Relationship between nifedipine sensitivity of aortae and blood pressure of stroke-prone spontaneously hypertensive rats.

1. We have previously described an increased sensitivity to inhibition by nifedipine of noradrenaline-induced contractures of blood vessels in hypertension. In this study we have investigated whether changes in blood pressure (BP) change the sensitivity to nifedipine and K+ of aortic rings from normotensive (Wistar-Kyoto rats, WKY) and stroke-prone spontaneously hypertensive rats (SHRSP). 2. SHRSP were treated with: hydralazine plus hydrochlorothiazide; captopril plus hydrochlorothiazide; hydralazine plus guanethidine; or captopril alone. WKY rats were treated with deoxycorticosterone acetate (DOCA) and NaCl. Treatment commenced from 5 weeks of age and continued until 13-15 weeks. 3. The SHRSP treatments produced similar reductions in BP, and the BP of all the treated groups were significantly lower than the mean BP of untreated SHRSP (201.0 +/- 7.7 mmHg). The mean BP of the treated WKY rats (134.2 +/- 7.6 mmHg) was significantly higher than the mean BP of the untreated WKY rats (86.8 +/- 7.4 mmHg). 4. An area-under-curve (AUC) analysis of the inhibitory effects of nifedipine on responses of aortae to noradrenaline showed no differences between treated and untreated SHRSP groups (overall mean 40.6 +/- 1.9% and 43.4 +/- 3.4% inhibition of control AUC, respectively), or between DOCA-salt treated WKY and untreated WKY groups (58.8 +/- 5.9 and 64.8 +/- 2.3, respectively). Noradrenaline-induced contractures of aortae from all SHRSP groups were significantly more sensitive to inhibition by nifedipine than aortae from both WKY groups. 5. The molar concentration of agonist required to evoke 50% of the maximum response (EC50) values for potassium chloride (KCl) were significantly increased in the aortae of all treated SHRSP groups in comparison to those from untreated SHRSP (treated SHRSP groups, 15.53 +/- 0.68 mmol/L vs untreated SHRSP group, 11.36 +/- 1.10 mmol/L). The EC50 values for KCl for the aortae from the DOCA-treated WKY rats were significantly less than those from aortae of the untreated WKY (11.80 +/- 0.80 and 17.08 +/- 1.50 mmol/L, respectively). 6. We conclude that reduction (in SHRSP) or increase (in WKY) of the BP has no effect on the sensitivity of aortic smooth muscle to the inhibitory effects of nifedipine on responses to noradrenaline, suggesting that alterations in voltage-dependent Ca2+ mechanisms may be a primary phenomenon in the SHRSP. In contrast, the fact that sensitivity to KCl changes in the treated SHRSP and WKY aortae suggests such sensitivity is secondary to the BP and thus a separate phenomenon from voltage-dependent Ca2+ mechanisms.

Increased sensitivity to inhibition by nifedipine of responses of the mesenteric artery bed of the SHRSP to noradrenaline is not dependent on alpha 1-adrenoceptor subtypes.

The effects of noradrenaline (NA) on the perfusion pressure of mesenteric vascular bed preparations from stroke-prone spontaneously hypertensive rats (SHRSP) or weight-matched normotensive Wistar-Kyoto (WKY) rats in the presence of chloroethylclonidine (CEC, alpha 1B-adrenoceptor antagonist) or WB4101 (WB, alpha 1A-adrenoceptor antagonist), with or without the addition of nifedipine, were studied. NA caused a greater maximum increase in the perfusion pressure of the SHRSP mesenteric bed than that of the WKY, and the EC50 for NA was lower in the SHRSP. There was no difference between the normotensive or hypertensive beds in the reduction of responses to NA produced by WB or CEC. Nifedipine, in the presence of either CEC or WB, further reduced responses to NA, but to a significantly greater extent in the SHRSP than in the WKY. There was no difference within either group between the additional inhibitory effect of nifedipine in combination with CEC or WB. These results confirm that following alpha 1-adrenoceptor subtype blockade, the response to NA by the mesenteric bed of the SHRSP is more sensitive to inhibition by nifedipine than its WKY counterpart. However, the increased sensitivity to the inhibitory effects of nifedipine on responses to NA is not a result of preferential linkage of Ca2+ entry mechanisms to one or other of the alpha 1-adrenoceptor subtypes.

