Pressor and mesenteric arterial hyporesponsiveness to angiotensin II is an early event in haemorrhagic hypotension in anaesthetised rats.

OBJECTIVE: Vascular responsiveness to vasoconstrictors is known to be attenuated in haemorrhagic shock. In this study we assessed the temporal development and the underlying mechanisms of haemorrhage-induced vascular hyporeactivity to pressor agents. METHODS: In phenobarbital-anaesthetised rats hypotension was induced by graded haemorrhage (8 ml blood total). Sham-manipulated rats served as controls. Blood flow (BF) was recorded with ultrasonic transit time flow probes. RESULTS: Following haemorrhage mean arterial pressure (MAP) fell by 25-45 mm Hg and was accompanied by a reduction in mesenteric BF without any alteration of mesenteric vascular conductance (VC). While pressor responses to arginine vasopressin remained unaltered, hyporesponsiveness to phenylephrine (10 nmol kg-1) developed 120-180 min after hypotension had been induced. Pressor and mesenteric constrictor responses to angiotensin II (30 pmol kg-1) became significantly blunted as early as 60 min post haemorrhage. The hypotensive effect of an angiotensin1 receptor antagonist, telmisartan (1 mg kg-1), was likewise blunted 3 h after haemorrhage. Pretreatment with the cyclooxygenase inhibitor indomethacin (10 mg kg-1) exaggerated the hypotensive reaction to haemorrhage but did not prevent the development of angiotensin II hyporesponsiveness. In contrast, the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methyl ester (10 mg kg-1), as investigated 3 h post haemorrhage, restored the systemic pressor responses to angiotensin II and phenylephrine as well as the mesenteric constrictor responses to phenylephrine to normal level and diminished the mesenteric hyporesponsiveness to angiotensin II. Glibenclamide (20 mg kg-1), an inhibitor of ATP-sensitive K- channels given 180 min post haemorrhage, partially reversed haemorrhage-induced hypotension but did not modify angiotensin II hyporesponsiveness. CONCLUSION: Systemic pressor responsiveness and mesenteric arterial reactivity to endogenous and exogenous angiotensin II is selectively impaired at an early stage of haemorrhagic hypotension. This phenomenon partially involves NO and is not related to ATP-sensitive K+ channels.

Lack of effect of a selective vasopressin V1A receptor antagonist SR 49,059, on potentiation by vasopressin of adrenoceptor-mediated pressor responses in the rat mesenteric arterial bed.

The vasopressin receptor subtype involved in the enhancement by vasopressin of adrenoceptor-mediated vasoconstriction was investigated in rat isolated perfused mesenteric arteries. [Arg8]vasopressin (1-10 nM) dose-dependently increased the perfusion pressure and enhanced the pressor response to the adrenoceptor agonist methoxamine (40 nmol) or electrical stimulation of periarterial nerves (16 Hz), at the concentration of 10 nM of [Arg8]vasopressin up to 4 and 3 fold, respectively. During prolonged exposure (45 min) the direct vasoconstrictor effect of [Arg8]vasopressin (10 nM) rapidly declined whereas the potentiation of methoxamine-induced vasoconstriction was maintained. The selective vasopressin V1A receptor antagonist SR 49,059 (1-3 nM) and the non-selective V1A/B and oxytocin receptor antagonist [deamino-Pen1,Tyr(Me)2,Arg8]vasopressin (15-45 nM) inhibited the direct vasoconstrictor action of [Arg8]vasopressin but had no effect on the enhancement of the pressor response to methoxamine or electrical stimulation. The V1B receptor agonist [deamino-Cys1,beta-(3-pyridyl)-D-Ala2,Arg8]vasopressin (100-1000 nM) and the V2 receptor agonist [deamino-Cys1,D-Arg8]vasopressin (1-10 nM) were devoid of any pressor activity and did not potentiate methoxamine-evoked vasoconstriction. In contrast, [1-triglycyl,Lys8]vasopressin (100 - 1000 nM) potentiated the methoxamine responses without per se inducing vasoconstriction. In arteries precontracted with methoxamine (7.5 microM) pressor responses to [Arg8]vasopressin (3-10 nM) were not inhibited by a dose of SR 49,059 (3 nM) which abolished the peptide's vasoconstrictor effect under control conditions. These data show that the direct vasoconstrictor effect of [Arg8]vasopressin is mediated by V1A receptors while the enhancement of adrenoceptor-mediated pressor responses is insensitive to V1A, V1B, and oxytocin receptor antagonists and is not mimicked by selective agonists of V1B and V2 receptors. In conclusion, an unusual interaction of vasopressin with V1A receptors, or even the existence of a novel receptor subtype, has to be considered.

5-HT2 and 5-HT3 receptor subtypes mediate cholera toxin-induced intestinal fluid secretion in the rat.

The mechanisms of diarrhea in Asiatic cholera have been studied extensively. Cyclic adenosine monophosphate, 5-hydroxytryptamine (5-HT), prostaglandins, and the function of neuronal structures have been implicated in the pathogenesis of cholera. To elucidate the action of 5-HT in mediating cholera secretion, in vivo experiments were performed in the rat jejunum. The inhibitory effects of the 5-HT2 receptor antagonist ketanserin and the 5-HT3 receptor antagonist ICS 205-930 were studied in cholera toxin- and 5-HT-induced fluid secretion. Both ketanserin and ICS 205-930 dose-dependently but only partially reduced the secretory effect of cholera toxin. The combination of the two blockers totally abolished cholera toxin-induced secretion without any influence on cholera toxin-induced increase in cyclic adenosine monophosphate. Prostaglandin E2- and bisacodyl-induced secretion was not affected by the combined administration of 5-HT2 and 5-HT3 antagonists. The present results provide evidence for an important role of 5-HT in cholera toxin-induced secretion. The data suggest a model in which cholera toxin may initiate the release of 5-HT from enterochromaffin cells. 5-Hydroxytryptamine may then cause prostaglandin E2 formation via 5-HT2 receptors and activation of neuronal structures via 5-HT3 receptors. These two effects may finally lead to the profuse fluid secretion which can be totally blocked by the combination of a 5-HT2 blocker and a 5-HT3 blocker.

Protein kinase C and intestinal fluid secretion: involvement of prostaglandin E2 but not of 5-hydroxytryptamine.

To determine the role of protein kinase C in the regulation of intestinal fluid transport, experiments were performed with the rat jejunum in vivo, using the active phorbol ester, 4-beta-phorbol 12-myristate 13-acetate (PMA), as stimulator of protein kinase C. Intraluminally administered PMA dose dependently reversed the net fluid absorption to net fluid secretion and significantly increased prostaglandin E2 (PGE2) but not 5-hydroxytryptamine (5-HT) output into the lumen. Mucosal cyclic AMP levels remained unchanged by PMA. Indomethacin inhibited the increase in PGE2 output and partially reduced the secretory response to PMA. Ketanserin was without effect whereas verapamil totally blocked the secretory response to PMA. It is concluded that intestinal fluid secretion, stimulated by activation of protein kinase C is partly mediated by PGE2 release. PGE2 may facilitate calcium entry rather than increase intracellular calcium through activation of cyclic AMP. Protein kinase C appears to play an important role as an intermediate in phosphoinositol hydrolysis, which is initiated by 5-HT, and finally induces fluid secretion via PGE2.