Effects of prostaglandins and thromboxane A2 on the coronary circulation of adult dogs and puppies

Intracoronary injections of prostaglandin endoperoxide (PGH2), prostacyclin (PGI2) and thromboxane A2 (TXA2) were given to adult dogs and puppies three months or younger. Both PGH2 and PGI2 caused dose-related coronary vasodilation in the adult as well as in the puppy dogs; and PGH2 was about twice as potent as PGI2. The threshold dose of PGH2 for coronary vasodilation, 0.01-0.02 microgram, was the same for the adult and puppy dogs although control coronary blood flow of the adult dog was 3-10 times higher. In the pupply, maximal coronary vasodilation was effected with high doses (1.0 microgram or more) of PGI2, but not with PGH2. TXA2 prepared from PGH2 up to 1.0 microgram, had no effects on the coronary circulation of the adult dog. In contrast, both the vasocon-stricting and platelet aggregating actions of TXA2 were demonstrated in the puppy. Mechanisms for the observed age-dependent differences of the canine coronary circulation to PGH2 quantitatively, and to TXA2 qualitatively, remain to be determined.