Norepinephrine-induced contractions of the rat aorta in the absence of extracellular calcium--I. Effects of alpha-adrenoreceptor blockers

Steroid sensitivity of norepinephrine uptake by human bronchial arterial and rabbit aortic smooth muscle cells

We have shown that an inhaled glucocorticosteroid (GS) causes alpha(1)-adrenergic antagonist-blockable, rapid, and transient bronchial vasoconstriction in healthy and asthmatic subjects. Steroids inhibit norepinephrine (NE) uptake by non-neuronal cells, thereby increasing NE concentration at alpha-adrenergic receptor sites. This could explain the GS-induced bronchial vasoconstriction. We therefore studied expression of the steroid-sensitive extraneuronal monoamine transporter (EMT) and steroid sensitivity of NE uptake in human bronchial artery and rabbit aorta (as a substitute for the limited supply of human bronchial artery). NE uptake was measured using a semiquantitative, sucrose-potassium phosphate-glyoxylic acid fluorescence method that we newly adapted for use in single cells. Both human bronchial arteries and rabbit aorta expressed messenger RNA for EMT, and steroids blocked NE uptake into freshly dissociated human bronchial arterial and rabbit aortic smooth-muscle cells (SMCs). In the latter, inhibition of NE uptake by steroids was not altered, either by a protein synthesis inhibitor (cycloheximide) or by a transcription inhibitor (actinomycin D), and corticosterone made membrane-impermeant by conjugation to bovine serum albumin inhibited NE uptake equipotently. These data show that NE uptake into bronchial arterial and rabbit aortic SMCs is sensitive to steroids, possibly mediated by EMT, and suggest a mechanism for GS-induced bronchial vasoconstriction.

Calcium antagonists inhibit contractions to norepinephrine in the rat aorta, in the absence, but not in the presence of the endothelium

1. To compare the effect of diltiazem and verapamil on the responsiveness of vascular smooth muscle to norepinephrine in the presence and absence of the endothelium, rings of rat aorta were studied in organ chambers. 2. The removal of the endothelium decreased the ED50 to norepinephrine and augmented the maximal response to the catecholamine. 3. The contraction to norepinephrine consisted of a rapid initial (phasic) and a (tonic) part. The tonic part was reduced in the presence of the endothelium. 4. Diltiazem shifted the concentration-response curve to norepinephrine to the right only in rings without endothelium and reduced the difference in maximal response between rings with and without endothelium. 5. Verapamil abolished the difference in sensitivity (ED50) between rings with and without endothelium. 6. Oxyhemoglobin prevented the inhibitory effect of the endothelium on the response to norepinephrine, and unmasked a shift of the ED50 to the catecholamine to the right by diltiazem in rings with endothelium. 7. These experiments suggest that spontaneously released endothelium-derived relaxing factor(s) is a functional antagonist of norepinephrine-induced contractions, presumably by reducing the stimulated influx of extracellular Ca2+.

Alpha 1 adrenergic receptor function in senescent Fischer 344 rat aorta

There have been numerous conflicting reports concerning alpha 1 adrenergic receptor-mediated blood vessel contraction during aging and possible changes in alpha 1 receptor transduction mechanisms have not been investigated. These studies assess capacity of the aging vascular alpha 1 receptor to stimulate production of inositol phosphates, which are its intracellular second messengers, and to elicit a contractile response via this pathway. Aortic ring segments from mature adult (6 month old) and senescent (24 month old) Fischer 344 rats were incubated with [3H]myo-inositol and then stimulated with the alpha 1 agonist norepinephrine (NE, 10(-7)M-3 x 10(-5)M) in the presence of LiCl (10mM), an inhibitor of inositol phosphate metabolism. There was a substantial increase in inositol phosphate accumulation throughout the dose range in aortic rings from 24 month old rats compared to 6 month old rats. This is an alpha 1 receptor response since it is blocked by the alpha 1 antagonist prazosin but not by the alpha 2 antagonist yohimbine. Aortic inositol phosphate accumulation in response to serotonin did not change with age. To assess second messenger stimulated contraction, aortic ring segments were placed in Ca++ free buffer and then stimulated with NE. Under these conditions Ca++ influx is eliminated and contraction depends on the actions of intracellular second messengers. There is an age-related reduction in aortic contraction in Ca++ free buffer. These results suggest that aortic alpha 1 receptor-mediated formation of inositol phosphate intracellular second messengers is enhanced during aging. Despite this, the capacity of senescent arteries to elicit contraction utilizing second messenger pathways seems to be deficient.

A comparison of the ability of various alpha-adrenoreceptor agonists to induce EGTA-resistant contractions of rat aorta

Phenylephrine (PE) and Guanfacine (GU) produced maximum contractions of rat isolated aorta which were 96.8 +/- 1.3% and 96.0 +/- 2.8% respectively of the maximum responses induced by noradrenaline (NA). The responses produced by maximum concentrations of NA following 2 min incubation of the preparations with 3.0 mM EGTA were 38.1 +/- 1.2% of the maxima achieved in normal Krebs. PE produced maximum EGTA-resistant responses which were significantly less than those seen with NA; GU produced EGTA-resistant responses which were significantly less than those to PE. The concentration response curves to NA and PE were shifted to the right by incubation with EGTA. The curve for PE was shifted by a significantly greater degree than that for NA. Amidephrine, UK-14,304 and St-587 produced only nominal EGTA-resistant responses even though one of these (UK-14,304) produced contractions in normal Krebs which were almost 70% of the maximum achieved by NA. The log dose ratio for all agonists (except amidephrine) were similar following the addition of 5 nM prazosin. Amidephrine had no effect in the presence of this concentration of prazosin. It is concluded that all agonists tested were acting on alpha 1-adrenoreceptors. Prazosin (5 nM) but not corynanthine (2.5 microM) slowed the rate of rise of contraction of all agonists tested. It is proposed that the rate of rise of agonist/receptor combinations is an important determinant of alpha-adrenoreceptor agonist induced EGTA-resistant responses in rat aorta.

Perfusion with calcium free solution potentiates prazosin blockade of noradrenaline induced pressor responses in the mesenteric bed of the rat

Dose response curves have been constructed from pressor responses to bolus injections of noradrenaline (NA) in the perfused mesenteric bed of the rat. Prazosin (1.0-50 nM) caused a shift to the right of these dose response curves and significantly depressed the maximum obtainable response without any significant change in slope, whereas corynanthine exhibited classical competitive antagonism. When perfused with calcium-free solution, the pressor responses to NA were of shorter duration and the maximum obtainable response was reduced by 26.9 +/- 3.1%. When perfused with calcium-free solution the antagonistic effect of corynanthine was unchanged but that of prazosin was enhanced such that 5.0 nM of prazosin now caused a much greater depression of the maximum NA-induced pressor responses and a marked reduction in slope of the dose response curve. Corynanthine (10 microM) was able to reverse these antagonistic effects of prazosin. These observations are consistent with the suggestion that prazosin selectively antagonizes NA-induced calcium release.