Lack of tolerance in forearm blood vessels in man to glyceryl trinitrate.

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Abstract
1. Nitrate tolerance is a clinical problem. The cause is not known but it has been suggested that tolerance to organic nitrates occurs within the blood vessels to reduce sensitivity to the drug. We have determined the sensitivity of human forearm resistance vessels and veins to glyceryl trinitrate (GTN) at the beginning and end of a 7 day period of GTN administration in healthy volunteers using a clinically relevant dose of transdermal drug. 2. Eight healthy volunteers completed the study which entailed measurement of change in forearm venous compliance and change in forearm blood flow following intraarterial infusions of two doses of glyceryl trinitrate (0.5 and 2.0 micrograms min-1) before, after 2 h and after 7 days of transdermal GTN administration using one 10 mg patch each 24 h. 3. Changes in venous compliance and blood flow were measured by venous occlusion plethysmography using a basal infusion of noradrenaline (1 microgram min-1) to increase venous tone. 4. Noradrenaline produced the expected decrease in forearm blood flow and venous compliance. The effect of locally infused GTN on venous compliance and forearm blood flow was similar on the three study days. In particular there was no significant difference in the response to GTN following 7 days transdermal administration compared with that after 2 h. 5. We conclude from this study that local vascular tolerance to GTN is unlikely to explain the clinical problem of nitrate tolerance, and that other mechanisms such as neurohumoral activation may be important.