Receptors for noradrenaline and histamine in the rabbit's posterior vena cava

Effect of captopril on bradykinin inactivation by human foetal placental circulation

The inactivation of bradykinin on passage across the human foetal placental circulation was investigated in six full-term human placentas. The placentas were perfused with a modified Krebs-Henseleit solution and placenta perfusion pressure was recorded. Samples collected at the arterial inflow and at the venous effluent were assayed on the isolated guinea-pig ileum as an estimation of bradykinin activity. Bradykinin (100 ng ml-1) was infused through the foetal placental vessels before and during captopril 4 X 10(-7) M. Bradykinin produced a transient increase in placental vascular resistance that was not potentiated by captopril. Bradykinin activity was completely abolished after passage through the foetal placental circulation, and the inactivation was blocked by captopril. These data suggest that angiotensin I converting enzyme (kininase II) might occur in the foetal placental vessels.

Some properties of 5-hydroxytryptamine receptors in the hindquarters of the rat

1 The rat hindquarter preparation, as described, responds with reproducible vasoconstriction to noradrenaline and tryptamines. 2 The receptors involved in these responses are distinct. 3 Evidence of heterogeneity of tryptamine receptors was not obtained. 4 The 5-hydroxytryptamine (5-HT) antagonists, methysergide and cyproheptadine, although very potent, displayed antagonism of a non-competitive type whereas a series of phenothiazines and phentolamine displayed competitive antagonism against 5-HT. 5 For the phenothiazines the order of increasing potency was promazine less than chlorpromazine less than triflupromazine.

The effects of uptake1 on alpha-adrenoceptor antagonist potency in dog saphenous vein

1 The effects of alpha-adrenoceptor antagonists on contractile responses to noradrenaline and methoxamine were examined in isolated saphenous vein strips of the dog. 2 Phentolamine, labetalol and thymoxamine antagonized responses to methoxamine to a greater extent than responses to noradrenaline. 3 Cocaine (3.0 x 10(-5) mol/l) increased the potency of noradrenaline by about eight fold, but had little or no effect on the potency of methoxamine. 4 In the presence of cocaine (3.0 x 10(-5) mol/l) the potency of the antagonists against noradrenaline was increased such that the pA2 values were then similar to those obtained against methoxamine. 5 It is concluded that the lower potency of phentolamine, labetalol and thymoxamine as antagonists of noradrenaline than of methoxamine is due to the presence of an avid saturable uptake1 process in the saphenous vein.

Receptors for 5-hydroxytryptamine and noradrenaline in rabbit isolated ear artery and aorta

5-Hydroxytryptamine (5-HT) is thought to be implicated in the vascular disturbances of the external carotid artery bed associated with migraine. As part of a study of the pharmacology of some 5-HT antagonists used in the treatment of migraine we have examined the interactions of these drugs with 5-HT and noradrenaline in rabbit isolated ear artery and aortic strip. The results provide new information on the distribution of 5-HT-receptors in these preparations. In the aorta, 5-HT and noradrenaline were of similar potency in producing contractions. Methysergide produced very small contractions and was about 1000 times less potent than the other two agonists. In the ear artery noradrenaline produced monophasic vasoconstrictor responses, whereas 5-HT and methysergide produced prolonged biphasic responses. 5-HT was about 700 times less potent and methysergide about 4500 times less potent than noradrenaline. Methysergide was a better agonist in the ear artery than in the aorta. Biphasic responses to 5-HT and methysergide were also obtained in ear arteries from reserpine-treated rabbits indicating that neither agonist was acting by releasing endogenous noradrenaline. Pizotifen, cyproheptadine and phentolamine had no agonistic actions in either the aorta or ear artery. In the aorta methysergide, pizotifen and cyproheptadine were potent antagonists of 5-HT and much weaker antagonists of noradrenaline. Phentolamine possessed the opposite profile of selectivity. These results show that there are distinct receptors for 5-HT and noradrenaline in rabbit aorta. In the ear artery the pA2 values for each of the four antagonists were virtually identical against 5-HT and noradrenaline and similar to those obtained on alpha-adrenoceptors in the aorta. We conclude that 5-HT and noradrenaline act directly at alpha-receptors to produce vasoconstriction in the ear artery and that this preparation does not contain specific 5-HT receptors. This insight into the distribution of 5-HT receptors and alpha-receptors allows interpretation of the various actions of methysergide. In the aorta, methysergide was a potent antagonist at 5-HT-receptors and a weak partial agonist at alpha-receptors. In the ear artery, methysergide was a partial agonist at alpha-receptors; it was only a weak antagonist of 5-HT because this preparation does not contain specific 5-HT-receptors. The cross-reactivity demonstrated throughout these experiments indicates that 5-HT-receptors and alpha-receptors, although distinct entities, have features in common. These results are discussed in relation to the mode of action of methysergide, pizotifen and cyproheptadine in the treatment of migraine.

Effect of synthetic bradykinin on contractile tension of human saphenous vein strips

The ability of synthetic bradykinin to contract isolated, helically-cut human saphenous vein strips was studied in vitro. Bradykinin (0.05-2.0,mug/ml) consistently failed to contract human vein strips. Similarly prepared rabbit aortic strips also failed to contract when exposed to bradykinin in the concentration range studied. Bradykinin-insensitive vein and aortic strips responded with substantial increases in contractile tension when exposed to prostaglandin E(2), noradrenaline, histamine and KC1. However, bradykinin (0.1,mug/ml) was, capable of relaxing human vein strips, which had contracted in response to prostaglandin E(2).