Ca2+ entry through the store-mediated pathway directly activates only the K+ current but the subsequent Ca2+ release from the store activates both K+ and Cl- currents in submandibular gland acinar cells of the rat

Differential effect of epidermal growth factor on serous and mucous cells in porcine airway submucosal gland

Using a patch-clamp technique, we found that the fresh porcine submucosal gland acinar cells contained two functionally distinct cell populations, i.e. physiologically relevant concentration of acetylcholine (ACh, 30 nM) induced two distinct patterns of electric response in tracheal gland acinar cells. One was characterized by an outstanding oscillatory Cl(-)-current activity, and the other was with poor Cl(-)-current response but with a comparable K(+)-current. We examined the effect of epidermal growth factor (EGF) on the ACh-induced electric responses in these cells. EGF affected only the latter (K(+)-prominent) cell type to potentiate significantly the ACh-induced K(+)-current. An immunohistochemistry revealed that the receptor for EGF was identified preferentially on the mucous, but not serous, cells. Genistein, one of the tyrosine-kinase inhibitors, abolished the augmentation effect of EGF on the ACh-induced current. Thus, we identified the serous cell with a Cl(-)-rich current in response to ACh and the mucous cell with a K(+)-dominant response. Moreover, EGF affected the mucous cells alone to potentiate the ACh-induced electric response. EGF may contribute to the pathophysiological alterations in chronic inflammatory airways both in morphological (mucous cell hypertrophy/hyperplasia) and functional (thick viscous hypersecretion) ways.

A novel function of thyrotropin as a potentiator of electrolyte secretion from the tracheal gland

Accumulating evidence suggests that thyrotropin (thyroid-stimulating hormone [TSH]) plays some roles in immunoregulation by an extrathyroidal action. Because airway submucosal glands are responsible for nonspecific and specific airway defense, we tested the effect of TSH on feline tracheal submucosal gland using a whole-cell patch-clamp technique, immunohistochemistry, and reverse transcription/polymerase chain reaction (RT-PCR). TSH potentiated neurotransmitter-induced ionic currents significantly in a dose-dependent manner. Acetylcholine (10(-)(8) M)- and norepinephrine (10(-)(7) M)-induced inward current (I(i)), which we previously showed to be a Cl(-) current, were increased to about 3-fold the pre-TSH control responses, respectively, by 2.0 ng/ml TSH; and to 6- and 23-fold the control values by 20.0 ng/ml TSH, respectively. TSH alone was without effect up to 20.0 ng/ml. Follicular stimulating hormone only slightly affected the I(i) (1. 5-fold the control). Analyses with immunohistochemistry and RT-PCR failed to identify TSH receptors on the glandular tissue. Maneuvers to raise the cellular adenosine 3',5'-cyclic monophosphate also failed to mimic the TSH-mediated potentiation. The TSH effect appeared to be mediated by a signaling pathway involving tyrosine kinase because its inhibitors (genistein and herbimycin A) abolished the augmentation completely, and interferon-gamma, a tyrosine kinase activator, imitated the TSH action on submucosal gland. Thus, TSH may be an important regulator of airway fluid secretion.

Kinetics of Ca2+ release evoked by photolysis of caged InsP3 in rat submandibular cells

Quantitative time-resolved measurements of cytosolic Ca2+ release by photolysis of caged InsP3 have been made in single rat submandibular cells using patch clamp whole-cell recording to measure the Ca2+-activated Cl- and K+ currents. Photolytic release of InsP3 from caged InsP3 at 100 Joules caused transient inward (V(H) = 60 mV) and outward (V(H) = 0 mV) currents, which were nearly symmetric in their time course. The inward current was reduced when pipette Cl- concentration was decreased, and the outward current was suppressed by K+ channel blockers, indicating that they were carried by Cl- and K+, respectively. Intracellular pre-loading of the InsP3 receptor antagonist heparin or the Ca2+ chelator EGTA clearly prevented both inward and outward currents, indicating that activation of Ca2+-dependent Cl- and K+ currents underlies the inward and the outward currents. At low flash intensities, InsP3 caused Ca2+ release which normally activated the K+ and Cl- currents in a mono-transient manner. At higher intensities, however, InsP3 induced an additional delayed outward K+ current (I[K,(delay)]). I[K(delay)] was independent of the initial K+ current, independent of extracellular Ca2+, inhibited by TEA, and gradually prolongated by repeated flashes. The photolytic release of Ca2+ from caged Ca2+ did not mimic the I[K(delay)]. It is suggested that Ca2+ releases from the InsP3-sensitive pools in an InsP3 concentration-dependent manner. Low concentrations of InsP3 induce the transient Ca2+-dependent Cl- and K+ currents, which reflects the local Ca2+ release, whereas high concentrations of InsP3 induce a delayed Ca2+-dependent K+ current, which may reflect the Ca2+ wave propagation.

