The interaction of cimetidine with various antihypertensive agents in the spontaneously hypertensive rat.

The H2-receptor antagonist cimetidine (250 micrograms) administered intracerebroventricularly (i.c.v.) 15 and 30 min before clonidine (25 micrograms kg-1 i.v.), significantly antagonized clonidine-induced hypotension in anaesthetized spontaneously hypertensive rats. The hypertensive response of cimetidine was correlated with the inhibition of clonidine-induced hypotension. In addition, cimetidine (250 micrograms i.c.v.) counteracted the hypotensive effects of pentolinium (5.0 mg kg-1 i.v.), guanethidine (5.0 mg kg-1 i.v.) and minoxidil (1.0 mg kg-1 i.v.) These data do not support previous suggestions that the hypotensive action of clonidine is caused by stimulation of the H2-receptor, but suggest that central administration of cimetidine causes peripheral vasoconstriction and this may offer resistance to the hypotensive action of different antihypertensive agents.

Trequinsin, a potent new antihypertensive vasodilator in the series of 2-(arylimino)-3-alkyl-9,10-dimethoxy-3,4,6,7-tetrahydro-2H-py rim ido [6,1-a]isoquinolin-4-ones.

Series of 3-substituted-9,10-dimethoxy-3,4,6,7-tetrahydro-2H-pyrimido [6,1-a]isoquinoline-2,4-diones and 2-substituted-9,10-dimethoxy-6,7-dihydro-4H-pyrimido [6,1-a]isoquinolin-4-ones were synthesized and tested for blood pressure lowering properties in anesthetized normotensive cats and conscious spontaneously hypertensive rats. Several compounds in the 2-(arylamino)-9,10-dimethoxy-6,7-dihydro-4H-pyrimido [6,1-a]isoquinolin-4-one series display a high order of activity. The most active compounds are the alkyl derivatives of the 2-mesitylamino/2-mesitylimino tautomeric forms. The 2-(mesitylimino)-3-methyl analogue trequinsin is a potent antihypertensive agent and displays a hemodynamic profile characteristic of an arteriolar dilator. It is also a potent inhibitor of both cAMP phosphodiesterase and platelet aggregation.

Central pressor activity of cimetidine in spontaneously hypertensive rats.

The systemic blood pressure effect of cimetidine given intracerebroventricularly (i.c.v.) in anaesthetized spontaneously hypertensive (SH) rats has been investigated. Cimetidine (250 micrograms i.c.v.) caused a gradual long lasting rise in mean arterial blood pressure with maximum of 31.6 +/- 4.5 mm Hg. Chemical degeneration of catecholaminergic neurons with 6-OHDA treatment, central administration of phentolamine and prazosin, and the bilateral adrenalectomy significantly inhibited the pressor response of cimetidine, while propranolol (i.c.v.) had no effect. From these results it appears that the hypertensive response of cimetidine is mediated by central catecholaminergic pathways and is due to an increase in efferent sympathetic outflow and release of catecholamine from the adrenal medulla.

The antihypertensive and positive inotropic diterpene forskolin: effects of structural modifications on its activity.

Four naturally occurring analogues of forskolin were isolated. Forty-nine semisynthetic derivatives were prepared, incorporating structural alterations at the 1-, 6-, 7-, 9-, 11-, and 14/15-positions. Blood pressure lowering properties of 53 compounds were assessed in anesthetized normotensive cats and of 31 compounds in conscious spontaneously hypertensive (SH) rats. The positive inotropic properties of 25 compounds were investigated in an isolated guinea pig atrial preparation. Forskolin was unique among the compounds in its hypotensive activity in cats and in its positive inotropic properties. Although several derivatives displayed oral antihypertensive activity in the SH rats, none was significantly more potent than forskolin. The optimal structural requirements for activity are apparent, since they are found in forskolin itself.

Effect of diphenylhydantoin on blood pressure of spontaneously hypertensive rats.

Diphenylhydantoin (DPH) has been shown to elicit direct peripheral vasodilatory effects in anaesthetised animals. Since spontaneously hypertensive (SH) rats exhibit many features similar to human essential hypertension, the effect of DPH on blood pressure of these rats was studied. DPH given orally for 5 days elicited dose-dependent fall in systolic blood pressure in conscious SH rats. In addition, repeated administrations of DPH increased the noradrenaline concentration in the hypothalamus. These results suggest that the central noradrenergic mechanisms might be involved in the hypotensive action of DPH in SH rats, probably at the supramedullary level.

Possible role of dopamine in central effects of cocaine as measured by apomorphine gnawing test in mice.

Apomorphine (10mg/kg, s.c.) does not induce in mice a compulsion to gnaw, but pretreatment with cocaine (10-40 mg/kg, i.p.) caused gnawing activity. This effect of cocaine was inhibited by pretreatment with alpha-methyl-p-tyrosine, haloperidol, and physostigmine, but not with FLA-63, phenoxybenzamine and tetrabenazine. These findings would suggest that dopaminergic mechanism plays a significant role in the potentiation of apomorphine gnawing activity by cocaine and also support the view that inhibition of dopamine uptake is responsible for the stimulatory action of cocaine.

Vascular reactivity of perfused vascular bed in spontaneously hypertensive and normotensive rats.

1 Hypertensive and normotensive rats of the same age group were isolated from an inbred colony of spontaneously hypertensive rats. 2 The perfused hindquarter and mesenteric artery preparations obtained from hypertensive and normotensive rats exhibited an increased reactivity to noradrenaline (NA) and angiotensin II. 3 Dose-response curves to NA obtained from hypertensive and normotensive rats exhibited a steeper slope and higher maximum than those from control rats. 4 These findings suggest that increased vascular reactivity of blood vessels is independent of the development or maintenance of elevated blood pressure.

Influence of pheniramine and chlorpheniramine on apomorphine induced compulsive gnawing in mice.

In mice, apomorphine (10 mg/kg s.c.) does not induce a compulsion to gnaw, but pretreatment with antihistamines, viz. pheniramine, chlorpheniramine and mepyramine, in doses ranging from 30 to 60 mg/kg i.p. caused gnawing activity. Mepyramine showed significantly less effect when compared to the other two agents. Antihistamines are known to influence the activity of biogenic amines in central nervous system. The potentiation of apomorphine-induced gnawing by antihistamines might depend upon the reciprocal balance between dopaminergic and cholinergic systems. This was tested by blocking biosynthesis of biogenic amines or by blocking their receptors. The potentiation of gnawing was antagonised by physostigmine (0.25 mg/kg) or blocked by pretreatment with alpha-methyl-p-tyrosine (alpha-MPT) (4 X 150 mg/kg) and bis-(4-methyl-1-homopiperazinylthiocarbonyl)-disulphide (FLA) (40 mg/kg), while p-chlorophenyl alanine (p-CPA) (3 X 100 mg/kg) had no effect. Similarly, phenoxybenzamine (30 mg/kg) and haloperidol (1.0 mg/kg) inhibited gnawing activity, but methysergide (10 mg/kg) had no effect. Furthermore, pretreatment with tetrabenazine (20 mg/kg) and L-Dopa (200 mg/kg) did not affect gnawing activity. It is concluded that both pheniramine and chlorpheniramine potentiate apomorphine gnawing by upsetting the cholinergic and dopaminergic balance in favour of dopaminergic dominance.