Vasoconstrictor activity of vasopressin in the pithed rat

Vasopressin caused a potent vasoconstrictor effect in pithed rat preparations, without significantly altering heart rate. The vasopressor/constrictor response to vasopressin was not influenced by bilateral adrenalectomy, reserpine pretreatment or the prior administration of the alpha-adrenoceptor antagonists prazosin and yohimbine. The minor impairments of vasopressin-induced vasoconstriction by bilateral nephrectomy, captopril and the calcium entry blockers verapamil and nifedipine could all be attributed to the vasodilator potency of these various pretreatments. In conclusion, the potent vasoconstrictor response to vasopressin in the pithed rat occurs independently of the peripheral sympathetic nervous system and its associated adrenoceptors and of the renin-angiotensin-aldosterone systems. Moreover, vasopressin-induced vasoconstriction does not require the influx of extracellular calcium ions and is hence insensitive to calcium entry blockade. These findings for vasopressin reveal an important difference with respect to alpha 1/alpha 2-adrenoceptor agonists and angiotensin II, which are known to cause vasoconstriction largely dependent upon the influx of extracellular calcium ions.