Studies with iontophoretic administration of drugs to human dermal vessels in vivo: cholinergic vasodilatation is mediated by dilator prostanoids rather than nitric oxide

Abstract

Aims: Impaired function of the vascular endothelium has been well documented in hypertension and hypercholesterolaemia. However, the 'gold standard' method for assessing endothelial function, using intra-arterial drug infusion, is invasive and has only been applied in the forearm and coronary circulations in vivo. The aim of the present study was to establish the non-invasive technique of transdermal drug iontophoresis to assess endothelial function in human dermal vessels in vivo.

Methods: In healthy male volunteers, we delivered acetylcholine (ACh) and sodium nitroprusside (SNP) to dermal vessels of the forearm using iontophoresis, and measured vasodilatation using laser Doppler fluximetry. Drugs were diluted in a methylcellulose gel vehicle which did not induce vasodilatation. To assess the contribution of nitric oxide and vasoactive prostanoids to cholinergic dilatation, the procedure was repeated during brachial artery infusion of the nitric oxide synthase inhibitor, L-N(G)-monomethyl-arginine (L-NMMA) and after intravenous administration of the cyclooxygenase inhibitor, aspirin. As a control for the vasoconstrictor effect of L-NMMA, which was measured by venous occlusion plethysmography, iontophoresis was repeated during brachial artery infusion of noradrenaline.

Results: Flux increased in response to iontophoresis of ACh (from 45 +/- 9 to 499 +/- 80 units; P < 0.0001) and SNP (from 32 +/- 11 to 607 +/- 82 units; P < 0.0001). Brachial artery infusions of L-NMMA or noradrenaline caused reductions in forearm blood flow (by 43 +/- 2% and 44 +/- 2%, respectively) but did not inhibit vasodilatation in response to iontophoresis of ACh or SNP. In contrast, aspirin inhibited the response to iontophoresis of ACh (from 473 +/- 81 to 222 +/- 43 units; P < 0.0001) but did not affect the response to SNP (from 348 +/- 59 to 355 +/- 58).

Conclusions: We conclude that in healthy subjects, in contrast to the forearm circulation, dermal vasodilatation in response to iontophoresis of ACh is mediated predominately by a dilator prostanoid rather than by nitric oxide generation. Furthermore, the non-invasive technique of iontophoresis could complement existing invasive tests of endothelial function in future clinical studies.

Figure 1

Schematic representation of the delivery of positive drug ions (D⁺), such as ACh, from the chamber. For negatively charged ions, such as SNP, the current is reversed. Current is delivered by an iontophoretic device connected to a computer that controls iontophoretic settings and collects the data.

Figure 2

Development of an inert vehicle for drug iontophoresis. Responses to saline (▪), tap water (•), distilled, de-ionised, h.p.l.c.-grade water (○), and 2% methylcellulose gel reconstituted in distilled, de-ionised, h.p.l.c.-grade water (□).

Figure 3

a) Vasodilatation during iontophoresis of acetylcholine before (▪) and during (□) brachial artery infusion of l-NMMA. Units of flux are given as arbitrary flux units (FU). b) Vasodilatation during iontophoresis of acetylcholine before (▪), and 30 min after (□), intravenous injection of aspirin (600 mg). Reduction in flow by aspirin was significant by ANOVA (P < 0.01). Units of flux are given as arbitrary flux units (FU).