Pharmacological comparison of isolated human cerebral and digital arteries

Effect of aging on contractile response to KCl, norepinephrine and 5-hydroxytryptamine in isolated human basilar artery

1. We investigated the effect of aging on contractile response in human basilar artery. 2. The maximal contraction caused by KCl, norepinephrine and 5-hydroxytryptamine decreased with age. 3. ED50 value for norepinephrine or 5-hydroxytryptamine did not correlate with age. 4. These results suggest that the decrease in contractile response is due to nonspecific changes in the medial structure of the artery.

Intervascular and stimulus selectivity of nitrendipine and related derivatives in KCl and prostaglandin F2 alpha precontracted porcine arteries

1. Dihydropyridine-type calcium entry blockers exhibit a different vasodilator potency depending on the arterial tissue (intervascular selectivity) as well as on the precontracting stimulus used (stimulus selectivity). In addition, the structure of their ester side chains seems to influence their activity. 2. Vascular activity of nitrendipine and six related 3-ester side chain derivatives was investigated in isolated coronary, ulnar and basilar arteries of the pig following precontraction with KCl or prostaglandin F2 alpha (PGF2 alpha). 3. After depolarization, all dihydropyridines exhibited a weak preferential action on coronary arteries. Bay E 6927 produced the strongest effect in all vessel types. By contrast, precontraction with PGF2 alpha resulted in a marked preferential action in basilar arteries, although higher concentrations of the dihydropyridines were required for half maximal vasorelaxation. In each case, ulnar arteries were less sensitive. 4. Except with Bay O 5572, the most bulky substituted and least active derivative, only moderate differences were observed within the dihydropyridines studied. On the other hand, there was a pronounced increase in the ratios of the half maximal active concentrations required after precontraction of the vessels with PGF2 alpha compared to KCl (stimulus selectivity) following a limited prolongation of the 3-ester side chain up to an isopropyl-group. 5. It is suggested that the observed shift in the intervascular selectivity after precontraction with PGF2 alpha is a consequence of different contractile mechanisms in the three vessel types studied. The degree of the stimulus selectivity may also depend on the structure of the dihydropyridines.

The influence of endothelium on the action of PGF2 alpha and some dihydropyridine-type calcium antagonists in porcine basilar arteries

Vascular endothelium modulates the effect of various vasoconstricting mediators as well as the affinity of dihydropyridine-type calcium entry blockers. To further investigate this influence, vasoconstriction by PGF2 alpha as opposed to KCl and the affinity of nitrendipine and some related 3-ester side-chain derivatives were determined in isolated porcine basilar arteries in the presence and in the absence of intact endothelium, as well as in the presence of methylene blue. Treatment with methylene blue or mechanical endothelial damage increased the contractile work of basilar arteries stimulated by PGF2 alpha and reduced the affinity of the dihydropyridines in such precontracted vessels. Both experimental conditions resulted in nearly the same effect. In addition, the degree of intact endothelium, as determined by substance-P-induced vasodilation, significantly correlated with the corresponding efficacy of all dihydropyridines examined. In contrast, KCl-mediated contractions remained unchanged. It is suggested that the endothelium (probably due to the production and release of endothelium-derived vasorelaxing factors, such as EDRF and/or prostacyclin) may attenuate PGF2 alpha-induced transmembrane calcium influx through receptor operated calcium channels, whereas potential operated calcium channels seems to be unaffected.

Pharmacological comparison of endothelium-dependent relaxation in isolated cerebral and extracerebral arteries

