Recovery of alpha-adrenoreceptor mediated responses and binding site number after intravenous benextramine in the rabbit

Further investigation of anticoccidial activity of 7-bromo-N-(2-imidazolidinylidene)-1H-indazol-6-amine

The clonidine analog 7-bromo-N-(2-imidazolidinylidene)-1H-indazol-6-amine exhibits potent activity against Eimeria tenella infections in chickens. Disease control was abrogated by a selective alpha 2 antagonist, which is consistent with the dependence of such activity upon binding to receptors with characteristics of the vertebrate alpha 2 adrenoceptor. Lack of significant activity against the parasite in tissue culture and our inability to detect significant binding of alpha 2 adrenergic ligands to E. tenella imply that the anticoccidial action may be an indirect effect mediated by the host. Efficacy varied, depending upon the Eimeria species, being greatest for the cecal species E. tenella and less for the intestinal species. The effects described differ substantially from previous accounts of adrenergic actions on parasitic protozoa. The evidence suggests that we have observed a new mechanism of action for antiparasitic drugs.

Inhibition of vasoconstriction to cirazoline by calcium-entry blockade after phenoxybenzamine in rat perfused hindquarters

The effects of phenoxybenzamine and benextramine were assessed with respect to the vasoconstriction to cirazoline in rat perfused hindquarters. Experiments were performed with and without restriction of inward calcium flux by addition of nifedipine (10(-9)-10(-6) M) to the standard physiological solution (PS), or by omission of calcium chloride from the PS. Addition of nifedipine or omission of Ca2+ did not affect the maximal response or potency of the selective but partial alpha 1-adrenoceptor agonist, cirazoline in rat perfused hindquarters. Upon pretreatment with phenoxybenzamine (0.03-30 micrograms/kg, i.v. at -1 h) or benextramine (1 mg/kg, i.v. at -2 h) both the slope and the maximal response to cirazoline were depressed. After phenoxybenzamine but not after benextramine the maximal response to cirazoline was depressed further upon addition of nifedipine or omission of Ca2+ from the PS. It is concluded that phenoxybenzamine selectively inhibits that part of the alpha 1-adrenoceptor mediated vasoconstriction that is independent of extracellular calcium, thereby unmasking a calcium-entry sensitive mechanism of vasoconstriction to cirazoline.

Lack of involvement of alpha-adrenoceptors in sympathetic neural vasoconstriction in the hindquarters of the rabbit

The hypothesis that sympathetic nerves in arterial blood vessels activate excitatory receptors distinct from alpha-adrenoceptors was investigated in vivo in the rabbit. In anaesthetized, ganglion-blocked rabbits, graded stimulation of the lumbar sympathetic nerve chains caused graded hind limb vasoconstriction. The responses to single pulses and short trains of stimuli were unaffected by benextramine (10 mg kg-1) and the longer trains were enhanced. Phenoxybenzamine (5 mg kg-1) slightly reduced the responses to short trains of stimuli and did not affect the responses to long trains. The dose-response curve to intra-arterial noradrenaline (after beta-adrenoceptor blockade) was shifted rightwards about ten fold by benextramine (10 mg kg-1) and by phenoxybenzamine (5 mg kg-1). In conscious rabbits the vasoconstriction caused by the nasopharyngeal reflex initiated by smoke inhalation was unaffected by benextramine (10 mg kg-1). Small mesenteric arteries (less than 250 microns) taken from untreated rabbits responded to noradrenaline with a threshold concentration of about 1 microM. Similar tissues from benextramine (10 mg kg-1)-treated rabbits were unresponsive to noradrenaline at concentrations up to 300 microM. However, these tissues were able to respond to potassium and angiotensin II. Aortic ring segments taken from the same rabbits were only about ten fold less sensitive to noradrenaline than segments from control rabbits. These results are in accord with the hypothesis that sympathetic nerves activate non-alpha-receptors in the vasculature of the rabbit.