Veratridine elicits histamine release and promotes calcium uptake in mast cells

Study of the activation mechanism of human GRF(1-29)NH2 on rat mast cell histamine release

Human growth releasing factor (GRF) (1-29)NH2 releases histamine from pleural and peritoneal rat mast cells by a non cytotoxic and non immunological mechanism. Pretreatment of cells with pertussis toxin markedly inhibits the secretion, suggesting a possible function of a Gi-protein in the activation pathway. In order to determine the role of cAMP on GRF mediated secretion, mast cells were preincubated with isobutylmethylxanthine (IBMX) or cholera toxin, since both drugs greatly and enhance cAMP levels. IBMX inhibits mediator secretion while, in contrast, cholera toxin is ineffective to modify histamine release. The PKC activator TPA amplifies the response of mast cells to human GRF, shifting the dose-response curve to the left. The pretreatment of mast cells with the phosphatase inhibitor okadaic acid exerts no effect on the dose-response function curve to GRF. The response to human GRF does not depend on extracellular calcium, but there is a good correlation between the percent of histamine released and 45calcium uptake. The kinetic of calcium uptake is fast, maximum uptake being reached in 30 seconds.

Study of the activation mechanism of adriamycin on rat mast cells

Adriamycin (ADR) induces nonimmunological and noncytotoxic histamine release from peritoneal and pleural rat mast cells. This secretion is unaffected by the pretreatment with pertussis toxin, cholera toxin and benzalkonium chloride. Histamine release induced by compound 48/80, was markedly inhibited by pertussis toxin, cholera toxin, benzalkonium chloride and neuraminidase. The ADR dose-response curve is significantly shifted to the right when cells were preincubated with the unspecific phosphodiesterase inhibitor IBMX. The activation of protein kinase C (PKC) with the phorbol esther TPA increases the response to ADR, while PKC inhibition with trifluoperazine decreases histamine release. The pretreatment of mast cells with okadaic acid did not modify the response to ADR. These results suggest that ADR elicits histamine release with a mechanism notably different from compound 48/80.

Effect of okadaic acid on immunologic and non-immunologic histamine release in rat mast cells

We have studied the effect of the protein phosphatase (PP) inhibitor, okadaic acid (OKA) on histamine release elicited by immunologic and non-immunologic stimuli in peritoneal and pleural rat mast cells. When cells were stimulated with antigen (egg albumin), OKA strongly inhibited histamine release. This finding suggests that PP1 and PP2A substrates mediate immunoglobulin E-(IgE) dependent secretion in mast cells. In contrast, after non-immunologic activation of mast cells with different drugs, such as the neuropeptide growth hormone releasing factor (GRF) and the cytostatic agents Adriamycin, navelbine and mitoxantrone, there is no effect of OKA on histamine secretion. These results indicate that IgE-dependent secretion is mediated by substrates of PP1 and PP2A, whereas following non-immunological stimuli, they activate pathways that lead to protein phosphorylation; these proteins are not substrates of PP1 and PP2A.

Veratridine-induced intoxication: an in vitro model for the characterization of anti-ischemic compounds?

Due to the complexity of ischemia-induced cellular dysfunction many different pharmacological approaches have been tested to improve cellular ischemia resistance. However, despite the importance of [Na+]i for ischemia-induced dysfunction, only very few studies have investigated the contribution of the Na+ channel to ischemia-induced failure of intracellular ion homeostasis. Since an activation of Na+ channels by veratridine also results in a failure of intracellular ion homeostasis, the veratridine- and ischemia-induced alterations of cellular function were compared. Moreover, despite the difference in the electrophysiological changes induced by veratridine and ischemia, the possible involvement of a slowly inactivating, less selective Na+ channel in both veratridine- and ischemia-induced cellular dysfunction is discussed. As a conclusion it is suggested that veratridine intoxication could be a helpful in vitro method for the characterization of putative anti-ischemic compounds.

Lack of specific saxitoxin binding to rat mast cells

We studied whether or not mast cells are endowed with specific sodium channels, by using tritiated saxitoxin which binds to site 1 of sodium channels on excitable tissues. Our results suggest that rat pleural and peritoneal mast cells lack specific sodium channels.

Changes in rat mast cell responses in sodium-free media. Lack of demonstrable sodium channel activity

Exposure of rat mast cells to isotonic sucrose (employed as a sodium free medium) increased several-fold the sensitivity to calcium, which itself became a stimulus for exocytosis. Concentrations of the cation as low as 25 microM permitted maximal histamine release. Preincubation of cells in sucrose to allow sodium efflux before adding the ionophore A23187 led to a slower release of histamine. We postulate that sodium efflux can generate a membrane potential that causes the increased sensitivity to calcium and the delay in response after preincubation. The response to A23187 is somewhat unspecific since the ionophore can release histamine from internal calcium reservoirs. Saxitoxin or veratridine did not affect cell responses, so that sodium activity is not mediated through defined sodium channels.