Role of BKCa in Stretch-Induced Relaxation of Colonic Smooth Muscle

Abstract

Stretch-induced relaxation has not been clearly identified in gastrointestinal tract. The present study is to explore the role of large conductance calcium-activated potassium channels (BK_Ca) in stretch-induced relaxation of colon. The expression and currents of BK_Ca were detected and the basal muscle tone and contraction amplitude of colonic smooth muscle strips were measured. The expression of BK_Ca in colon is higher than other GI segments (P < 0.05). The density of BK_Ca currents was very high in colonic smooth muscle cells (SMCs). BK_Ca in rat colonic SMCs were sensitive to stretch. The relaxation response of colonic SM strips to stretch was attenuated by charybdotoxin (ChTX), a nonspecific BK_Ca blocker (P < 0.05). After blocking enteric nervous activities by tetrodotoxin (TTX), the stretch-induced relaxation did not change (P > 0.05). Still, ChTX and iberiotoxin (IbTX, a specific BK_Ca blocker) attenuated the relaxation of the colonic muscle strips enduring stretch (P < 0.05). These results suggest stretch-activation of BK_Ca in SMCs was involved in the stretch-induced relaxation of colon. Our study highlights the role of mechanosensitive ion channels in SMCs in colon motility regulation and their physiological and pathophysiological significance is worth further study.

Figure 1

Expression of BK_Ca in smooth muscle tissues from different GI segments of rats. (a) and (b) Protein expression levels of BK_Ca in GI tract. All values are shown as means ± SEM, n = 4; ^∗∗ P < 0.01 versus duodenum; ^# P < 0.05 versus ileum; ^## P < 0.01 versus ileum. (c) A single-channel recording was performed in high-K⁺ solutions in the cell-attached mode at +100 mV. The lower diagram is an extension of the underlined part of the upper diagram. Seven open levels were recorded in the patch.

Figure 2

Activation of BK_Ca by membrane stretch in rat colonic SMCs. (a) The pressure-dependent activation of BK_Ca channels was recorded from a cell-attached patch. The patches were held at +60 mV. (b) The expanded data show the amplitude of the single-channel current was not changed compared with that in other panels (C: closed state). (c) Statistical data of BK_Ca channel open probability (NPo) challenged by negative pressure on cell-attached patch. Values are shown as means ± SEM, n = 5; ^∗ P < 0.05 versus control group; ^∙ P < 0.05 versus −30 mmHg group.

Figure 3

Effects of ChTX, TEA, and TTX on the muscle tone and contraction amplitude of colonic smooth muscle strips. (a) Effects of ChTX, a nonspecific blocker of BK_Ca channels, on the activities of colonic longitudinal muscle. (b) Active contraction amplitude increased dose-dependently by ChTX. (c) ChTX had no effect on the muscle tone of the colon. (d) The effects of TEA, a nonspecific potassium channel blocker, on the activities of colonic longitudinal muscle. (e) Active contraction amplitude increased by TEA. (f) TEA had no effect on the muscle tone of the colon. (g) The effect of TTX, a specific voltage-gated sodium channel blocker, on the activities of the colonic longitudinal muscle. (h) and (i) TTX had no effect on the active contraction amplitude and the muscle tone of the colon. Values are shown as means ± SEM, n = 6; ^∗ P < 0.05.

Figure 4

The tension changes of colon smooth muscle strips induced by stretch stimulation. (a) The longitudinal muscle tension changes in the control group after stretch stimulation. (b) The effect of ChTX on the tension changes induced by stretch. (c) The effect of TEA on the tension changes induced by stretch. (d) and (e) Statistical data of the ratio of the stable muscle tone and the relevant stretched peak tension for longitudinal muscle (d) and circular muscle (e), respectively. Values are shown as means ± SEM, n = 10; ^∗ P < 0.05, ^∗∗ P < 0.01, and ^∗∗∗ P < 0.001 versus control group; ^# P < 0.05 versus ChTX group.

Figure 5

The tension changes induced by stretch stimulations after blocking the nervous system by TTX. (a) The longitudinal muscle tension changes induced by stretch stimulation in the control group. (b) The longitudinal muscle tension changes induced by stretch stimulation in the TTX group. (c) The effects of TTX + ChTX on the longitudinal muscle tension changes induced by stretch. (d) The effects of TTX + IbTX on the longitudinal muscle tension changes induced by stretch. (e) The effects of TTX + TEA on the stretch-induced longitudinal muscle tension changes. (f) and (g) Statistical data of the ratio of stable muscle tone and stretched peak tension from TTX and control groups for longitudinal muscle (f) and circular muscle (g). (h) and (i) Statistical data of the ratio of stable muscle tone and stretched peak tension from TTX, TTX + ChTX, TTX + IbTX, and TTX + TEA groups for longitudinal muscle (h) and circular muscle (i). Values are shown as means ± SEM, n = 9 for TTX, TTX + ChTX, and TTX + TEA groups and n = 6 for TTX + IbTX group; ^∗ P < 0.05, ^∗∗ P < 0.01, and ^∗∗∗ P < 0.001 versus TTX group; ^# P < 0.05 versus TTX + ChTX group.