14C-histamine release during vasodilatation induced by lumbar ventral root stimulation

Endogenous calcitonin gene-related peptide mediates nonadrenergic noncholinergic depressor response to spinal cord stimulation in the pithed rat

The role of endogenous calcitonin gene-related peptide (CGRP) in the nonadrenergic noncholinergic depressor response to spinal cord stimulation was studied in the pithed rat in vivo. Pithed rats were given hexamethonium (2 mg/kg per minute i.v.) to block autonomic outflow, and mean blood pressure was artificially maintained at approximately 100 mm Hg with methoxamine (10-15 micrograms/kg per minute i.v.). Electrical stimulation of the spinal cord at the level of the lower thoracic vertebra (T9-12) caused a fall in blood pressure in a frequency-dependent (0.5-10 Hz), voltage-dependent (2.5-50 V), and pulse duration-dependent (0.25-8 msec) manner. The heart rate did not change during the depressor response. The depressor response was long lasting, and the maximum response was elicited by stimulation at 4-6 Hz. The neurotoxin tetrodotoxin (100 micrograms/kg i.v.) abolished the depressor response to spinal cord stimulation, whereas treatment with propranolol (0.5 mg/kg per minute i.v.), atropine (0.05 mg/kg per minute i.v.), or a combination of pyrilamine (0.5 mg/kg per minute i.v.) and cimetidine (0.5 mg/kg per minute i.v.) did not affect the response. In pithed rats treated with capsaicin (total dose of 500 mg/kg s.c.), spinal cord stimulation caused a slight depressor response. Exogenous CGRP, but not acetylcholine, isoproterenol, histamine, or substance P, caused a sustained fall in blood pressure that mimicked the spinal cord stimulation-induced depressor response. Continuous infusion of CGRP[8-37] (60 nmol/kg per minute i.v.), a CGRP receptor antagonist, markedly inhibited the depressor responses not only to spinal cord stimulation but also to exogenous CGRP.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of histamine and the histamine antagonists mepyramine and cimetidine on human coronary arteries in vitro

The effects of histamine have been studied on human isolated coronary artery preparations taken from hearts ranging in age from 9 to 73 years. Histamine in large concentrations (100 microM) contracted arteries which were without tone or spontaneous activity and sometimes induced rhythmic contractile activity. If spontaneous rhythmic activity was present it was enhanced by histamine. The contractile effects of histamine were inhibited by mepyramine but not by cimetidine. Arteries which were contracted by depolarization responded with relaxation to histamine concentrations lower than those required to evoke a contraction; arteries from younger hearts were more sensitive than those from older hearts. Mepyramine potentiated the maximal relaxant effect of histamine in arteries from hearts of all ages but cimetidine had very little effect. In the presence of mepyramine, cimetidine antagonized the relaxant effect of histamine, shifting the concentration-effect curve to the right. It is concluded that human coronary arteries contain both H1- and H2-type receptors, the H1-receptors mediating contraction. The relaxant effects of histamine can only be inhibited by a combination of both H1- and H2-receptor antagonists.

Vasodilator neurons supplying skin and skeletal muscle of the limbs

Three types of neural vasodilator responses can be identified in the vascular beds of limbs. The first is almost certainly cholinergic, is restricted to precapillary resistance vessels in skeletal muscle, and appears to be activated during emotionally-oriented arousal. The second is non-cholinergic, is attenuated by histamine H1-receptor antagonists, and occurs in both skeletal muscle and skin. This response may also be evoked by emotional stimuli. The third response is localized to arteriovenous shunts in the digits, and is generally insensitive to both anticholinergic and antihistaminic drugs. It is probably implicated in thermoregulatory control, and may mediate cold vasodilation.

Effects of histamine on the resting and stimulation-induced release of [3H]-noradrenaline in guinea-pig isolated atria

1 The sympathetic transmitter stores of guinea-pig isolated atria were labelled with [3H]-noradrenaline. The effects of histamine (0.3 to 100 mumol/l) on resting and stimulation-induced (S-I, 2 Hz for 10 s) release of radioactivity were investigated. 2 Histamine, in low concentrations (0.3 and 1 mumol/l) had no effect on resting release but inhibited S-I release of radioactivity. The inhibition was abolished by the H2-receptor antagonist, cimetidine (10 mumol/l) and also by the H1-receptor antagonist, mepyramine (1 mumol/l). 3 The inhibitory actions of histamine on S-I release were not due to indirect effects involving alpha-adrenoceptors, beta-adrenoceptors, muscarinic cholinoceptors or prostaglandin synthesis. 4 Histamine in a high concentration (100 mumol/l) increased the resting and S-I release of radioactivity. The increase in resting release was abolished by the neuronal uptake blocking drug cocaine (30 mumol/l) but the increase in S-I release was only partially blocked by cocaine.

Inhibition of adrenergic neurotransmission in canine vascular smooth muscle by histamine: mediation by H2-receptors

Histamine depressed the contractions of dog saphenous vein strips caused by stimulation of their sympathetic nerves. This was due to a decrease in the release of norepinephrine which appears to be mediated by histamine H2-receptors. The evidence for this is as follows: (1) Contractions of the strips caused by activating the nerve endings electrically or by depolarization with potassium ions were depressed by histamine, whereas contractions caused by tyramine and norepinephrine were either unchanged or augmented. (2) Strips were incubated with norepinephrine[7-3H] and mounted for superfusion and isometric tension recording. The perfusate was collected for estimation of total radioactivity and for column chromatographic separation of norpinephrine and its metabolites. Histamine (0.9 muM) depressed the release of norepinphrine[7-3H] during contractions caused by electric stimulation, whereas the release of radioactive compounds caused by tyramine was unaffected. (3) The depression by histamine of the contractions and the efflux of radioactive compounds caused by electric stimulation were inhibited by an H2-receptor antagonist (metiamide), but were unaffected by an H1-receptor antagonist (pyrilamine). (4) Contractions caused by electric stimulation were inhibited by an H2-receptor agonist (4-methylhistamine) and augmented by an H1-receptor agonist (2-methylhistamine). These findings suggest the possibility that histamine, which is abundant in sympathetic nerves, might have a regulatory role in the release of the neurotransmitter.

Cutaneous and muscular rasodilation in the canine hindlimb evoked by central stimulation

Using stereotaxic procedures, we electrically stimulated specific sites in the hypothalamus and midbrain of anesthetized dogs pretreated with guanethidine and atropine methonitrate. A tract in which stimulation caused noncholinergic dilator responses in the hindlimbs was identified. The course of this trace was different from that subserving cholinergic vasodilation in the hindlimb musculature. In a number of experiments we studied the proportional distribution of blood flow to leg and paw. Responses restricted to the paw were regarded as occurring mainly in cutaneous vessels; those restricted to the leg were regarded as occurring mainly in the skeletal muscle vessels. Some dilator responses in both beds were abolished by intra-arterial administration of antihistamines: other dilator responses were abolished by intra-arterial injections of dopamine antagonists. Centrally evoked dilation of leg and paw vessels by noncholinergic pathways suggests physiological roles for these fibers in the regulation of cardiovascular function.