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The vascular role of vasopressin and sympathetic nervous system Tabrizchi, Reza


The aim of my study was to investigate the influence of arginine vasopressin (AVP) and the sympathetic nervous system in the control of peripheral resistance and to examine the constrictor actions of various pressor agents in capacitance vessels. In the first set of experiments, the vascular effect of AVP in the presence and absence of influence from angiotensin II (Ag II) or a-adrenergic system was investigated in pentobarbital anaesthetized rats. Cardiac output (CO) and the distribution of blood flow (BF) were determined by the microspheres technique prior to and following the injection of an AVP pressor antagonist [d(CH₂)₅Tyr(Me)AVP] in three Groups of rats: saline-treated (I), saralasin-treated (II) and phentolamine-treated (III). The AVP antagonist decreased MAP and TPR in all Groups and it caused a greater depressor effect in Groups II and III than in I. In Group I, AVP antagonist increased BF to the stomach and skin. In Group II, AVP antagonist increased BF to the muscle and skin. In Group III, AVP antagonist markedly increased BF to the muscle. The second set of experiments investigates the physiological role of the α-adrenoceptors in the vasculature. The effects of prazosin (α₂-blocker), rauwolscine (α₁-blocker) and phentolamine (nonselective α -blocker) on MAP, CO and its distribution were investigated in halothane anaesthetized rats. All three a-antagonists caused similar decreases of MAP and all increased of % distribution of CO to the lungs and muscle. During the infusion of prazo- sin, TPR was decreased but CO was not changed. In contrast, CO was decreased but TPR was not altered during the infusions of rauwolscine and phentolamine. Since CO was reduced after the blockade of α₂-but not α₁ receptors, it appears that α₂ receptors are responsible for the control of venous capacitance. A final set of experiments were carried out to investigate the actions of various vasoconstrictor agents on the capacitance vessels of conscious rats. We investigated the dose-response relationships of methoxamine (α₁-agonist), B-HT 920 (α₂--agonist), noradrenaline (NA, non-selective α-agonist), AVP and Ag II on MAP, mean circulatory filling pressure (MCFP) and heart rate (HR) in conscious rats. The infusions of all the agonists, but not saline, caused dose-dependent increases in MAP and decreases in HR. The infusions of saline and methoxamine did not affect MCFP while the infusions of B-HT 920, NA and Ag II increased MCFP. Therefore, receptors for α₂ adrenergic agonists and Ag II are involved in the control of venous tone.

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