Cromone Mechanism of Action

The exact mechanism of action of cromolyn and nedocromil has not been determined. Multiple mechanisms involving ion channel blockade, blockade of signaling of heat shock protein or G-protein, or even blockade of capsaicin receptor have been identified. However, the final common mechanism appears to be an inhibition of mast cell activation.

Studies have reported that the phosphorylation of a 78-kDa-molecular-weight protein prevents mediator release in mast cells (17). More specifically in rat peritoneal mast cells, both medications are reported to phosphorylate a 78-kDa protein from the p and g subunits of the IgE binding protein (FCeRI), which may impair a cell volume-dependent chloride current (17,18). Wang et al. (19) reported that protein kinase C inhibitors prevented phosphorylation of the 78-kDa protein by cromolyn and that this protein was insensitive to protein kinase C activators and Ca2+. This suggests that regulation of an atypical protein kinase C may be involved as an additional mechanism where cromolyn inhibits mast cell activation. The protein kinase C isoenzymes are an important step in signaling cascade involved in the process of mast cell degranulation. Other proteins with molecular weights of 42, 59, and 68-kDa are activated in 10 seconds after the mast cell is challenged with allergen or compound 48/80, whereas the 78-kDa protein responds after 30 to 60 seconds (8). It is possible that termination of mediator release may be associated with phosphorylation of this 78-kDa protein (7). More recently this 78-kDa protein has been identified as moesin, a member of the 4.1 ERM superfamily, which includes ezrin, radixin, and merlin (20,21). These ERM proteins possess actin-binding domains and co-localize with actin at the plasma membrane surface (2). Thus, it is possible that moesin may interact with the cytoskeleton and prevent mast cell activation and secretion of mediators (7).

Furthermore, Garland and Mongor reported that cromolyn inhibited histamine release from rat peritoneal mast cells using phosphatidylserine and calcium (22). Both calcium and phosphatidylserine are required for the action of protein kinase C (8). This suggests that cromolyn may inhibit protein kinase C, which prevents mediator release in the mast cell.

Another study reported that chromones act to inhibit the activation of a chloride current in cells undergoing shape and volume changes (23). Both cromolyn and nedocromil can inhibit chloride transport (24). In rat mucosal mast cells cromolyn has been reported to block an ''intermediate conductance'' chloride channel, which may inhibit the antigen-induced mediator secretion (25). In addition Heinke et al. (23) reported that both medications inhibit chloride current in activated pulmonary endothelial cells exposed to hypotonic saline and reduce open-channel availability of single chloride channels in sheep airway epithelial cells (Fig. 4). Thus if the chloride current isn't activated the membrane will not be hyperpolar-ized to allow for subsequent mast cell degranulation.

Kay et al. (27) have reported that cromolyn can prevent extracellular calcium influx into the cytoplasm of the mast cell. The calcium channel activation that occurs after cross-linking membrane-bound IgE by antigen can be inhibited when mast cells are incubated with cromolyn (28). Thus, by inhibiting calcium influx and mediator release cromolyn may prevent allergic inflammatory responses.

As previously described, cromolyn and nedocromil do not enter the intracellular space due to their physiochemical properties. It is likely that the effects of cromolyn are due to the binding of a membrane receptor at the cell surface. A specific binding site has been reported on rat basophil leukemia cells (RBL-2H3) for cromolyn by Mazurek et al. (28). Later work by other investigators reported that these RBL-2H3 cells were insensitive to the inhibitory effects of CS (8).

Phramacology Astma

Figure 4 Concentration-dependence of the effects of intracellular and extracellular sodium cromoglycate. (A) Hypotonic saline (HTS)-activated current from three different cells under control conditions (o) and after intracellular loading with 5 (V) and 50 mM (A) sodium cromoglycate. Currents are expressed per unit membrane capacitance (measured before HTS). Note the slower activation of the current in the presence of sodium cromoglycate. (B) Synopsis of the data with extracellular sodium cromoglycate (SCG ■) and nedocromil sodium (o), as well as those by intracellular loading with sodium cromoglycate (4). For extracellular sodium cromoglycate, a K1 value of 310 mM was obtained (see text). The value for intracellular sodium cromoglycate is in the range of 5-10 mM, i.e., nearly two orders of magnitude smaller.

Figure 4 Concentration-dependence of the effects of intracellular and extracellular sodium cromoglycate. (A) Hypotonic saline (HTS)-activated current from three different cells under control conditions (o) and after intracellular loading with 5 (V) and 50 mM (A) sodium cromoglycate. Currents are expressed per unit membrane capacitance (measured before HTS). Note the slower activation of the current in the presence of sodium cromoglycate. (B) Synopsis of the data with extracellular sodium cromoglycate (SCG ■) and nedocromil sodium (o), as well as those by intracellular loading with sodium cromoglycate (4). For extracellular sodium cromoglycate, a K1 value of 310 mM was obtained (see text). The value for intracellular sodium cromoglycate is in the range of 5-10 mM, i.e., nearly two orders of magnitude smaller.

The inhalation of adenosine results in bronchoconstriction in asthmatic patients. Tamaoki et al. (29) reported that inhaled adenosine also caused microvascular leakage in sensitized rats. Pretreatment with capsaicin or the tachykinin neurokinin-1 receptor antagonist FK888 prevents this microvas-cular leakage with inhaled adenosine. Moreover, cromolyn also prevents this adenosine-induced vascular extravasation of fluid (29).

Okada et al. (30) reported that cromolyn inhibited part of the heat shock protein 90 (Hsp 90) complex in vitro. The Hsp90 protein may be involved in signaling cascade, leading to mast cell degranulation. This protein can act to prevent protein aggregation and promote refolding in vitro.

Both morphine and certain anesthetic muscle relaxants are known mast cell activators, but the mechanism of this effect has not been completely elucidated. One possible mechanism of morphine and d-Tubocurarine mast cell activation may be through activation of G-proteins. At concentrations of 10 mM and 100 mM DSCG reduced the stimulation of these

G-proteins by morphine by 50% and 80%, respectively, possibly through direct inhibition of the G-proteins and resultant suppression of mast cell activation (31).

Another possible mechanism for cromolyn may involve guanosine 3', 5' cyclic monophosphate (cGMP). A study with rat peritoneal mast cells showed that exogenously applied cGMP and treatment with DSCG produced a potent inhibition of histamine release (32).

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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