Spontaneous phasic activity of the pig urinary bladder smooth muscle: characteristics and sensitivity to potassium channel modulators

Abstract

A hallmark for unstable bladder contractions is hyperexcitability and changes in the nature of spontaneous phasic activity of the bladder smooth muscle. In this study, we have characterized the spontaneous activity of the urinary bladder smooth muscle from the pig, a widely used model for studying human bladder function. Our studies demonstrate that phasic activity of the pig detrusor is myogenic and is influenced by the presence of urothelium. Denuded strips exhibit robust spontaneous activity measured as mean area under the contraction curve (AUC=188.9+/-15.63 mNs) compared to intact strips (AUC=7.3+/-1.94 mNs). Spontaneous phasic activity, particularly the amplitude, is dependent on both calcium entry through voltage-dependent calcium channels and release from ryanodine receptors as shown by inhibition of spontaneous activity by nifedipine and ryanodine respectively. Inhibition of BK(Ca) channels by iberiotoxin (100 nM) resulted in an increase in contraction amplitude (89.1+/-20.4%) and frequency (92.5+/-31.0%). The SK(Ca) channel blocker apamin (100 nM) also increased contraction amplitude (69.1+/-24.3%) and frequency (53.5+/-13.6%) demonstrating that these mechanisms are critical to the regulation of phasic spontaneous activity. Inhibition of K(ATP) channels by glyburide (10 microM) did not significantly alter myogenic contractions (AUC=18.5+/-12.3%). However, K(ATP) channel openers (KCOs) showed an exquisite sensitivity for suppression of spontaneous myogenic activity. KCOs were generally 15 fold more potent in suppressing spontaneous activity compared to contractions evoked by electrical field-stimulation. These studies suggest that potassium channel modulation, particularly K(ATP) channels, may offer a unique mechanism for controlling spontaneous myogenic activity especially those associated with the hyperexcitability occurring in unstable bladders.

Figure 1

Functional role of the urothelium in modulating spontaneous phasic activity of pig bladder. Response of isolated strips taken from away-dome area without urothelium (A) and with urothelium (B). Data shown as the means±s.e.mean (n⩾4).

Figure 2

The effect of L-type Ca²⁺ channel inhibition on spontaneous phasic activity in the pig bladder. (A) Concentration response curve for nifedipine. Shown is a representative tracing. (B) Mean reductions in spontaneous phasic activity by 300 nM nifedipine expressed as a percentage change from baseline response. Data were analysed for changes in overall contractions (AUC), frequency, duration and amplitude. ^*Represents significant differences from baseline responses (P<0.05; n=4).

Figure 3

Effect of ryanodine on the spontaneous phasic activity of the pig bladder. (A) Representative tracing showing reduction in spontaneous phasic activity by 10 μM ryanodine. (B) Mean reductions in spontaneous activity expressed as a percentage change from baseline response. ^*Represents significant differences from baseline responses (P<0.05; n=4).

Figure 4

Effects of iberiotoxin on the spontaneous phasic activity of the pig bladder. (A) Iberiotoxin, (100 nM) increases the spontaneous phasic activity in the pig bladder. Shown is a representative tracing. (B) Mean increases in spontaneous contractions expressed as a percentage change from baseline response. ^*Represents significant differences from baseline responses (P<0.05; n=4).

Figure 5

Effects of apamin on the spontaneous phasic activity of the pig bladder. (A) Apamin, (100 nM) increases the spontaneous phasic activity in the pig bladder. Shown is a representative tracing. (B) Mean increases in spontaneous contractions expressed as a percentage change from baseline response. ^*Represents significant differences from baseline responses (P<0.05; n=4).

Figure 6

The effect of glyburide on the spontaneous phasic activity of the pig bladder. (A) Glyburide (10 μM) has no significant effect on the spontaneous phasic activity of the pig bladder. Shown is a representative tracing. (B) Mean responses in spontaneous contractions expressed as a percentage change from baseline response.

Figure 7

The inhibition of spontaneous phasic activity of the pig bladder by K_ATP channel openers. (A) Concentration response curve for the K_ATP channel opener, P1075 and its reversal by glyburide. Shown is a representative tracing. (B) Mean concentration response curves for P1075 (n=8) and DMSO (n=8). Data were analysed as a percentage of maximum response. (C) The overall inhibition caused by P1075 (30 nM) expressed as a percentage change from baseline was due to a reduction in frequency with no substantial change in duration or amplitude. ^*Represents significant differences from baseline responses (P<0.05; n=4).

Figure 8

Comparison of the potencies of K_ATP channel openers for the inhibition of spontaneous phasic activity with potencies obtained for the suppression of electrical field-stimulated contractions. The correlation coefficient was r²=0.89 and slope=1.12. The dashed line represents 1 : 1 correlation. The K_ATP channel openers are 15 fold more potent in suppressing spontaneous phasic activity than contractions evoked by electrical field-stimulation. The −log IC₅₀ data of KCOs for suppression of electrical field-stimulation taken from Buckner et al., 2000.