Stimulation of adenosine 3,5-monophosphate formation in mast cells by 5-hydroxytryptamine and guanethidine

The effect of in vivo hyperoxic exposure on the release of endogenous histamine from the rat isolated perfused lung

Female rats were exposed to either 1 atm air or 100% O2 for 12, 24, or 48 hr. The rats were killed, the lungs were removed, and an isolated perfused lung (IPL) system was prepared. The isolated lung preparation was perfused with a modified Krebs-Henseleit buffer in a recirculating system, and the effect of the O2 exposures on histamine release from the IPL was determined. The effect of these O2 exposures on malondialdehyde formation in the IPL also was examined. Maximal release of histamine occurred after 20 min of perfusion. A linear relationship was found between the maximal histamine concentration released into the perfusate and the length of time the rats were exposed to normobaric hyperoxia. Malondialdehyde in lung perfusate also increased in a linear manner with increasing O2 exposure time. Addition of the H1-receptor antagonist, d-chlorpheniramine, to the perfusate completely inhibited basal histamine release from the IPL of both air- and O2-exposed rats, while addition of the H2-receptor antagonist, metamide, potentiated the release process. There was no significant effect demonstrated when an equimolar concentration of atropine was added to the perfusate. Arterial plasma histamine from rats exposed to 100% O2 for 24 hr increased 40% when compared to air-exposed controls, while histamine release from the IPL increased 75%. In conclusion, exposure of rats to normobaric hyperoxia caused both histamine release and malondialdehyde formation. Histamine release probably occurred as a result of a free radical-induced peroxidation of the lipid membrane of histamine-containing mast cells. Release of histamine from the IPL may be an early biochemical marker of damage by O2.

Action of 5-hydroxytryptamine on intestinal ion transport in the rat

1. 5-HT increased the electrical activity of rat jejunum both in vivo and in vitro. The increased potential difference and short-circuit current resulted from a stimulation of electrogenic chloride secretion. NaCl absorption may also have been inhibited. 2. 5-HT did not alter cyclic AMP levels in isolated enterocytes. 3. The 5-HT response in vivo was unaffected by atropine, cyproheptadine, propranolol and hexamethonium. Phenoxybenzamine reduced the maximum response without affecting the dose required to produce a 50% maximum response. Methysergide, at a dose of 40 mg/kg, had a similar effect while a lower dose of 2 mg/kg produced no change. Mianserin competitively antagonized the response to 5-HT, a dose of 2 mg/kg producing a fourfold increase in the amount of 5-HT required to produce a 50% maximum response. 4. Acetylcholine and 5-HT seem to act independently in inducing intestinal secretion since atropine did not block the response to 5-HT and Mianserin did not alter the response to acetylcholine. 5. Experiments in which the intestinal villi or crypts were subjected to preferential damage suggested that 5-HT primarily produced its response at the crypt cell level.

Inhibition by 5-hydroxytryptamine of anaphylactic histamine release from bovine granulocytes

A suspension of granulocytes from calves sensitised to horse plasma released histamine when exposed to the antigen. Histamine release was strongly inhibited by 5-hydroxytryptamine in a dose-related fashion. 5-HT is known to be released from bovine lungs in vitro, and may regulate the release of histamine in vivo during anaphylaxis.

Inhibition of reserpine, guanethidine and imiparmine of the uptake of 5-hydroxytryptamine by rat peritoneal mast cells in vitro

The effect of reserpine, guanethidine and imipramine of the uptake of tritiated 5-hydroxytryptamine by rat peritoneal mast cells was studied in vitro. The Km value for uptake of 5-hydroxytryptamine was 3 x 10(-7) M in this study. Kinetic analysis of the data obtained suggests that the drugs inhibits the uptake in a competitive manner, in the order of decreasing potency: reserpine, imipramine, guanethidine. For reserpine the Dixon plot resulted in two curves, each composed of 2 linear components suggesting inhibition of uptake of 5-hydroxytryptamine at 2 different levels or by 2 different mechanisms with different K1 values. It is suggested that the higher K1 value, representing an immediate effect of reserpine, stands for inhibition of uptake at the cell membrane. The lower K1 value possibly represents inhibition at the level of the membrane surrounding the intracellular storage organelles.