Endothelium-dependent relaxation was induced by acetylcholine (ACh), adenosine triphosphate (ATP), and thrombin in isolated cerebral and extracerebral arteries obtained from rabbits and dogs. Using an isometric tension-recording method, the authors then examined the difference in the extent of relaxation between the cerebral and extracerebral arteries. In rabbits, the dose-response curve of the basilar artery for ACh was significantly different (p less than 0.05) from curves of the femoral and common carotid arteries. The IC50 value (the concentration inducing a one-half inhibition of the initial contractile tone) for the basilar artery in ACh-induced relaxation was significantly higher (p less than 0.05) than for the common carotid artery, although the mean maximum relaxation of the basilar artery to ACh was not significantly different from that seen in extracerebral arteries. The relaxing effect of ACh in dogs was much less in the middle cerebral and basilar arteries than in the common carotid, vertebral, and femoral arteries. On the other hand, both ATP (in rabbits and dogs) and thrombin (in dogs) induced significantly more (p less than 0.05) relaxation in the cerebral arteries than in the extracerebral arteries. Endothelium-dependent relaxation induced by ACh or ATP has been demonstrated in a wide range of arteries from a variety of animals. The present results suggest that ATP has a more important role than ACh in the regulation of the vascular tone of the major cerebral arteries in these two species.

Differential effects of prostaglandins on mesenteric and cerebral arteries isolated from premature newborn baboons

Responses to prostaglandins (PGs) E1, E2, F2 alpha and I2 at a wide range of concentrations were compared in the cerebral and mesenteric arteries isolated from premature newborn baboons. PGE1 at low concentrations relaxed the cerebral artery but not the mesenteric artery. PGI2 produced a relaxation of similar magnitude in both arteries. PGF2 alpha in high concentrations produced a much greater contraction in the mesenteric artery than in the cerebral artery. Thus, during the perinatal stage, the baboon cerebral artery has distinctive PG-reactivity compared to the extracerebral artery. Clinical use of PGE1 or indomethacin may result in circulatory derangement or redistribution of blood flow in the cerebral and mesenteric circulations, and may produce side effects of these drugs.

Vascular responsiveness of simian digital artery to various vasoactive substances

The stainless steel cannula inserting method was used to observe vascular effects of 7 vasoactive substances on the isolated and cold-stored digital artery of the monkey. The arterial preparation was suspended in a bath at 37 degrees C and perfused with Ringer's solution under constant flow rate. Drugs were administered in the endothelial side of the artery through a cannulated tubing, and the response was obtained as changes in perfusion pressure. All used agonists increased perfusion pressure in a dose-related manner. The order of potency for inducing vasoconstriction was norepinephrine greater than serotonin = phenylephrine = clonidine greater than xylazine greater than tyramine greater than potassium chloride. Inhibiting effect of prazosin (0.001-0.01 microgram) was greater on phenylephrine (0.03 microgram)-induced contractile responses than on clonidine (0.03 microgram)-induced responses, and the reverse was true in the case of yohimbine (0.01-1.0 microgram). Thus, it was suspected that the simian digital artery would be a good model for in vitro investigation of vascular responsiveness as a conduit artery which mainly supplies blood flow to the skin, and that this artery might be contracted not only via postjunctional alpha 1- but also alpha 2-adrenoceptors.

On the postjunctional alpha-adrenoreceptors in rat cerebral and mesenteric arteries

The contractile response to exogenously applied noradrenaline (NA) was examined in vitro in tubal segments (0.2-0.1 mm in diameter) of rat middle cerebral (MCA), basilar (BA) and mesenteric (MA) arteries. In the MCA, the maximum contractile response to NA (10(-4)M) was considerably smaller than that induced by K+ (124 mM) or 5-hydroxytryptamine (10(-5)M), whereas the inverse relationship was found in the MA. NA usually failed to elicit contraction in the BA even in the presence of propranolol and cocaine. In the MCA, propranolol (3 X 10(-7)M) enhanced the maximum contractile response to NA by approximately 100% without affecting the potency of the agonist. In the MA, propranolol had no effect on the concentration-response relationship for NA. Cocaine (10(-5)M) or 6-hydroxydopamine pretreatment increased the NA sensitivity of the MA by a factor of three, whereas these procedures failed to influence the NA sensitivity of the MCA. A marked stereoselectivity was found in the MCA, as (-)-NA was more than 100 times more potent than (+)-NA as a contractile agent. The order of potency of a series of alpha-adrenoreceptor agonists was (-)-adrenaline greater than oxymetazoline greater than (+/-)-NA approximately (-)-phenylephrine greater than methoxamine in the MCA and (+/-)-NA greater than (-)-phenylephrine in the MA. Clonidine failed to elicit contraction in concentrations lower than 3 X 10(-4)M in both types of artery. Prazosin was between three and four orders of magnitude more potent than rauwolscine in inhibiting NA-induced contractions in the MCA and MA. The pA2 values for, respectively, prazosin and rauwolscine were 9.3 and 5.4 in the MCA and 9.7 and 6.8 in the MA. The slope of the Schild plot deviated significantly from unity only for rauwolscine in the MA (0.64). It is concluded that the contractile response to exogenous NA in the MCA and MA is mediated mainly by stimulation of alpha 1-adrenoreceptors, although a small contribution of postjunctional alpha 2-adrenoreceptors in the MA cannot be excluded. In contrast to the MCA, the BA appears to lack contraction-mediating alpha-adrenoreceptors, indicating regional differences in the alpha-adrenoreceptor distribution in the rat cerebrovascular bed.

