Dissociation of intracellular Ca2+ release and Ca2+ entry response to 5-hydroxytryptamine in cultured canine tracheal smooth muscle cells

G-Protein-coupled receptor agonists differentially regulate basal or tumor necrosis factor-α-stimulated activation of interleukin-6 and RANTES in human airway smooth muscle cells

Using thapsigargin (Tg), an agent that mobilizes calcium by directly emptying intracellular stores, we previously showed that intracellular calcium may play an important role in the regulation of intercellular adhesion molecule (ICAM)-1 gene expression induced by cytokines in human airway smooth muscle (ASM) cells. In the present study, we extended this previous observation by comparing the effect of Tg and other calcium-mobilizing G-protein-coupled receptor (GPCR) agonists on the expression of different pro-inflammatory genes in response to tumor necrosis factor (TNF)-alpha in ASM cells. We found that in resting cells, Tg (10-100 nM) or the bradykinin (BK) (1-10 muM) and thrombin (Thr) (1 U/ml) stimulated interleukin (IL)-6 secretion but had no effect on regulated on activation, normal T cells expressed and secreted (RANTES) levels. More importantly, such calcium-mobilizing agents significantly enhanced TNF-alpha-induced IL-6 secretion while RANTES secretion was abrogated. The use of luciferase-tagged IL-6 and RANTES promoter constructs demonstrated similar effects of Tg on IL-6 and RANTES genes in basal and TNF-alpha-stimulated conditions. The cyclic adenosine monophosphate (cAMP)-dependent pathway plays a minor role in this differential regulation of IL-6 and RANTES genes expression. 2-Aminoethoxydiphenyl borate (APB), a blocker of store-operated calcium channels (SOCs), and bisindolylmaleimide I (Bis I), a broad-spectrum protein kinase C (PKC) inhibitor, inhibited the basal and synergic effects of IL-6 secretion in response to calcium-mobilizing agents and TNF-alpha, but did not prevent the abrogated effect of RANTES secretion. We also found that Go-6976, a selective calcium-dependent PKC isozyme inhibitor, did not inhibit IL-6 secretion in response to GPCR agonist and TNF-alpha; whereas Rottlerlin, a PKC-delta inhibitor, inhibited both Thr- and TNF-alpha-induced expression of IL-6, while BK-induced IL-6 secretion was not affected. Interestingly, TNF-alpha-induced interferon regulatory factor (IRF)-1 activation was significantly inhibited by all calcium-mobilizing agents, BK, Thr and Tg. These results show that calcium-mobilizing GPCR agonists functionally interact with TNF-alpha to differentially regulate pro-inflammatory genes expression in human ASM cells, possibly by involving Tg-sensitive intracellular calcium stores, SOC and PKC.

The frequency of calcium oscillations induced by 5-HT, ACH, and KCl determine the contraction of smooth muscle cells of intrapulmonary bronchioles

Increased resistance of airways or blood vessels within the lung is associated with asthma or pulmonary hypertension and results from contraction of smooth muscle cells (SMCs). To study the mechanisms regulating these contractions, we developed a mouse lung slice preparation containing bronchioles and arterioles and used phase-contrast and confocal microscopy to correlate the contractile responses with changes in [Ca²⁺]_i of the SMCs. The airways are the focus of this study. The agonists, 5-hydroxytrypamine (5-HT) and acetylcholine (ACH) induced a concentration-dependent contraction of the airways. High concentrations of KCl induced twitching of the airway SMCs but had little effect on airway size. 5-HT and ACH induced asynchronous oscillations in [Ca²⁺]_i that propagated as Ca²⁺ waves within the airway SMCs. The frequency of the Ca²⁺ oscillations was dependent on the agonist concentration and correlated with the extent of sustained airway contraction. In the absence of extracellular Ca²⁺ or in the presence of Ni²⁺, the frequency of the Ca²⁺ oscillations declined and the airway relaxed. By contrast, KCl induced low frequency Ca²⁺ oscillations that were associated with SMC twitching. Each KCl-induced Ca²⁺ oscillation consisted of a large Ca²⁺ wave that was preceded by multiple localized Ca²⁺ transients. KCl-induced responses were resistant to neurotransmitter blockers but were abolished by Ni²⁺ or nifedipine and the absence of extracellular Ca²⁺. Caffeine abolished the contractile effects of 5-HT, ACH, and KCl. These results indicate that (a) 5-HT and ACH induce airway SMC contraction by initiating Ca²⁺ oscillations, (b) KCl induces Ca²⁺ transients and twitching by overloading and releasing Ca²⁺ from intracellular stores, (c) a sustained, Ni²⁺-sensitive, influx of Ca²⁺ mediates the refilling of stores to maintain Ca²⁺ oscillations and, in turn, SMC contraction, and (d) the magnitude of sustained airway SMC contraction is regulated by the frequency of Ca²⁺ oscillations.

