Pharmacological evidence that calcitonin gene-related peptide is implicated in cerebral autoregulation

In anesthetized rats, we examined the possibility that calcitonin gene-related peptide (CGRP, a neuropeptide) released in response to transient hypotension may contribute to the reflex autoregulation of cerebral blood flow. Changes in pial arterial diameter (mean 33.0 +/- 1.1 microns) with changes in systemic arterial blood pressure (mean 101.9 +/- 2.7 mmHg) were observed directly through a closed cranial window. In capsaicin-treated rats (depletor of CGRP and substance P, 50 nmol capsaicin injected intracisternally 24 h before experiment), vasodilatation, which was evoked on transient hypotension, and vasoconstriction on reverse of hypotension were markedly attenuated or almost abolished. When changes in pial arterial diameter were plotted as a function of changes in blood pressure, the slopes of regression lines for vasodilatation and vasoconstriction were markedly reduced after capsaicin treatment. Similar reductions were evidenced under suffusion of CGRP antibody serum (1:1,000) and after CGRP receptor desensitization but not after substance P receptor desensitization. Pretreatment with glibenclamide, a K(+)-channel antagonist, also caused severe alterations in the autoregulatory vasomotor responses to hypotension and its reverse. Suffusion with mock cerebrospinal fluid, containing either CGRP or cromakalim, a K(+)-channel opener, dilated the pial artery in a concentration-dependent manner, and their effects were antagonized by glibenclamide. Substance P produced a vasodilatation, which was unaffected by glibenclamide.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of ouabain on relaxation induced by cromakalim in human and canine mesenteric arteries

We investigated the effect of cromakalim, a K+ channel opener, that activates indirectly the Na(+)-K+ pump, in association with increased K+ conductance in the mesenteric arteries. In 65% of human mesenteric arteries tested, the concentration-dependent relaxation curves for cromakalim were biphasic: the low concentration (< 10(-7) M) effect was preferentially inhibited by ouabain, whereas the higher concentration effect was significantly inhibited by glibenclamide. In branches of canine mesenteric artery, the cromakalim-induced relaxation was inhibited by pretreatment with ouabain (1 microM) as well as by glibenclamide (1 microM). The reduction in contraction of human and canine mesenteric arterial strips caused by cromakalim was totally reversed by pretreatment with ouabain (1 microM) or glibenclamide (1 microM). On the other hand, in canine mesenteric artery, cromakalim caused a significant stimulation of 22Na+ influx and ouabain-sensitive 86Rb+ uptake in association with increased 86Rb+ efflux, all of which were inhibited by glibenclamide (1 microM). Thus, it is suggested that cromakalim possesses the additional property to stimulate the Na(+)-K+ pump through an elevation in intracellular Na+, resulting in strong relaxation of blood vessels.

Disturbances in autoregulatory responses of rat pial arteries by sulfonylureas

It was aimed to test the role of ATP-sensitive K+ channels in the autoregulatory response of cerebral arterioles in vivo. Changes in pial arterial caliber (mean, 43.2 +/- 2.3 microns in diameter) in response to changes in systemic arterial blood pressure (mean, 104.3 +/- 1.4 mmHg) were observed directly through closed cranial windows in anesthetized normotensive rats. During superfusion with vehicle, pial arterial caliber automatically increased in response to hypotension induced by arterial bleeding into a reservoir and decreased on reverse of arterial blood pressure by infusion of blood. After pretreatment with sulfonylureas, glibenclamide (1 and 3 microM) and glipizide (30 and 100 microM), arteriolar dilatation and constriction observed during hypotension and its reverse were disturbed. A similarity was evidenced when hypotension was induced by sodium nitroprusside (750 nmol kg-1min-1, i.v.). Cromakalim, a K+ channel opener, exerted a concentration-dependent vasodilatation of the pial artery and its effect was antagonized by glibenclamide. These data suggest that the endogenous glibenclamide-sensitive K+ channel opener is involved in the modulation of cerebral microvascular autoregulation.