Vascular effects of ergot alkaloids: a study on human basilar arteries

Postmortem examinations on human basilar arteries were performed in vitro by recording isometric tension from spiral strips in tissue baths at 37 degrees C. Antagonism of 5-HT by ergotamine, dihydroergotamine and methysergide was of the non-competitive type, that of noradrenaline was either absent (i.e. ergotamine) or considerably weaker. In addition each ergot alkaloid acted as a partial agonist in the following order of efficacy: ergotamine greater than dihydroergotamine greater than methysergide, producing pD2 values of 9.30, 9.60 and 7.10 respectively. Evidence is provided that the stimulant activity of ergotamine is mediated through 5-HT receptors. The present results are compared to those obtained on isolated human vein strips as well as to those on canine blood vessels.

The human adrenergic neurovascular mechanism

Although evidence is limited and derived from many studies with many different objectives on a variety of blood vessels under varying experimental conditions, it appears that the human adrenergic vascular neuroeffector mechanism is qualitatively similar to that defined in studies on animals. The characteristics of the alpha-adrenoceptor based upon pA2 measurements and other pharmacological characteristics show it to be similar to adrenoreceptors defined in animals. The sensitivity of vascular smooth muscle to norepinephrine is of the same order throughout the human vasculature and is similar to that seen in most animal vessels. In most blood vessels the alpha-adrenoreceptor is the dominant type. By contrast the beta-adrenoceptor component is usually small with a high threshold except in the coronary artery and facial vein. Sympathetic frequency-response relationships of both arteries and veins are similar to those from animal studies. The human alpha-adrenoceptor is coupled both to sequestered calcium sites and slow calcium channels, and these two mechanisms are responsible for the biphasic response of human blood vessels to norepinephrine. The slow calcium channel but not the sequestered calcium system is sensitive to calcium antagonists. There is evidence that the human adrenergic mechanism is altered in disease.

Techniques for testing isolated blood vessels

There are many different techniques described for studying isolated blood vessels, either human or animal. 1. For postsynaptic responses, spiral strip preparations using isometric tension monitoring probably allow the greatest flexibility for drug concentration-effect relationships. Isotonic tension may be monitored if extreme sensitivity is necessary and ring preparations probably better mimic the situation in vivo, but are harder to use. 2. Minor differences in salt solutions, pH, temperature and degree of oxygenation are probably not important except in special situations such as pH alterations when studying cerebral vessels and pO2 alterations when studying pulmonary or umbilical vessels. It is most important that the calcium concentrations remain constant as altered contractile effect may occur with small alterations in the calcium concentration of the physiological salt solution. 3. Electrical stimulation is necessary for studying presynaptic mechanisms. Most commonly, vessels are canulated and perfused through their lumen and superfused to remove the transmitter which has overflowed from the perivascular nerve stimulation. Neurotransmitter is then measured after preloading with tritiated noradrenaline. Electrical stimulation can also be performed on spiral strips which are immersed in small volume tissue baths allowing rapid changes in the bathing fluid, and although this method allows better monitoring of the postsynaptic responses associated with the electrical stimulation, it is more difficult to perform than perfusion and superfusion.