The difference in nifedipine sensitivity between hypertensive and normotensive rat aortae is dependent on age.

1. In order to investigate further whether the increased sensitivity to inhibition by nifedipine of the responses to noradrenaline of aortae from spontaneously hypertensive stroke-prone (SHRSP) rats is related to the development of the hypertension, we have compared the sensitivity to noradrenaline, potassium chloride (KCl) and nifedipine of aortae from SHRSP and control (Wistar-Kyoto) WKY rats of different age groups (young: 3-5 weeks, and adult: 13-16 weeks). 2. The sensitivity to KCl was found to be less in the aortae from the adult WKY group than in any of the other three groups. Responses to noradrenaline of the adult WKY aortae were also less sensitive to inhibition by nifedipine in comparison to each of the other three groups. 3. The changes in sensitivity were not due to the changes in the populations of alpha 1-adrenoceptor subtypes as responses of the adult SHRSP aortae to noradrenaline were more sensitive to nifedipine in the presence of either the alpha 1-adrenoceptor subtype antagonists chloroethylclonidine or WB4101 than were those of the adult WKY aortae, but aortae from the young SHRSP were not. 4. These results suggest that, rather than the SHRSP aorta becoming more sensitive to nifedipine and potassium depolarization as hypertension develops, it is the WKY aorta that becomes more resistant as it matures.

Effects of fentanyl on the rat aorta are mediated by alpha-adrenoceptors rather than by the endothelium.

We have assessed if fentanyl interacts with the endothelium to affect vessel tone. In the presence or absence of endothelium, fentanyl in concentrations greater than 10(-7) mol litre-1 decreased the sensitivity of the rat aortic rings to phenylephrine, but fentanyl in smaller concentrations had no significant effect. Rings, with or without endothelium, and pre-contracted by phenylephrine were relaxed by fentanyl, and this relaxation was not inhibited by the opioid receptor antagonist, naloxone. Pretreatment of the rings with either fentanyl or phentolamine protected alpha-adrenoceptors from block by the alpha-adrenoceptor antagonist, phenoxybenzamine. We conclude that, in the rat aorta, fentanyl-induced relaxation was mediated by alpha-adrenergic receptors, and that the endothelium modulated, but did not mediate, this relaxation.

Sufentanil and alfentanil cause vasorelaxation by mechanisms independent of the endothelium.

1. The aim of these experiments was to determine if the vasorelaxation of the rat isolated aorta induced by sufentanil or alfentanil is mediated by the endothelium, and, if not, by alpha-adrenoceptor blockade, or a direct effect on the smooth muscle. 2. Both sufentanil (from 10(-7) mol/L to 10(-4) mol/L) and alfentanil (from 10(-7) mol/L to 3 x 10(-4) mol/L) relaxed rings, where endothelium was intact and precontracted with 40 mmol/L KCl, in a concentration-related manner. Similarly, sufentanil and alfentanil relaxed rings, in the presence or absence of endothelium, which had been precontracted with phenylephrine. 3. Naloxone (10(-4) mol/L) had no significant effect on the relaxation induced by either sufentanil or alfentanil. 4. In a similar manner as phentolamine, pretreatment with sufentanil protected alpha-adrenoceptors from blockade by phenoxybenzamine (PBZ) in both endothelium intact and denuded rings, but the estimated potency of sufentanil was approximately 100-fold less than that of phentolamine in alpha-adrenoceptor protection. Treatment with alfentanil did not produce any receptor protection. 5. We concluded that, in the rat aorta, vascular relaxation induced by sufentanil is mediated by both alpha-adrenoceptor blockade and a direct effect on smooth muscle, whilst the relaxant effect of alfentanil is caused by direct effects alone. We also concluded that the endothelium has little role in relaxation produced by either drug.

Differences between rat and guinea pig aorta in postreceptor mechanisms of alpha 1-adrenoceptors.

