Spontaneous oscillations of cytoplasmic free calcium ion concentration in cultured smooth muscle cells from guinea pig ileum

P2Y receptor-mediated transient relaxation of rat longitudinal ileum preparations involves phospholipase C activation, intracellular Ca(2+) release and SK channel activation

AIM

Purinergic signaling plays a major role in the enteric nervous system, where it governs gut motility through a number of P2X and P2Y receptors. The aim of this study was to investigate the P2Y receptor-mediated motility in rat longitudinal ileum preparations.

METHODS

Ileum smooth muscle strips were prepared from rats, and fixed in an organ bath. Isometric contraction and relaxation responses of the muscle strips were measured with force transducers. Drugs were applied by adding of stock solutions to the organ bath to yield the individual final concentrations.

RESULTS

Application of the non-hydrolyzable P2 receptor agonists α,β-Me-ATP or 2-Me-S-ADP (10, 100 μmol/L) dose-dependently elicited a transient relaxation response followed by a sustained contraction. The relaxation response was largely blocked by SK channel blockers apamin (500 nmol/L) and UCL1684 (10 μmol/L), PLC inhibitor U73122 (100 μmol/L), IP3 receptor blocker 2-APB (100 μmol/L) or sarcoendoplasmic Ca(2+) ATPase inhibitor thapsigargin (1 μmol/L), but not affected by atropine, NO synthase blocker L-NAME or tetrodotoxin. Furthermore, α,β-Me-ATP-induced relaxation was suppressed by P2Y1 receptor antagonist MRS2179 (50 μmol/L) or P2Y13 receptor antagonist MRS2211 (100 μmol/L), and was abolished by co-application of the two antagonists, whereas 2-Me-S-ADP-induced relaxation was abolished by P2Y6 receptor antagonist MRS2578 (50 μmol/L). In addition, P2Y1 receptor antagonist MRS2500 (1 μmol/L) not only abolished α,β-Me-ATP-induced relaxation, but also suppressed 2-Me-S-ADP-induced relaxation.

CONCLUSION

P2Y receptor agonist-induced transient relaxation of rat ileum smooth muscle strips is mediated predominantly by P2Y1 receptor, but also by P2Y6 and P2Y13 receptors, and involves PLC, IP3, Ca(2+) release and SK channel activation, but is independent of acetylcholine and NO release.

Activation of tyrosine kinase pathway by vanadate in gallbladder smooth muscle

Vanadate, an inhibitor of tyrosine phosphatase activity, might induce gallbladder contraction through the stimulation of the tyrosine kinase pathway. The aim of this study was to characterize the effects of vanadate in the guinea pig gallbladder smooth muscle. Vanadate exerts contractile effects which are not mediated by neurotransmitter release. The tyrosine kinase inhibitor genistein nearly abolished vanadate contraction, suggesting that an increase in protein tyrosine phosphorylation mediates the actions of vanadate. This suggestion was confirmed by Western blot analysis. Vanadate contractions were reduced in the presence of methoxyverapamil or in Ca(2+)-free medium, suggesting that vanadate may induce Ca(2+) influx. Neither inactivation of the Na(+)/K(+) pump nor reversal of the Na(+)/Ca(2+) exchanger can account for vanadate's actions. Vanadate contractile effects were reduced by indomethacin, as well as mepacrine, the inhibitor of phospholipase A(2), but were not affected by phospholipase C inhibitors. Neither inhibitors of diacylglycerol lipase nor protein kinase C reduced the response induced by vanadate. These data indicate that the effects of vanadate on smooth muscle are mainly mediated by protein tyrosine phosphorylation and reveal a new link between tyrosine phosphorylation and arachidonic acid metabolism in the control of gallbladder smooth muscle contraction.

Phenylephrine-induced Ca2+ oscillations in canine pulmonary artery smooth muscle cells

Modulation of [Ca2+]i in response to receptor activation is a critical determinant of vascular smooth muscle tone. In this study, we examined the effect of continuous stimulation of alpha 1-adrenoceptors with phenylephrine (PE) on [Ca2+]i in single pulmonary artery smooth muscle cells (PASMCs) cultured from explants of canine intrapulmonary artery. Fura 2-loaded PASMCs pretreated with propranolol (5 mumol/L) were continuously superfused with PE at 37 degrees C on the stage of an inverted fluorescence microscope, and [Ca2+]i was measured using a dual-wavelength spectrofluorometer. Resting values of [Ca2+]i were 96 +/- 4 nmol/L. PE (10 mumol/L) stimulated oscillations in [Ca2+]i at a frequency of 1.35 +/- 0.07/min, which reached a peak [Ca2+]i of 650 +/- 26 nmol/L (n = 69 cells). The oscillations lasted for > 30 minutes and were constant in amplitude and frequency. Both the amplitude and frequency of PE-induced [Ca2+]i oscillations increased in a dose-dependent (3 x 10(-8) to 10(-4) mol/L) manner. Pretreatment with the alpha 1-adrenoceptor antagonist prazosin (50 nmol/L) or removal of extracellular Ca2+ abolished the repetitive [Ca2+]i oscillations induced by PE. The voltage-operated Ca2+ channel blockers nifedipine (1 mumol/L) and verapamil (1 mumol/L) had no effect on the [Ca2+]i oscillations. In contrast, inhibition of phospholipase C with U73122 (10(-7) to 10(-5) mol/L) attenuated the oscillations in a dose-dependent fashion. The nonselective protein kinase inhibitor staurosporine (10(-9) to 10(-7) mol/L) had a minimal inhibitory effect on the oscillations. Caffeine (30 mmol/L) and thapsigargin (1 mumol/L) abolished the oscillations, whereas pretreatment with ryanodine (1 to 100 mumol/L) had no effect. In freshly dispersed PASMCs, PE (10 mumol/L) induced oscillations in [Ca2+]i similar to those observed in cultured cells, and patch-clamp experiments revealed oscillations in membrane potential. These results indicate that PE induces [Ca2+]i oscillations in PASMCs via stimulation of alpha 1-adrenoceptors coupled to phospholipase C activation. Voltage-operated Ca2+ channels and protein kinases are not required for the oscillations. The requirement for extracellular Ca2+ and intracellular Ca2+ stores indicates that both Ca2+ influx and intracellular Ca2+ release play a role in the maintenance of the oscillations.