Similarities between effects of superoxide-mediated endothelium-derived relaxing factor and cromakalim

A stable endothelium-derived relaxing factor has been reported to be generated on exposure of endothelial cells to the superoxide anion. In this study, we first evaluated the effects of the relaxing factor and cromakalim on mechanical tone and, second, assessed their consequences on the 86Rb efflux rate. On application of hypoxanthine-xanthine oxidase to a bath for generating superoxide anion, the precontracted rabbit mesenteric artery exhibited another transient increase in contraction, followed by sustained relaxation. This relaxation was lost in the K(+)-physiological salt solution (PSS) (greater than 35 mM) and was inhibited by glibenclamide (10 microM) but not by N-methyl-L-arginine or methylene blue. Hypoxanthine-xanthine oxidase application did not increase either basal or stimulated synthesis of guanosine 3',5'-cyclic monophosphate. In the presence of 2 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and 10 mM MgCl2, the relaxing factor caused a significant increase in 86Rb efflux from the aortic and mesenteric arterial segments, as did the cromakalim. The increased 86Rb efflux, either by the relaxing factor or by cromakalim, was wholly inhibited by glibenclamide. These results suggest that superoxide-mediated endogenous relaxing factor may have a similar mechanism of action to cromakalim in vasodilatation.

Release of superoxide-dependent relaxing factor(s) from endothelial cells

In the present work, an experimental system was designed to study superoxide anion radical, implicated as the cause of vascular dilatation. To circumvent its direct effect, we employed a two-bath system. When the endothelial cells (EC) were exposed to electrical field stimulation (EFS) or to a hypoxanthine-xanthine oxidase system in bath A plus its physiological buffer solution suffused on a helical strip of cat basilar artery in bath B, the contraction to 5-hydroxytryptamine (5-HT) was depressed to approximately 40-50% of the control value. The reduction was not elicited on EFS in a state of calcium deficiency or in the absence of EC. The depression could be prevented by pretreatment with superoxide dismutase (SOD), but not with an effective dose of catalase, dimethyl sulfoxide (DMSO), mannitol, or indomethacin. The percent depression of contraction was paralleled by an increase in SOD-inhibitable cytochrome c reduction, which was not associated with cyclic guanosine 3',5'-monophosphate formation. These results suggest that superoxide-dependent relaxing factor is released from EC differently than the endothelium-derived relaxing factor mediated by acetylcholine.

Assessment of superoxide-mediated release of vascular-inhibitory factor(s) from endothelial cells by using a two-bath system

Release of a vascular-inhibitory factor from endothelial cells (EC), different from endothelium-derived relaxant factor (EDRF), was identified through use of a two-bath system. This two-bath system precluded the effects of oxygen-free radicals that appear when electrical field stimulation (EFS) is directly imposed on detector muscle.

Increased alpha 1- and decreased alpha 2-adrenoceptor sensitivities upon chronic treatment with imipramine in mediating cardiovascular responses in pithed rats

After chronic treatment with imipramine (20 mg/kg, i.p., twice daily for 14 days) the pressor dose-response curves to phenylephrine, methoxamine and cirazoline (alpha 1-adrenoceptor agonists) significantly shifted to the left with decreased PD50 values in pithed rats; however, the dose-response curve to Sgd 101/75, a selective alpha 1-adrenoceptor agonist was not affected. On the other hand, the alpha 2-adrenoceptor agonists such as B-HT 920, xylazine and clonidine produced a rightward shift for both the pressor (increased PD50) and cardioinhibition (increased ID50) dose-response curves in these rats. These results required treatment with imipramine over 2 weeks. Chronic treatment with imipramine has reduced the antagonism by prazosin of the pressor effect of phenylephrine when compared with the dose-ratios between the 2 groups. On the contrary, the antagonism by piperoxan of the cardioinhibitory effect of B-HT 920 was rather enhanced by the treatment, but that of the pressor effect of B-HT 920 was little changed. In cerebrocortical membrane fractions obtained from rats pretreated with imipramine, Ki of phenylephrine to displace [3H]prazosin was decreased, whereas that of clonidine and yohimbine to displace [3H]yohimbine was increased. In conclusion, it is demonstrated that after chronic imipramine treatment the peripheral alpha 2-adrenoceptors (both presynaptic and postsynaptic sites) as well as central alpha 2-adrenoceptors respond with a decreased sensitivity to the alpha 2-adrenoceptor agonists, and moreover, this treatment produces an increased sensitivity of the central and peripheral alpha 1-adrenoceptors to the alpha 1-adrenoceptor full agonists.

