[Analysis of the mode of action of ifenprodil on cerebral and peripheral circulation in dogs]

Pharmacological properties of ifenprodil, a vasodilating agent similar in chemical structure to isoxsuprine were investigated. In anesthetized dogs and cats, ifenprodil (0.3 approximately 1 mg/kg, i.v.) caused a temporary prominent depressor response with an increment in heart rate and an occasional enlargement of pulse pressure followed by a mildly durable hypotension. These two phases of hypotension of ifenprodil could not be demonstrated in spinal rats and cats. Ifenprodil also decreased the pressor response of norepinephrine and reversed the pressor effect of epinephrine to a depressor one. The lowering effect of ifenprodil on the constant-rate perfused pressure of vertebral artery in dogs was half as potent as isoxsuprine. Ifenprodil significantly reduced the contraction to norepinephrine in the isolated guinea-pig vas deferens. This was greater than the constrictor response produced by preganglionic sympathetic nerve stimulation. The inhibitory effect of ifenprodil was less potent than that of phentolamine on the responses to norepinephrine in isolated rabbit ascending aorta strips. In the isolated guinea-pig right atrium, ifenprodil (10(-5) g/ml) showed slightly negative inotropic and chronotropic effects, and reduced the responses to isoproterenol. It is suggested that the two phases of lowering effects of ifenprodil on the constant-rate perfusion pressure of either vertebral or femoral artery in dogs are due to a possible neurogenic and an alpha-adrenolytic action, but the precise mechanism is not clear. Ifenprodil like isoxsuprine has a weak beta-sympathetic effect.

[Pharmacological analysis on the mode of action of mandelamidine, a long-acting hypotensive agent (1). Comparison with guanethidine]

MA produced a temporary and long lasting hypotension in anesthetized animals. MA like guanethindine caused relaxation of the nictitating membrane in unanesthetized cats and inhibited the contraction of the nictitating membrane elicited by electrical stimulation at various frequencies of preganglionic cervical sympathetic nerve in anesthetized cats. The mode of neuron blocking action of MA differed from guanethidine on the contraction of the nictitating membrane elicited by stimulation at high frequencies. Neuron blocking action of MA was also shown on rat inferior eyelid, dog carotid occlusion, rabbit jejunum and guinea-pig vas deferens. Moreover that of MA like guanethidine on the cat nictitating membrane was reversed by a following administration of methamphetamine. MA slightly potentiated the effects of norepinephrine, epinephrine and reduced the effects of tyramine on the responses of the cat nictitating membrane and blood pressure, but did not potentiate the effects of norepinephrine on the guinea-pig vas deferens and rabbit pulmonary artery. MA produced a contraction of the cat nictitating membrane and this contraction was inhibited by pretreatment with reserpine or phentolamine. However, MA did not produce a rise in blood pressure in spinal cats nor did it reduce the contractile force and heart rate in guinea-pig atria. These findings suggest that sympathomimetic action and uptake inhibition of the catecholamine of MA was weaker than that of guanethidine.

[Mode of action of clonidine, a central hypotensive agent, analyzed by the interaction with various sympathomimetic agents]

Injection of a low dose of clonidine showed the biphasic blood pressure response which comprises a delayed onset of long lasting hypotension and a quick onset of hypertension of short duration followed by the former, in the alpha-chloralose-urethanized rat. A second injection of low dose of clonidine administered 90 min after the first showed only two peaks of pressor response, no longer a depressor one, and tachyphylaxis occurred with repeated injections of the same dose of clonidine, together with a gradual elevation of blood pressure level. Furthermore, the depressor action was inhibited by pretreatment with cocaine or imipramine. In contrast, injection of a high dose of clonidine exhibited pressor response with only two peaks, duration of which was markedly potentiated by pretreatment with cocaine or imipramine, and also by guanethidine, but inhibited in reserpinized and spinal rats. While the blood pressure was sustained at a high level after the injection of a high dose os clonidine, blood pressure reversal was produced by tyramine, due to possible beta-mimetic action of much greater amounts of catecholamines released by tyramine. The isolated guinea-pig vas deferens contracted when treated with clonidine, and this contraction was inhibited in the reserpinized preparation. This inhibition was recovered by incubation with norepinephrine. Interaction between tyramine and clonidine was also seen in vitro, but it disappeared in the reserpinized, denervated or propranolol-pretreated guinea-pig vas deferens. The contraction of the vas deferens induced by clonidine was potentiated by pretreatment with cocaine, but not by pretreatment with guanethidine, ans was inhibited by treatment with alpha-adrenergic blocking agents phentolamine and phentolame and phenoxybenzamine, but not by tolazoline.

[Effect of dopamine on perfusion pressure in rat peripheral blood vessels]

Rat mesenteric and hind limb arteries were perfused with the rat's own blood under constant perfusion rate, in situ. The effect of dopamine (DA) administered intraarterially was investigated and compared with effects of norepinephrine (NE), tyramine (Tyr), and phenylephrine (PHE). The magnitude of the vasoconstrictive maximum response of both vasculatures to NE and PHE was larger than those to DA and Tyr, and those to DA were larger than those to Tyr in hind limb, but those to Tyr were more potent than those to DA in the mesenteric artery. The vasoconstriction evoked by celiac ganglionic stimulation was much the same to that evoked by NE and PHE. Under cocaine infusion, the rise in perfusion pressure of both vasculatures evoked by NE and PHE. Under cocaine infusion, the rise in perfusion pressure of both vasculatures evoked by NE was slight, but the duration was markedly potentiated. However, the effect of Tyr was suppressed and that of DA was not changed. In rats on alpha-methyl-rho-tyrosine, the responses of both vasculatures to Tyr were reduced and those to NE and DA were not changed. In the reserpinized rat, the effect of NE was not changed and that of Tyr was reduced, while that of D was more evident in both vasculatures. After phenoxybenzamine treatment, vasoconstrictive effect of DA was reversed in mesenteric and reduced in hind limb artery. Vasodilating effect of DA after phenoxybenzamine, was not changed by treatment with propranolol, atropine, and diphenhydramine but was reduced with reserpine and abolished with haloperidol. These results suggest that the vasoconstrictive effect of DA depends partly on indirect sympathomimetic action, and after phenoxybenzamine, DA acts by vasodilating the DA receptor. Potentiation with reserpine may depend on a post-synaptic mechanism and increase in dopamine-beta-hydroxylase activity.