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

AIM: To study the origin of calcium necessary for agonist-induced contraction of the distal colon in rats. METHODS: The change in intracellular calcium concentration ([Ca2+]i) evoked by elevating external Ca2+ 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 Ca2+ influx had an effect on [Ca2+]i. In nominally Ca2+-free medium, the sarco-endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin (1 mumol/L) increased [Ca2+]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 Ca2+ was reintroduced. Furthermore, the change in [Ca2+]i was observed with verapamil (5 micromol/L), La3+ (1 mmol/L) or KCl (40 mmol/L) in the bathing solution. These channels were sensitive to La3+ (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 Ca2+-free Krebs solution, ACh induced transient contraction due to Ca2+ release from the intracellular stores. The transient contraction lasted until the Ca2+ store was depleted. Restoration of extracellular Ca2+ in the presence of atropine produced contraction, mainly due to Ca2+ influx. Such contraction was not inhibited by verapamil, but was decreased by La3+ (50 micromol/L) from 0.96 to 0.72 g (P < 0.01). CONCLUSION: The predominant source of activator Ca2+ for the contractile response to agonist is extracellular Ca2+, and intracellular Ca2+ has little role to play in mediating excitation-contraction coupling by agonists in rat distal colon smooth muscle in vitro. The influx of extracellular Ca2+ is mainly mediated through voltage-, receptor- and store-operated Ca2+ channels, which can be used as an alternative to develop new drugs targeted on the dysfunction of digestive tract motility.

Canonical transient receptor potential 1 channel is involved in contractile function of glomerular mesangial cells.

Contractility of mesangial cells (MC) is tightly controlled by [Ca(2+)](i). Ca(2+) influx across the plasma membrane constitutes a major component of mesangial responses to vasoconstrictors. Canonical transient receptor potential 1 (TRPC1) is a Ca(2+)-permeable cation channel in a variety of cell types. This study was performed to investigate whether TRPC1 takes part in vasoconstrictor-induced mesangial contraction by mediating Ca(2+) entry. It was found that angiotensin II (AngII) evoked remarkable contraction of the cultured MC. Downregulation of TRPC1 using RNA interference significantly attenuated the contractile response. Infusion of AngII or endothelin-1 in rats caused a decrease in GFR. The GFR decline was significantly reduced by infusion of TRPC1 antibody that targets an extracellular domain in the pore region of TRPC1 channel. However, the treatment of TRPC1 antibody did not affect the AngII-induced vasopressing effect. Electrophysiologic experiments revealed that functional or biologic inhibition of TRPC1 significantly depressed AngII-induced channel activation. Fura-2 fluorescence-indicated that Ca(2+) entry in response to AngII stimulation was also dramatically inhibited by TRPC1 antibody and TRPC1-specific RNA interference. These results suggest that TRPC1 plays an important role in controlling contractile function of MC. Mediation of Ca(2+) entry might be the underlying mechanism for the TRPC1-associated MC contraction.

[Capacitative Ca²âº entry is involved in ACh-induced distal colon smooth muscle contraction in rats].

Contraction of smooth muscle cells is triggered by an increase in cytosolic Ca(2+) upon agonist stimulation. Ca(2+) influx across the plasma membrane constitutes a major component of the agonist-induced response in smooth muscle cells. Traditionally, voltage-operated Ca(2+) channel (VOCC) is considered as the channel mediating the Ca(2+) entry. However, this view has been challenged by recent discoveries, which demonstrated that other types of ion channels, such as store-operated and/or receptor-operated Ca(2+) channels (SOCC and/or ROCC), also participate in Ca(2+) response induced by agonists in smooth muscle cells. SOCC is defined as the channel activated in response to the depletion of the internal Ca(2+) stores, an event secondary to G protein coupled receptor or receptor tyrosine kinase stimulation. The Ca(2+) flow mediated by SOCC is termed as capacitative Ca(2+) entry (CCE). Previous study from other group has demonstrated that VOCC played a predominant role in ACh-induced contraction of distal colon smooth muscle in guinea pig. However, whether SOCC participates in the agonist-induced contractile response in this particular tissue is unknown. The present study was performed to investigate the role of CCE in ACh-induced mechanical activity of distal colon smooth muscle in rats. The contractile function of the smooth muscle was assessed by measuring isometric force of isolated rat distal colon rings. We showed that both high extracellular K(+) (40 mmol/L) and ACh (5 mumol/L) evoked striking contraction of the smooth muscle. The contractile responses were almost abolished by removal of extracellular Ca(2+) with ethylene glycol-bis(2-aminoethylether)-N,N,N',N' tetraacetic acid (EGTA), suggesting a critical contribution of extracellular source of Ca(2+) to the contraction. Verapamil (5 mumol/L), an L-type VOCC blocker, significantly attenuated, but didn't completely eliminate the high K(+)- and ACh-induced contraction (74% and 41% for high K(+) and ACh, respectively), indicating that additional channels might be involved in the contractile mechanism. Furthermore, ACh only induced transient contractions in the absence of extracellular Ca(2+). Readmission of Ca(2+) into the extracellular compartment resulted in a significant and sustained increase in the tension of the smooth muscle. This response was not affected by verapamil (5 mumol/L) and Cd(2+) (5 mumol/L), both of which efficiently block VOCC at the doses. However, La(3+), a known inhibitor of SOCC, significantly suppressed the Ca(2+) readdition-induced contraction in a dose-dependent manner. On the basis of these results, we conclude that contraction of smooth muscle in the distal colon is regulated by multiple Ca(2+) channels. In addition to VOCC-mediated Ca(2+) influx, SOCC-mediated CCE participates in agonist-induced contractile response of distal colon smooth muscle in rats.

[Store-operated Ca2+ channels in rat colonic smooth muscle cells].

