Influence of SQ 29,548 on vasoconstrictor responses in the mesenteric vascular bed of the cat

PGH(2)-TxA(2)-receptor blockade restores vasoreactivity in a new rodent model of genetic hypertension

The purpose of this study was to determine whether activation of prostaglandin H(2)-thromboxane A(2) (PGH(2)-TxA(2)) receptors impedes vasodilation in the in situ peripheral microcirculation of spontaneously hypertensive hamsters, a new rodent model of high-renin genetic hypertension. Using intravital microscopy, we found that vasodilation elicited by suffusion of acetylcholine and vasoactive intestinal peptide (VIP), two neurotransmitters localized in perivascular nerves in the peripheral circulation, on the in situ cheek pouch was significantly attenuated in spontaneously hypertensive hamsters relative to age- and genetically matched normotensive hamsters (P < 0.05). However, nitroglycerin-induced vasodilation was similar in both groups. Pretreatment with SQ-29548, a selective and potent PGH(2)-TxA(2)-receptor antagonist, restored acetylcholine- and VIP-induced vasodilation in spontaneously hypertensive hamsters. SQ-29548 had no significant effects on resting arteriolar diameter and on nitroglycerin-induced vasodilation in both groups. SQ-29548 slightly but significantly potentiated VIP- but not acetylcholine-induced vasodilation in normotensive hamsters. Collectively, these data indicate that activation of PGH(2)-TxA(2) receptors impedes agonist-induced vasodilation in the in situ cheek pouch of spontaneously hypertensive hamsters. We suggest that this model is suitable for studying the role of prostanoids in mediating vasomotor dysfunction observed in genetic hypertension.

Prostacyclin and thromboxane production of rat and cat arterial tissue is altered independently by several vasoactive substances

The modulation of the production of prostacyclin and thromboxane from cat and cat aortic tissue slices by different vasoactive agents has been studied in order to reveal whether the release of these main two vasoactive prostanoids goes in parallel or may be controlled independently. Norepinephrine, isoproterenol, phentolamine, propranolol, angiotensin II, vasopressin, bradykinin, thrombin, verapamil, gallopamil, dopamine or methionin enkephalin were added to the incubation medium and 6-keto-PGF1 alpha (the stable metabolite of prostacyclin) and TxB2 (the stable metabolite of thromboxane) were determined in the supernatant by radioimmunoassay. The ratio of the release of prostacyclin and thromboxane was computed. Norepinephrine increased both prostacyclin and thromboxane release. Isoproterenol increased the ratio of prostacyclin and thromboxane released in cat aortic tissue slices. Phentolamine and propranolol had no effects. Angiotensin II induced a slight but statistically insignificant increase in the ratio of the two prostanoids released. Vasopressin increased thromboxane release only. Bradykinin stimulated the prostacyclin while thrombin stimulated the thromboxane release. Verapamil decreased both prostacyclin and thromboxane production. Gallopamil decreased prostacyclin release and increased thromboxane release from vessel wall slices in a certain concentration range causing a characteristic dose dependent minimum in the ratio of prostacyclin and thromboxane release. Dopamine separately increased prostacyclin release while enkephalin had no significant effect. The data obtained show that in vascular tissue some unidentified yet cytophysiological mechanisms might exist which specifically control the activities of the prostacyclin synthase and thromboxane synthase enzymes.

Influence of SQ 29,548 on vasoconstrictor responses in the hindquarters vascular bed of the cat

The effects of SQ 29,548, a thromboxane (TX) receptor blocking agent, on vasoconstrictor responses were investigated under conditions of controlled blood flow in the hindquarters vascular bed of the cat. Intravenous injection of SQ 29,548 at a dose of 0.1 mg/kg had no significant effect on systemic arterial pressure but caused a significant reduction in hindquarters perfusion pressure. Injection of the TXA2 mimics, U44069 and U46619, into the perfusion circuit caused dose-dependent increases in hindquarters perfusion pressure with U46619 being approximately 3 times more potent than U44069. Following the administration of SQ 29,548, pressor responses to both U44069 and U46619 were reduced significantly, and the dose-response curves for both TXA2 mimics were shifted to the right in a parallel fashion. SQ 29,548 had no significant effect on the dose-dependent increases in hindquarters perfusion pressure in response to angiotensin II or BAY K8644, a nifedipine analog which promotes calcium entry. The TXA2 receptor blocking agent had no significant effect on increases in hindquarters perfusion pressure in response to angiotensin II or BAY K8644, a nifedipine analog which promotes calcium entry. The TXA2 receptor blocking agent had no significant effect on increases in hindquarters perfusion pressure in response to sympathetic nerve stimulation or injections of norepinephrine and tyramine. These findings suggest that SQ 29,548 blocks responses to the TXA2 mimics in a competitive manner, and that this inhibitory effect is selective.(ABSTRACT TRUNCATED AT 250 WORDS)