Role of central alpha 2-adrenoceptors on the development of muricidal behavior in olfactory bulbectomized rats: effect of alpha 2-adrenoceptor antagonists

Upon surgical removal of olfactory bulbs, by the 8th to 10th day after the surgery muricidal behavior has been demonstrated in 75.4% of rats. In the homogenates of forebrain obtained from rats demonstrating muricidal behavior, levels of noradrenaline, 5-hydroxytryptamine and their metabolites (i.e., 3-methoxy-4-hydroxyphenylglycol sulfate and 5-hydroxyindoleacetic acid) were not changed. However, the fractional turnover rate of noradrenaline in the forebrain of the olfactory bulbectomized rats was significantly decreased without alterations in the metabolism of 5-hydroxytryptamine when compared to that of the sham rats. The muricidal behavior and the aggressiveness were suppressed by antagonists of alpha 2-adrenoceptors (yohimbine, idazoxan and rauwolscine), but not by the alpha 1-adrenoceptor antagonist (corynanthine). Furthermore, upon administration of yohimbine or idazoxan to rats demonstrating the muricidal behavior, the level of noradrenaline in the homogenates of forebrain was decreased while that of 3-methoxy-4-hydroxyphenylglycol sulfate was increased. The maximum binding capacity (Bmax) of [3H]yohimbine to the forebrain membranes obtained from the olfactory bulbectomized rats was significantly higher than that from the sham rats without demonstrating any differences in dissociation constants (Kd) between the two brain membranes. Based on these results, it was suggested that olfactory bulbectomy has caused some functional changes in central alpha 2-adrenoceptors.

Enhancement of central and peripheral alpha 1-adrenoceptor sensitivity and reduction of alpha 2-adrenoceptor sensitivity following chronic imipramine treatment in rats

After chronic imipramine treatment (20 mg/kg i.p., once daily for 14 days) the dose-response curve of the isolated rat anococcygeal muscle to phenylephrine shifted to the left, and furthermore, the -log KA value (affinity) for phenylephrine was significantly increased without affecting the affinity for guanfacine. On the other hand, such treatment caused a shift to the right of the dose-response curve to guanfacine on aortic strips and the affinity of the alpha-adrenoceptor for guanfacine was lowered without any accompanying changes in the affinity value for phenylephrine. However the relative efficacies of phenylephrine or guanfacine were not influenced by imipramine in either preparation. The ability of phenylephrine to displace [3H]prazosin from its specific binding sites was significantly enhanced after chronic imipramine treatment. These results may indicate that following chronic imipramine treatment the alpha 1-adrenoceptors of both central and peripheral tissues responded with supersensitivity to an alpha 1-preferential agonist, and the alpha 2-adrenoceptors with reduced sensitivity to an alpha 2-preferential agonist.

"On" and "off" responses of guinea pig ureter

The in vitro guinea pig ureter responds to 5-s trains of electrical stimuli with two contractions: the first, an "on" response, occurs within 0.1-0.3 s after the onset of the stimulus train; the second, an "off" response, occurs 0.2-1.0 s after the termination of the stimulus train. Force decreases between the two responses during a time when the stimulus is still being delivered. Longer duration and/or higher frequencies of stimuli within the train are required to elicit the off response than the on response. Neither the on nor the off response appears to be neurally mediated, since both responses are unchanged by tetrodotoxin, phentolamine, atropine, and pyrilamine. Decreasing temperature from 37 to 22 degrees C decreases the amplitude of the on response and increases the amplitude of the off response. Calcium-free solution, 2 mM ethylenediaminetetraacetic acid (EDTA), 1 mM Mn2+, and 1 microM verapamil abolish the on response at a time at which the off response continues to persist. Conversely, 0.5 mM caffeine and 0.1 mM theophylline abolish the off response, whereas they only slightly reduce the on response. These data suggest that the on response depends on extracellular free calcium, whereas the off response is more dependent on bound or stored calcium.

Effect of age on contractility of guinea pig ureter

Longitudinal force-length relationships of guinea pig ureter were studied in vitro in animals 3 weeks, 3 months, and 3 years of age. Distal segments of 3-week-olds underwent a greater per cent increase in length in response to an applied resting force than those from older age groups. Resting force-length relations of proximal segments were unaffected by age. Active force was higher in distal segments at all ages and at all lengths. Maximum active force increased with age. The increase in force developed between 3 weeks and 3 months of age seemed to be attributable to an increase in contractility since there was an associated increase in active stress; the increase developed between the 3-month- and 3-year-olds was associated with increased muscle cross-sectional area but with no change in active stress. This finding suggests that no change in contractility occurred during this time interval.