AIM: To study whether store-operated Ca2+ channel (SOC) is present in rat colonic smooth muscle cells. METHODS: Intracellular Ca2+ ([Ca2+]i) changes induced by thapsigargin- or caffeine-activated SOC channel were measured in enzymatically dissociated rat colonic smooth muscle cells with the fluorescent indicator Fura-2/AM. RESULTS: In the absence of external Ca2+ , the sarco-endoplasmic reticulum Ca2+ pump inhibitor thapsigargin (1 micromol/L) and ryanodine receptor (RyR) activator caffeine both transiently elevated [Ca2+]i from (68.32 +/- 3.43) nmol/L to (240.85 +/- 12.65 ) nmol/L, (481.25 +/- 34.77) nmol/L. A subsequent reintroduction of Ca2+ into the extracellular solution resulted in [Ca2+]i further elevated to (457.55 +/- 19.80) nmol/L, (1005.93 +/- 54.62) nmol/L; (643.88 +/- 34.65) nmol/L, (920.16 +/- 43.25) nmol/L, respectively. And the elevated response was blocked by lanthanum (1 mmol/L), but was insensitive to L-type voltage calcium channels blocker verapamil and membrane depolarization. CONCLUSION: SOC activated by store depletion are present in rat colonic smooth muscle cells. And we further prove the existence of such Ca2+ channels in excitable cells.

[Bombesin-mediated non-cholinergic late slow excitatory postsynaptic potentials in guinea pig inferior mesenteric ganglion in vitro].

The effect of bombesin (BOM) on non-cholinergic excitatory synaptic transmission of the guinea pig inferior mesenteric ganglion (IMG) was investigated by intracellular recording. Repetitive stimulation of the colon nerves (1 ms, 25 Hz, 4 s) elicited a burst of action potentials, which was followed by a long-lasting depolarization in 74.3% (52/70) of the IMG neurons. The depolarization was not blocked by nicotinic (d-tubocurarine, 100 micromol/L) and muscarinic (atropine, 1 micromol/L) antagonists, but was eliminated in a low Ca(2+)/high Mg(2+) Krebs solution, indicating that the depolarization was due to the release of non-cholinergic transmitters. Superfusing the ganglia with BOM (10 micromol/L, 1 min) induced a slow depolarization in 66.5% (109/164) neurons tested. The BOM response was not appreciably changed in low Ca(2+)/high Mg(2+) Krebs solution (n=6, P>0.05), suggesting that BOM depolarized the neurons by acting directly on the postsynaptic membrane rather than via a release of other endogenous depolarizing substances. In a total of 102 cells that exhibited late slow excitatory postsynaptic potential (ls-EPSP), superfusion of the ganglia with BOM produced a membrane depolarization in 82 neurons (80%), while the remaining 20 cells (20%) exhibited no response to BOM. In 18 neurons with ls-EPSP, 4 (22%) neurons were sensitive to both BOM and SP; 6 (33%) and 5 (28%) neurons were only sensitive to BOM and SP, respectively. The remaining 3 (17%) neurons were insensitive to both BOM and SP. Membrane resistance (Rm) had no apparent change in 47.3%, 59.5 % of the neurons tested during the ls-EPSP (n=55) and BOM depolarization (n=84), respectively, but had a marked decrease in 38.2%, 27.4%, and a marked increase in the remaining 14.5%, 13.1% of the neurons. However, when the Rm change accompanying ls-EPSP was compared with that accompanying BOM depolarization (n=20) in the same neuron, the changes in Rm were always parallel. Moreover, ls-EPSP (n=6) and BOM depolarization (n=8) were all augmented by conditioning hyperpolarization. The extrapolated values of the reversal potentials of ls-EPSP and BOM depolarization were 46.0+/-8.0 and 50.0+/-7.0 mV (n=8, P>0.05), respectively. In 14 BOM-sensitive neurons, a ls-EPSP was elicited by repetitive colon nerve stimulation. Superfusion of BOM (10 micromol/L) in these cells initially caused a large depolarization and then membrane potential gradually subsided to resting level in the continuous presence of BOM. Stimulation of the presynaptic nerves at this time failed to elicit a detecable ls-EPSP in 2 neurons and induced a much smaller one in 10 cells, while the ls-EPSP in the remaining 2 neurons was not appreciably affected. On the other hand, prolonged superfusion of BOM had no effect on the amplitude and duration of ls-EPSP in 6 BOM-insensititive neurons studied (P>0.05). The amplitude and duration of SP-induced depolarization were not altered by prolonged superfusion of BOM (n=4, P>0.05) Superfusion of tyr(4) D-phe(12) bombesin (1 micromol/L, 10 15 min), a BOM receptor antagonist, did not cause any noticeable changes in passive membrane properties nor block nicotinic f-EPSPs, but markedly suppressed (n=5) or completely abolished (n=11) BOM depolarization in all 16 neurons tested Similarly, tyr(4) D-phe(12) bombesin partially or completely antagonized the ls-EPSP in 9 out of a total of BOM sensitive neurons (n=11). The ls-EPSP elicited in the remaining two neurons was insignificantly affected by this drug. However, following 10 20 min of wash with Krebs solution the ls-EPSP was reversed. In contrast, superfusion of the ganglia with tyr(4) D-phe(12) bombesin did not change the amplitude and duration (P>0.05) of ls-EPSP in 10 BOM-insensitive cells. Similarly, the amplitude and duration of SP-induced depolarization were not appreciably affected by tyr(4) D-phe(12) bombesin (n=6, P>0.05). In conclusion, our results indicate that BOM may be another transmitter mediating the ls-EPSP in the guinea pig IMG and that there is no cross-desensitization of BOM receptors and SP receptors.