The relative importance of different postreceptor mechanisms associated with alpha 1-adrenoceptor activation in guinea pig aorta and rat aorta was compared. In both tissues, a concentration-dependent correlation was observed between contractile responses produced by high concentrations of norepinephrine (NE) and inositol-1-phosphate (IP1) accumulation. Blockade of Ca2+ entry through voltage-dependent membrane channels by nifedipine had no inhibitory action on NE-induced contractile responses in guinea pig aorta, but significantly inhibited NE-induced contractile responses in rat aorta. Nifedipine had no major effect on NE-induced IP1 accumulation in either tissue. A medium with no Ca2+ inhibited NE-induced contractile responses and IP1 accumulation in guinea pig aorta, but had a more marked effect on contractile responses and IP1 accumulation in rat aorta. The combination of a high concentration of nifedipine and a medium with no Ca2+ almost completely inhibited NE-induced contractile responses and IP1 accumulation in both tissues. Exposure to EGTA also virtually completely inhibited NE-induced contractile response and IP1 accumulation in both tissues. Significant 45Ca2+ entry was stimulated in rat aorta by NE, and this entry was completely blocked by nifedipine. No significant 45Ca2+ entry was stimulated by NE in guinea pig aorta. We conclude that the most important postreceptor event linked to alpha 1-adrenoceptor stimulation in guinea pig aorta is activation of the phosphatidylinositol pathway, whereas in rat aorta Ca2+ entry through voltage-dependent membrane channels is as important as activation of the phosphatidylinositol pathway. We also conclude that activation of the phosphatidylinositol pathway in both tissues is critically dependent on the presence of a small amount of Ca2+, which may enter from the external medium.

Differing calcium sensitivities of human cerebral and digital arteries, human metatarsal veins, and rat aorta.

1. The effects of the voltage dependent calcium channel blocking agent nifedipine, and of a calcium free bathing medium, on the responses of human blood vessels obtained postmortem to various agonists have been compared with those of the rat aorta. The human vessels studied were digital arteries, basilar arteries and metatarsal veins. 2. Responses to potassium chloride (5-80 mM), noradrenaline (10(-9)-10(-4) M), 5-hydroxytryptamine (10(-8)-10(-4) M) and U46619 (10(-11)-10(-6) M), in the presence and absence of nifedipine (1, 10, and 100 nM) or in a calcium-free bathing medium, were assessed using an area-under-curve analysis. 3. In general, the order of sensitivity of the vessels to inhibition of agonist induced contractures by nifedipine was basilar arteries greater than metatarsal veins = digital arteries = rat aorta. 4. For all the vessels, the order of sensitivity for antagonism of responses to the agonists by nifedipine was potassium chloride greater than 5-hydroxytryptamine = noradrenaline greater than U46619. 5. A calcium free bath inhibited responses of digital arteries to potassium chloride more than noradrenaline, 5-hydroxytryptamine or U46619, and responses of rat aorta to a greater extent than responses of the digital arteries. 6. In the rat aorta, a calcium-free bath inhibited responses to all agonists (except KCl) to a greater degree than did nifedipine. 7. We conclude that inhibition of extracellular calcium entry through voltage dependent calcium channels affects contractile responses of different blood vessels to different extents, and, within the same blood vessel, responses to different contractile agonists to different extents.

Interaction between platelet-released serotonin and thromboxane A2 on human digital arteries.

Synergism between the contractile effects of platelet-derived serotonin (5HT) and thromboxane A2 (TXA2) on a human blood vessel has been investigated by incubating strips of digital arteries in subcontractile concentrations of either 5HT or the TXA2-mimetic agent U46619. Either agonist U46619 or 5HT, in subcontractile concentrations, significantly potentiated the contractile response to the other. The 5HT antagonist ketanserin (10 mumol/l), the Ca2+ antagonist drugs verapamil (3 mumol/l), or nifedipine (10 nmol/l), or a Ca2+-free bathing medium, reduced the contractile responses to 5HT, but had no effect on the potentiation mediated by U46619. The interaction between TXA2 and 5HT derived from platelets was studied by measuring responses to platelets 1 min after aggregation (in the absence or the presence of ketanserin 10 mumol/l), and 20 min after aggregation. The results indicated that the response to platelets mediated by TXA2 and 5HT was greater than the sum of those mediated by TXA2 or 5HT separately. It is concluded that synergism between the contractile effects of 5HT and U46619 occurs in human blood vessels; that this is mediated by enhanced utilization of intracellular, rather than extracellular calcium; and that synergism can also occur when 5HT and TXA2 are released from stimulated human platelets.

