Inhibition of Norepinephrine Transport and Reserpine Binding by Reserpine Derivatives

Abstract

Reserpine, a competitive inhibitor of catecholamine transport into adrenal medullary chromaffin vesicles, consists of a trimethoxybenzoyl group esterified to an alkaloid ring system. Reserpine inhibits norepinephrine transport with a Ki of approximately 1 nM and binds to chromaffin-vesicle membranes with a KD of about the same value. Methyl reserpate and reserpinediol, derivatives that incorporate the alkaloid ring system, also competitively inhibit norepinephrine transport into chromaffin vesicles with Ki values of 38 +/- 10 nM and 440 +/- 240 nM, respectively. Similar concentrations inhibit [3H]reserpine binding to chromaffin-vesicle membranes. 3,4,5-Trimethoxybenzyl alcohol and 3,4,5-trimethoxybenzoic acid, derivatives of the other part of the reserpine molecule, do not inhibit either norepinephrine transport or [3H]reserpine binding at concentrations up to 100 microM. Moreover, trimethoxybenzyl alcohol does not potentiate the inhibitory action of methyl reserpate. Therefore, the amine binding site of the catecholamine transporter appears to bind the alkaloid ring system of reserpine rather than the trimethoxybenzoyl moiety. The more potent inhibitors are more hydrophobic compounds, suggesting that the reserpine binding site is hydrophobic.

Transport of Monoamine Transmitters by the Organic Cation Transporter Type 2, OCT2

“…Cocaine is a known inhibitor of the neuronal dopamine transporter (DAT), the K i being about 0.1 mol/liter (22). OMethylisoprenaline inhibits the extraneuronal monoamine transporter with a K i of 1.5 mol/liter (21) and reserpine the vesicular monoamine transporters with a K i of 1 nmol/liter (23).…”

Vesicular Ca2+ Participates in the Catalysis of Exocytosis

“…We have examined the effects of inhibitors of the vesicular monoamine transporter on Ca 2ϩ mobilization from vesicles and its effects upon release. Reserpine, tetrabenazine, and two reserpine derivatives were employed, all of which are specific inhibitors of catecholamine transport (11)(12)(13)(14). The results indicate that VMAT 1 inhibitors are capable of mobilizing vesicular Ca 2ϩ in addition to catecholamine, that the effects of this mobilization are reflected in altered characteristics of stimulated secretion, and that inhibitors with high affinity for VMAT can trigger exocytosis in the absence of any other secretagogue.…”

“…These drugs are less effective than reserpine in inhibiting transport of catecholamines by VMAT. Neither is a substrate for transport by VMAT, and both are relatively impermeant when compared with reserpine (12,27). The reserpine derivatives (1 M) generated only one or two spikes when pressure-ejected onto the cells.…”

“…In this work, we examined the effects of reserpine and tetrabenazine, specific inhibitors of the vesicular monoamine transporter (11)(12)(13)(14), upon release and Ca 2ϩ mobilization from vesicles. Incubations with these agents for 15 min decreased both the exocytotic frequency and amount released per event evoked by exposure to depolarizing agents (Fig.…”

Catecholamine transport by the organic cation transporter type 1 (OCT1)

“…reserpine. Desipramine, corticosterone and reserpine inhibit the neuronal noradrenaline transporter, the extraneuronal monoamine transporter and the vesicular monoamine transporter with K i 's of 5, 140 and 1 nmol l 71 , respectively (Parti et al, 1987;SchoÈ mig & Bo È nisch, 1986;SchoÈ mig & SchoÈ nfeld, 1990). Monoamine transport in 293 OCT1r cells was resistant to all these known monoamine transport inhibitors but sensitive to the OCT1 inhibitor cyanine863 ( Figure 5).…”