EFFECTS OF DIAZOXIDE AND ETHACRYNIC ACID ON SEQUENTIAL VASCULAR SEGMENTS IN THE CANINE GRACILIS MUSCLE

¹ Clinical Pharmacology Division, The Montreal General Hospital, and the Departments of Medicine, Pharmacology and Therapeutics, McGill University, Montreal, Canada

The effects of i.a. administration of ethacrynic acid (EA) and diazoxide (DZ) on vascular pressure gradients were compared in the gracilis muscle. Five-minute infusions of EA or DZ (1 mg/min) briefly reduced perfusion pressure. During 25-minute infusions of EA, arterial resistance was reduced an average of 31% from base line. The maximal effect of EA was noted late in the infusion period and the reduction in resistance persisted throughout the postinfusion period. During the 25-minute infusions of DZ, arterial resistance was reduced an average of 36% from base line. This effect of DZ was noted early in the infusion period but was diminished during the postinfusion period. Changes in postcapillary resistance were noted only in preparations infused with DZ. The post- to precapillary resistance ratio during DZ infusions was reduced markedly whereas this ratio was increased from base line during EA infusions. Thus precapillary resistance vessels were affected directly by both EA and DZ, but only DZ had an effect on postcapillary resistance vessels. There was a greater effect of DZ on post- than on precapillary vessels and the maximal vasodilatory effect of DZ was more rapid in onset and less persistent than that of EA in the gracilis muscle vascular bed. We conclude that the vasodilatory mechanism for EA is probably different from that for DZ.