The effects of platelet-derived contractile agents on human digital arteries.

The contractile responses of spiral strips of human digital arteries to samples of a suspension of human platelets aggregated by thrombin have been studied at different time intervals after aggregation. Platelets added to the arterial strips 1 min after aggregation of the platelets produced contractile responses which were significantly greater than those produced by corresponding platelets added 20 min after aggregation. When platelets were aggregated in the presence of indomethacin or the thromboxane synthetase antagonist 1-benzylimidazole, contractile responses produced by the platelets 1 min after aggregation were significantly reduced. They were then not significantly different from those produced by addition of the aliquots 20 min after aggregation, which were unaffected. Pretreatment of the arteries with the serotonin antagonist ketanserin nearly abolished the contractile responses produced by addition of the platelets 20 min after aggregation, and significantly reduced those produced by addition of the platelets 1 min after aggregation. Ketanserin did not affect the contractile responses of the arteries to potassium chloride, prostaglandin F2 alpha, or the endoperoxide analogue U-46619, but antagonized the contractile effects of exogenous serotonin. Combination of pretreatment of the arteries with ketanserin and aggregation of the platelets in indomethacin or 1-benzylimidazole virtually abolished contractile responses to platelets added both 1 min and 20 min after aggregation. Tensions developed to different dilutions of platelets added 1 min after aggregation to arteries pretreated with ketanserin were not significantly different from those obtained to the same dilutions added 20 min after aggregation to arteries not pretreated with ketanserin.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses of human arterial muscle to stable vasoactive substances released from human platelets by collagen and heat aggregated IgG.

The supernatant solutions obtained after aggregation or sonication of washed human platelets were superfused over preparations of human isolated digital arteries using a small volume bioassay method. The agents released from the platelets caused strong contractions of the artery strips. Platelet aggregation induced by 10 micrograms/ml collagen or by 100 micrograms/ml heat aggregated IgG, released 31.5% and 38.5% respectively, of the contractile activity produced by sonication of the platelets. The quantitative contractile effect of supernatants from platelets aggregated by 50 micrograms/ml IgG was significantly less than that for 100 micrograms/ml HA IgG. Similarly, the maximum contractile effect of supernatants from platelets aggregated by 300 ng/ml collagen was significantly less than that for 1 microgram/ml collagen. This suggests that the concentration of contractile agents released from platelets depends on the concentration of aggregating stimulus. Comparison with concentration-effect curves for exogenous serotonin suggests that if the contractility of the platelet supernatant occurring after sonication of platelets is solely due to serotonin, then it is present in a concentration of approximately 3.3 X 10(-6) mol/l (6.6 nmol per 10(9) platelets). It is suggested from this study that in certain clinical situations characterized by hypertension, and in which circulating immune complexes have been found, in vivo platelet activation by immune complexes may be releasing sufficient concentrations of serotonin to constrict peripheral blood vessels and contribute to the hypertension.

Vasoactivity generated from platelets by immune complexes in pre-eclampsia.

Circulating immune complexes in excess of the equivalent of 20 micrograms/ml heat-aggregated IgG were found in fourteen out of twenty patients diagnosed as having preeclampsia. Only six of the nineteen controls tested had similar levels of immune complexes. Recent studies have established that concentrations of heat aggregated IgG in excess of 20 micrograms/ml activate human platelets to release sufficient concentrations of vasoactive agents to constrict a human blood vessel in vitro. It is therefore suggested that in vivo platelet activation by circulating immune complexes may release sufficient concentrations of vasoactive agents to contribute to the hypertension in pre-eclampsia.