P2u purinoceptor modulation of intracellular Ca2+ in a human lung adenocarcinoma cell line: down-regulation of Ca2+ influx by protein kinase C

The human lung small cell adenocarcinoma cell line, A549, demonstrates a concentration-dependent rise in [Ca2+]i in response to extracellular nucleotides. The cells show Ca2+ mobilization on addition of various nucleotides, with an order of agonist potency: UTP > or = ATP > ADP > ADP beta S > AMP; adenosine is ineffective. The EC50 values for UTP and ATP are 12.5 +/- 0.4 microM and 18.9 +/- 0.5 microM, respectively. Together, these results are strongly indicative of the P2U subclass being the major nucleotide receptor expressed in these cells. The Ca2+ response was typically biphasic consisting of an initial spike, representing release of Ca2+ from internal stores, and a subsequent plateau representing Ca2+ influx. The majority of cells showed an agonist-induced Ca2+ increase that was unaffected by pretreatment with the Ca(2+)-ATPase inhibitors 2,5-di(tert-butyl)1,4-benzohydroquinone or thapsigargin. Caffeine did not raise [Ca2+]i above basal levels and applied in conjunction with nucleotide did not attenuate the agonist-mediated response. The Ca2+ influx was sensitive to protein kinase C, and agonist addition in the presence of a protein kinase C inhibitor, D-erythrosphingosine, produced a significantly potentiated Ca2+ influx. Furthermore, agonist-mediated Ca2+ influx was abolished in the presence of a protein kinase C activator, phorbol 12,13-dibutyrate. It is concluded that these cells posses a functional P2U receptor that, upon activation, causes Ca2+ mobilization from TBQ and thapsigargin insensitive stores followed by protein kinase C regulated Ca2+ influx.

Role of extracellular Ca2+ in acetylcholine-induced repetitive Ca2+ release in submandibular gland acinar cells of the rat

Acetylcholine (ACh) caused repetitive transient Cl- currents activated by intracellular Ca2+ in single rat submandibular grand acinar cells. As the concentration of ACh increased the amplitude and the frequency of the transient Cl- currents increased. These responses occurred also in the absence of extracellular Ca2+ but disappeared after several minutes. Repetitive transient Cl- currents were restored by readmission of Ca2+ to the extracellular solution. The higher the concentration of extracellular Ca2+ readmitted, the larger the amplitude of the transient Cl- currents. Ca2+ entry through a store-coupled pathway was detected by application of Ca2+ to the extracellular solution during a brief cessation of stimulation with ACh. In these experiments too, the higher the concentration of Ca2+, the larger the transient Cl- currents activated by Ca2+ released from the stores. The time course of decrease in total charge movements of repetitive transient responses to ACh with removal of extracellular Ca2+ depended on a decrease in charge movements of each transient event rather than a decrease in frequency of the repetitive events. The decrease of charge movements of each transient event was due to a decrease in its amplitude rather than its duration. The results suggest that in this cell type and amplitude-modulated mechanism is involved in repetitive Ca2+ release and that Ca2+ entry is essential to maintain the repetitive release of Ca2+. The results further suggest that the magnitude of Ca2+ entry determines the number of unitary stores filled with Ca2+ which can synchronously respond to ACh.

Depletion of ryanodine-sensitive Ca2+ store activates Ca2+ entry in rat submandibular gland acinar cells

The existence of ryanodine-sensitive Ca2+ stores and their role in the Ca2+ entry mechanism were examined in the rat submandibular gland acinar cells, using the microfluorimetry of intracellular Ca2+ concentration ([Ca2+]i). In the presence of thapsigargin, a Ca(2+)-ATPase inhibitor of inositol (1, 4, 5) triphosphate (InsP3)-sensitive Ca2+ stores, caffeine caused an increase in [Ca2+]i, which was inhibited by treatment with ryanodine (a ligand to the Ca(2+)-induced Ca2+ release channels). In the cells treated with ryanodine, 1 mM Ca2+ addition to a Ca(2+)-free solution caused a marked increase in [Ca2+]i, which was eliminated by application of Ni2+ or SK & F 96365, suggesting a Ca2+ entry triggered by ryanodine. The maximal change in the net increase in [Ca2+]i caused by the ryanodine-coupled Ca2+ entry, was 104.0 +/- 16.0 nM, which intense was caused by 10 microM ryanodine. Emptying the InsP3-sensitive stores by treatment with thapsigargin also caused Ca2+ entry, which maximally changed [Ca2+]i by 349.6 +/- 15.1 nM. Ten mumol/liter ryanodine was confirmed to cause a release of 45Ca2+ from the parotidic microsomal fraction enriched in endopalsmic reticulum. We propose that ryanodine-sensitive Ca2+ stores are present in rat submandibular gland acinar cells. We further propose that release of Ca2+ from the ryanodine-sensitive stores, which means eventually depletion of the ryanodine-sensitive Ca2+ stores, can activate the Ca2+ entry. The ability for Ca2+ entry coupled with the ryanodine-sensitive Ca2+ stores seems to be about 30% of the ability for Ca2+ entry coupled with the thapsigargin-sensitive Ca2+ stores.