Potentiation of 5-hydroxytryptamine release from platelets by desmethylation of chlorpromazine and related agents

The effect of narcotic analgesics on the uptake of 5-hydroxytryptamine and (-)-metaraminol by blood platelets

1. The effects of narcotic analgesic and related drugs were studied on the uptake of 5-hydroxytryptamine (5-HT) and (-)-metaraminol by blood platelets.2. The most potent drug in inhibiting the uptake of 5-HT (10 muM) by human platelets was methadone, followed by pentazocine>piminodine approximately pethidine approximately anileridine approximately cyclazocine approximately thebaine > dextropropoxyphene. Alphaprodine, papaverine, apomorphine, nalorphine, codeine, and morphine were almost without effect. Methadone was slightly less active than desipramine, and had 10% of the activity of imipramine under similar conditions. Naloxone did not antagonize the effect of methadone on 5-HT uptake.3. The most potent inhibitor of metaraminol (3 muM) uptake by human platelets was piminodine, followed by pentazocine>/=anileridine>cyclazocine=methadone > dextropropoxyphene approximately thebaine >/= papaverine approximately alphaprodine >pethidine>morphine. The activity of morphine was 1% of that of piminodine. Piminodine was more potent than desipramine and protriptyline under similar conditions. The order of potency of drugs studied in inhibiting the uptake of metaraminol by rabbit platelets was similar to that obtained with human platelets.4. The effects of the analgesics studied on inhibiting uptake of monoamines did not correlate with their pain-relieving properties.

The effect of drugs upon the uptake of 5-hydroxytryptamine and metaraminol by human platelets

The abilities of some tricyclic and bicyclic antidepressive drugs and an α-receptor blocking agent, phenoxybenzamine, to inhibit the uptake of 5-hydroxytryptamine (5-HT) and (—)-metaraminol into human platelets have been compared in vitro. All the drugs inhibited the uptake both of 5-HT and of metaraminol into platelets. But there were differences in their abilities to inhibit the uptake of these two monoamines. The desmethylated antidepressive drugs were more potent inhibitors of metaraminol uptake than were their tertiary analogues, whereas imipramine, a tertiary amine, was by far the best inhibitor of 5-HT uptake. The order of the activities of the antidepressive drugs in inhibiting the uptake of 5-HT and metaraminol into platelets paralleled their potencies in blocking the uptake of 5-HT, and noradrenaline or metaraminol into nerve endings. It is suggested that the uptake of 5-HT and metaraminol into platelets is a useful model for the neuronal uptake of 5-HT and noradrenaline, respectively.