model for the production of non-diabetic bladder hypertrophy. The data were examined to determine whether the mode of expression influenced the results. Active tension developed in response to 32 Hz stimulation was measured after equilibration at 40.5, 1, 2, 3, 4, 5, 7, 9, and 11 g resting tension. Strip lengths were also measured. In addition, the responses to bethanechol and KC1 were examined to exclude the possible effects of diabetes-induced neuropathy. The data were compared using three different methods of expression: 1) absolute grams tension developed; 2) grams tension per mm and 3) grams tension per 100 mg tissue. Diabetes and sucrose&inking causedsignificant increases in bladder mass (Con: 154 it 14; SW: 240 f 12; STZ: 259 f 34 tug). The cross-sectional areas of strips from diabetic rats were significantly increased compared to controls and sucrose-drinking rats (Con: 3.95 jr 0.14; Sue: 4.54 f 0.29; S’IZ: 5.35 f 0.32 mm2) The magnitudes of the passive-active tension curves for all three groups increased with increases in resting tension to a pIateau which was etch from appro~ately I g to 11 g resting tension. No opts resting tension for generation of active tension was found. This isin contrast to skeletal or vascular smooth muscle where, as the tissue is stretched past an optimal resting tension, the active tension developed is reduced. Strips from diabetic or sucrose-drinking rats reached the
675
plateau at a slightly larger resting tension than did strips from control rats. In all instances, the plateau was reached within 2 g resting tension. In addition, strips from diabetic or sucrose&inking rats generally developed a greater active tension than did strips from control rats (see table). Contractile responses to bethanechol and KC1 were of a similar magnitude to nerve stimulation, and showed a similar relationship between the groups. Table 1 Passiv~ac~vetension ~la~ons~p for bladder body strips from control, sucro~-ding,
and diabetic rats.
Tension developed in response to 32 Hz expressed as:
Control (n = 6) Sucrose (n = 7) Diabetic (n = 8)
W=n2)
11.3f19 20.1 f 2.3 * 18.Q 2.0
7.3f0.7 12.2f1.2 10.5f1.5
tg/lO@w) *
27.0&4.1 35.2rt2.5 27.3jr 1.7
’ Active tension at 11 g resting tension. * p c 0.05 (vs. controls, Newman-Keuls test).
The data indicate that comparisons of contractile function of bladder strips from control and diabetic rats should be done at resting tensions of z 2 g to ensure that maximal active tension is generated. Under these seditions, increased responses to nerve st~ulation and to agonists seem to be due to h~~rophy, rather than due to other diabetes-indu~ effects. Our findings suggest that the complex non-linear arrangement of smooth muscle fibers in the bladder wall results in the unusual length-tension curves generated by urinary bladder strips, which maintain maximal active tension at lengths greater than Lo. This relationship could be of benefit to early-stage diabetic patients before neuropa~y development, when the stretching of the bladder wall might allow activation without any compromise of bladder function.
Longmore, J., Weston, A.H. and Trezise, D. SEIZE Mm&eReset&
Grump,~e~~r~rne~~of P~~~ioiogicu~Sciences, ~niuersil~ of ~a~ch~fer,
~an~hes~~r~MI3 PPT, U.#
The measurement of the effects of potassium (K) channel opening drugs on the efflux of K or rubidium (Rb) from smooth muscle tissues generally underestimates the absolute change in K permeability (PK) since these drugs produce membrane hyperpolarisation thus reducing the force driving K out of the cells. In addition, Rb may not be a good substitute for K, since so-meK channels may discriminate between K and Rb (Quast and I3aumlim, 1988). We have exa~n~ the relations~p between changes in PK and PKb and the electrical and rn~h~~c~ cffecps of crom~~m and d&oxide in bovine tracheal smooth muscle. Strips of bovine tracheal smooth muscle were mounted on a manifold fos stmultaneous % and ?Rb efflux studies (Edwards and Weston, 1989) or in a tissue bath for the recording of either isometric tension or membrane potential. The relaxant effects of ~orn~~rn and diazoxide were assessed by pre-contract~g the tissues with 25 mhl KCl. The results are shown in Table 1. Par details of the calculation of pe~eabi~ties see Quast and Baumlin, 1988. Pot cromakalim (1 and 10 PM) the rate of rise and the maximum changes in PK were greate: thaa those for PRb. T,,,, values for cromakalim-induced changes in membrane potential and tension corresponded more closely to the t,, value for PK rather than that for PRb. In contrast, diazoxide produces similar changes in PK and PRb and the t,, values for diazoxide-induced changes in membrane potential and tension corresponded more closely to the t,, value
for PRb. The results may suggest that the effects of cromakalim on PK, membrane potential and tension largely reflect the opening of Rb-~permeable K channels whereas the effects of diazoxide may involve a K channel which is permeable to Rb.