Sources of calcium in agonist-induced contraction of rat distal colon smooth muscle in vitro

Abstract

AIM:To study the origin of calcium necessary for agonist-induced contraction of the distal colon in rats. METHODS:The change in intracel lular calcium concentration ([Ca 2+ ]i) evoked by elevating external Ca 2+ was detected by fura 2/AM fluorescence. Contractile activity was measured with a force displacement transducer. Tension was continuously monitored and recorded using a Powerlab 4/25T data acquisition system with an ML110 bridge bioelectric physiographic amplifier. RESULTS:Store depletion induced Ca 2+ influx had an effect on [Ca 2+ ]i. In nominally Ca 2+ -free medium, the sarco-endoplasmic reticulum Ca 2+ -ATPase inhibitor thapsigargin (1 µmol/L) increased [Ca 2+ ]i from 68 to 241 nmol/L, and to 458 (P < 0.01) and 1006 nmol/L (P < 0.01), respectively, when 1.5 mmol/L and 3.0 mmol/L extracellular Ca 2+ was reintroduced. Furthermore, the change in [Ca 2+ ]i was observed with verapamil (5 µmol/L), La 3+ (1 mmol/L) or KCl (40 mmol/L) in the bathing solution. These channels were sensitive to La 3+ (P < 0.01), insensitive to verapamil, and voltage independent. In isolated distal colons we found that in normal Krebs solution, contraction induced by acetylcholine (ACh) was partially inhibited by verapamil, and the inhibitory rate was 41% (P < 0.05). On the other hand, in Ca 2+ -free Krebs solution, ACh induced transient contraction due to Ca 2+ release from the intracellular stores. The transient contraction lasted until the Ca 2+ store was depleted. Restoration of extracellular Ca 2+ in the presence of atropine produced contraction, mainly due to Ca 2+ influx. Such contraction was not inhibited by verapamil, but was decreased by La 3+ (50 µmol/L) from 0.96 to 0.72 g (P < 0.01). CONCLUSION:The predominant source of activator C a 2 + f o r t h e c o n t ra c t i l e r e s p o n s e t o a g o n i s t i s extracellular Ca 2+ , and intracellular Ca 2+ has little role to play in mediating excitation-contraction coupling by agonists in rat distal colon smooth muscle in vitro . The influx of extracellular Ca 2+ is mainly mediated through voltage-, receptor-and store-operated Ca 2+ channels, which can be used as an alternative to develop new drugs targeted on the dysfunction of digestive tract motility.

Phenytoin inhibits contractions of rat gastrointestinal and portal vein smooth muscle by inhibiting calcium entry

“…This is consistent with the finding that phenytoin produces a strong shift in the [Ca 2+ ]‐response curve of permanently depolarized fundic smooth muscle. The amplitude of ACh‐induced contractions is affected less than high [K + ]‐induced contractions. As it is well‐accepted that ACh mediates contractions in the investigated tissues not only by activating Ca 2+ ‐influx from the extracellular space by opening voltage‐dependent calcium channels, but also by promoting the release of stored Ca 2+ , this finding gives further evidence that phenytoin inhibits calcium channel function The relative inhibition of fundic smooth muscle responses to EFS was more pronounced than the inhibition of ACh‐induced contractions (ACh contractions reduced to 63% of control, EFS‐contractions to 30%).…”

“…As it is well-accepted that ACh mediates contractions in the investigated tissues not only by activating Ca 2+ -influx from the extracellular space by opening voltage-dependent calcium channels, but also by promoting the release of stored Ca 2+ , this finding gives further evidence that phenytoin inhibits calcium channel function. 20 4 The relative inhibition of fundic smooth muscle responses to EFS was more pronounced than the inhibition of ACh-induced contractions (ACh contractions reduced to 63% of control, EFS-contractions to 30%). As the responses were fully abolished by TTX, it can be stated that they were caused by enteric nerve stimulation with subsequent neurotransmitter release.…”

Antispasmodic effects of a new kaurene diterpene isolated from Croton argyrophylloides on rat airway smooth muscle

“…[32,33] Ca 2+ mobilization from M3-receptor stimulation consists of two components: an initial component (phasic component) resulting from IP3-induced Ca 2+ release from the SR, [34] and a sustained component (tonic component) resulting from a dihydropyridine-insensitive Ca 2+ influx through ROCCs [35] and VOCCs. [36] Under Ca 2+ -free conditions in the presence of both nifedipine (to remove the VOCC-mediated Ca 2+ influx) and Ach (to maintain ROCCs opening), TCCA reduced the contractions of tracheal preparations induced by the addition of CaCl2 to the extracellular milieu (Figure 7). This finding, together with the ability of TCCA to reduce ACh-induced contractions in both phasic and tonic components, suggests that its antispasmodic activity is also partially mediated by a decrease in Ca 2+ entry through ROCCs.…”

“…First, TCCA was able to significantly inhibit the 5-HTinduced contraction (Figure 4d). Similar to muscarinic receptor activation, Ca 2+ mobilization by 5-HT through 5-HT2 activation also consists of two components: an initial one resulting from IP3-induced Ca 2+ release from the SR, [36] and a late component due to a Ca 2+ influx from the extracellular medium. [37] Second, TCCA significantly inhibited the phasic component of ACh-induced contractions that mainly results from IP3-induced Ca 2+ release from the SR.…”

Depolarization-Stimulated Contractility of Gastrointestinal Smooth Muscle in Calcium-Free Solution: A Review

“…In rat ileal tissue, neither depolarization nor acetylcholine was able to induce a contraction during exposure to calcium-free solution with 1 mM EGTA [2]. Similar effects were found in Bufo gastric smooth muscle strips [3], rat colonic muscle strips [4], and guinea pig ileal longitudinal muscle strips [5]. …”