Lysophosphatidic acid sensitizes mechanical stress-induced Ca2+ response via activation of phospholipase C and tyrosine kinase in cultured smooth muscle cells

We previously reported that lysophosphatidic acid (LPA) sensitized mechanical stress-induced intracellular free Ca2+ concentration response (Biochem. Biophys. Res. Commun. 208, 19-25, 1995). In the present study, the signal transduction pathway of the sensitizing effect of LPA was investigated in cultured longitudinal muscle cells from guinea pig ileum. Suramin, a putative LPA receptor antagonist, did not affect the response in the presence of 30 nM LPA, suggesting that the response is induced via activation of suramin-insensitive LPA receptor. Neither pertussis toxin nor wortmannin inhibited the LPA-sensitized response, indicating that G(i/o)- and phosphatidylinositol 3-kinase (PI3-kinase)-mediated pathways are not involved in the sensitizing effect. C3 ADP ribosyltransferase had no effect on the response, whereas formation of actin-stress fiber in the presence of LPA was completely inhibited, suggesting rho-related cytoskeletal change is not involved in the response. In contrast, a phospholipase C (PLC) inhibitor, U73122, completely inhibited the response, but broad spectrum kinase inhibitors, staurosporine and H7, had no effect on the response. In addition, tyrosine kinase inhibitor, genistein, but not tyrphostin partially inhibited the response. These results suggest that LPA sensitizes the mechanical stress-induced response via activation of PLC, but not protein kinase C. Additionally, tyrphostin-insensitive tyrosine kinase, which is related to other pathway than G(i/o)- and rho-mediated pathways, may be involved in the response.

Mechanisms of mechanical stress-induced Ca(2+)-mobilization sensitized by lysophosphatidic acid in cultured smooth muscle cells

We have previously reported that lysophosphatidic acid (LPA) sensitizes mechanical stress-induced cytosolic free Ca2+ concentration ([Ca2+]i) response related to Ca2+ entry through Gd(3+)-sensitive ion channels (Biochem. Biophys. Res. Commun. 208 19-25 1995). Here we examined the contribution of Ca2 release from intracellular stores to the mechanical stress-induced [Ca2+]i response sensitized by LPA in cultured longitudinal muscle cells from guinea pig ileum. Although the percentage of responsive cells to the mechanical stress in the presence of 30 nM LPA declined by decreasing extracellular Ca2+ concentration to less than 20 microM, the amplitude of the mechanical stress-induced [Ca2+]i transient did not depend on extracellular Ca2+ concentrations (10 microM-1.8 mM). The [Ca2+]i transient was completely abolished by treatment with thapsigargin. In addition, the amplitude of the [Ca2+]i transient gradually decreased after ryanodine and caffeine treatment. These results indicate that the mechanical stress-induced [Ca2+]i transient in the presence of LPA is mainly due to Ca2+ release from ryanodine-sensitive intracellular stores and may be triggered by Ca2+ influx through Gd(3+)-sensitive ion channels.

Functional coupling of the Na+/Ca2+ exchanger with Ca2+ release from intracellular stores in cultured smooth muscle cells of guinea pig ileum

The mechanism of increase in intracellular Ca2+ concentration ([Ca2+]i) by removal of extracellular Na+, which phenomena were reported previously (Japan. J. Pharmacol. 63 83-91 1993), was investigated in cultured guinea pig ileum longitudinal muscle cells loaded with a fluorescent Ca2+ indicator, fura-2, by digital ratio imaging microscopy. Isotonic substitution of choline chloride for NaCl induced a transient increase in [Ca2+]i. The pretreatment of thapsigargin (0.5 microM), but not nicardipine (10 microM), suppressed the transient increase completely. In solutions containing micromolar concentrations of free Ca2+ (nominally Ca2+-free solution), the Na+-free induced transient increase was observed, but neither the second cell exposure to the Na+-free solution nor the following application of histamine increased [Ca2+]i, indicating that removal of extracellular Na+ releases Ca2+ from intracellular stores including inositol 1,4,5-trisphosphate (IP3)-releasable pools. The Na+-free induced transient increase required the presence of more than micromolar concentrations of extracellular free Ca2+ and releasable Ca2+ within the stores, but ryanodine did not affect the transient increase. These results suggest that undetectable influx of Ca2+ by the reverse-mode action of the Na+/Ca2+ exchanger can release Ca2+ from the thapsigargin-sensitive intracellular stores including IP3-releasable pools.