Role of sarcoplasmic reticulum Ca2+ content in Ca2+ entry of bovine airway smooth muscle cells

Depletion of intracellular Ca(2+) stores induces the opening of an unknown Ca(2+ )entry pathway to the cell. We measured the intracellular free-Ca(2+) concentration ([Ca(2+)]i) at different sarcoplasmic reticulum (SR) Ca(2+) content in fura-2-loaded smooth muscle cells isolated from bovine tracheas. The absence of Ca(2+) in the extracellular medium generated a time-dependent decrement in [Ca(2+)]i which was proportional to the reduction in the SR-Ca(2+) content. This SR-Ca(2+) level was indirectly determined by measuring the amount of Ca(2+) released by caffeine. Ca(2+) restoration at different times after Ca(2+)-free incubation (2, 4, 6 and 10 min) induced an increment of [Ca(2+)]i. This increase in [Ca(2+)]i was considered as Ca(2+) entry to the cell. The rate of this entry was slow (~0.3 nM/s) when SR-Ca(2+) content was higher than 50% (2 and 4 min in Ca(2+)-free medium), and significantly ( p<0.01) accelerated (>1.0 nM/s) when SR-Ca(2+) content was lower than 50% (6 and 10 min in Ca(2+)-free medium). Thapsigargin significantly induced a higher rate of this Ca(2+) entry ( p<0.01). Variations in Ca(2+) influx after SR-Ca(2+) depletion were estimated more directly by a Mn(2+) quench approach. Ca(2+) restoration to the medium 4 min after Ca(2+) removal did not modify the Mn(2+) influx. However, when Ca(2+) was added after 10 min in Ca(2+)-free medium, an increment of Mn(2+) influx was observed, corroborating an increase in Ca(2+) entry. The fast Ca(2+) influx was Ni(2+) sensitive but was not affected by other known capacitative Ca(2+) entry blockers such as La(3+), Mg(2+), SKF 96365 and 2-APB. It was also not affected by the blockage of L-type Ca2(+) channels with methoxyverapamil or by the sustained K(+)-induced depolarisation. The slow Ca(2+) influx was only sensitive to SKF 96365. In conclusion, our results indicate that in bovine airway smooth muscle cells Ca(2+) influx after SR-Ca(2+) depletion has two rates: A) The slow Ca(2+) influx, which occurred in cells with more than 50% of their SR-Ca(2+) content, is sensitive to SKF 96365 and appears to be a non-capacitative Ca(2+) entry; and B) The fast Ca(2+) influx, observed in cells with less than 50% of their SR-Ca(2+) content, is probably a capacitative Ca(2+) entry and was only Ni(2+)-sensitive.

Voltage-dependent inhibition of the muscarinic cationic current in guinea-pig ileal cells by SK&F 96365

The effects of SK&F 96365 on cationic current evoked either by activating muscarinic receptors with carbachol or by intracellularly applied GTPgammaS (in the absence of carbachol) were studied using patch-clamp recording techniques in single guinea-pig ileal smooth muscle cells. SK&F 96365 reversibly inhibited the muscarinic receptor cationic current in a concentration-, time- and voltage-dependent manner producing concomitant alteration of the steady-state I-V relationship shape which could be explained by assuming that increasing membrane positivity increased the affinity of the blocker. The inhibition was similar for both carbachol- and GTPgammaS-evoked currents suggesting that the cationic channel rather than the muscarinic receptor was the primary site of the SK&F 96365 action. Increased membrane positivity induced additional rapid inhibition of the cationic current by SK&F 96365 which was more slowly relieved during membrane repolarization. Both the inhibition and disinhibition time course could be well fitted by a single exponential function with the time constants decreasing with increasing positivity for the inhibition (e-fold per about 12 mV) and approximately linearly decreasing with increasing negativity for the disinhibition. At a constant SK&F 96365 concentration, the degree of cationic current inhibition was a sigmoidal function of the membrane potential with a potential of half-maximal increase positive to about +30 mV and a slope factor of about -13 mV. Increasing the duration of voltage steps at -80 or at 80 mV, increased the percentage inhibition; the degree of inhibition was almost identical at both potentials providing evidence that the same cationic channel was responsible for the cationic current both at negative and at positive potentials. It is concluded that the distinctive and unique mode of SK&F 96365 action on the muscarinic receptor cationic channel is a valuable tool in future molecular biology studies of this channel.