(-)-Adrenaline-induced, calcium-dependent phosphorylation of proteins in human platelets.

In human platelets, adrenaline stimulated, approximately four-fold, as compared with controls, the phosphorylation of primarily two proteins of apparent molecular weights of 20,000 and 40,000, respectively. Maximum phosphorylation occurred after incubation for 1 min and was inhibited by the addition of either yohimbine, prostaglandin E1, or EGTA. Phosphorylation of the two proteins was accompanied by diacylglycerol formation. The (-)-adrenaline-induced phosphorylation of proteins corresponds to the activation of a calcium-dependent protein kinase partially purified by DEAE-cellulose and Sephadex G150 column chromatography. The enzymatic activity was modulated by addition of (-)-adrenaline and CaCl2, by diolein, and in the presence of membranes or phosphatidylinositol but not phosphatidylethanolamine and phosphatidylcholine. A phospholipid-dependent reaction appears to be involved in the molecular mechanism of action of adrenaline.

Calmodulin regulation of adenylate cyclase activity in human platelet membranes.

The mechanism of calmodulin dependent regulation of adenylate cyclase has been studied in human platelet membranes. Calmodulin activated adenylate cyclase exhibited a biphasic response to both Mg2+ and Ca2+. A stimulatory effect of Mg2 on adenylate cyclase was observed at all Mg2+ concentrations employed, although the degree of activation by calmodulin was progressively decreased with increasing concentrations of Mg2+. These results demonstrate that the Vmax of calmodulin dependent platelet adenylate cyclase can be manipulated by varying the relative concentrations of Mg2+ and Ca2+. The activity of calmodulin stimulated adenylate cyclase was always increased 2-fold above respective levels of activity induced by GTP, Gpp(NH)p and/or PGE. The stimulatory influence of calmodulin was not additive but synergistic to the effects of PGE1, GTP and Gpp(NH)p. GDP beta S inhibited GTP-and Gpp(NH)p stimulation of adenylate cyclase but was without effect on calmodulin stimulation. Since the inhibitory effects of GDP beta S have been ascribed to apparent reduction of active N-protein-catalytic unit (C) complex formation, these results suggest that the magnitude of calmodulin dependent adenylate cyclase activity is proportional to the number of N-protein-C complexes, and that calmodulin interacts with preformed N-protein-C complex to increase its catalytic turnover. Our data do not support existence of two isoenzymes of adenylate cyclase (calmodulin sensitive and calmodulin insensitive) in human platelets.

Age and cardiovascular response adaptation. Determinants of an antihypertensive treatment concept primarily based on beta-blockers and calcium entry blockers.

The patient's age has great impact on the development of hypertension, its duration, and severity. In patients with essential hypertension, sympathetic cardiovascular control changes from an early phase with increased beta-adrenoceptor-mediated responses, e.g., cardiac output and renin, into a later phase where these responses are blunted and alpha-adrenoceptor-mediated vasoconstriction prevails, associated with higher intracellular free sodium and calcium concentration. This pathophysiological view of essential hypertension has its corollary in the pharmacotherapeutic approach. Younger patients, who often have high renin levels, respond better to monotherapy with a beta-blocker or with a converting-enzyme inhibitor. Older patients, who often have low renin levels, respond less well to beta-blockers but particularly well to calcium entry blockers as an alternative to diuretics. Therefore, beta-blockers and calcium entry blockers form new cornerstones for antihypertensive treatment and strategy, with the potential of cardioprotection.

Enhanced platelet cyclic AMP response to prostaglandin E1 in essential hypertension.

Platelets provide an accessible and homogeneous cellular system for investigative studies on hypertension. Hypertension-associated abnormalities of cyclic adenosine 3',5'-monophosphate (AMP) metabolism were studied in human platelets. Platelets from hypertensive subjects had an enhanced cyclic AMP accumulation response to prostaglandin E1 (twofold increase in prostaglandin E1 sensitivity). The degree of adenylate cyclase activation in response to both prostaglandin E1 (receptor-mediated) and forskolin (non-receptor-mediated) was greater in hypertensive than normotensive subjects, and prostaglandin E1-stimulated and forskolin-stimulated adenylate cyclase activity correlated directly (r = 0.71, p less than 0.001, n = 26). This finding suggests that the catalytic subunit of the enzyme is the rate-limiting step of this hormonal information transduction. Platelets from hypertensive subjects were more sensitive to epinephrine-induced inhibition of the stimulatory effects of prostaglandin E1 on both cyclic AMP accumulation (fourfold) and activation of cyclic AMP-dependent protein kinase. These findings suggest that the enhanced cyclic AMP metabolic response to prostaglandin E1 in platelets from subjects with established essential hypertension may function as a negative feedback mechanism to protect the cells against calcium overload and to reduce their stimulated participation in hemostatic and thrombotic processes.

Platelet membrane calmodulin-stimulated calcium-adenosine triphosphatase. Altered activity in essential hypertension.

Platelet free Ca2+ concentration has been found to be elevated in essential hypertension and to correlate with blood pressure level. Free cytoplasmic calcium concentration is determined by calcium influx, pooling, and efflux. The present study found a Ca2+-ATPase in platelet membranes that has a high affinity for Ca2+ (Km approximately 1 microM), is inhibited by low concentrations of orthovanadate (Ki approximately 1 microM), and can be stimulated by calmodulin (Km approximately 5 nM). The absolute increase in calmodulin-stimulated Ca2+-ATPase activity was not different between normotensive and hypertensive subjects; however, the degree of stimulation of Ca2+-ATPase activity at saturating calmodulin concentrations apparently was diminished in calmodulin-deficient membranes from subjects with established essential hypertension (40%) as compared to that in normotensive subjects of similar age (135%; p less than 0.001). Affinities for calmodulin and Ca2+ were comparable between the two groups, while the capacity for Ca2+-ATPase activity (basal and calmodulin-stimulated) was markedly greater (1.5- to 1.8-fold) in both native and calmodulin-deficient membranes from hypertensive subjects. On the other hand, the defective calcium efflux pump activity, as assessed by a decreased degree of calmodulin stimulation, may have contributed to elevated cytoplasmic calcium concentrations and the associated enhanced hormone sensitivity in platelets from essential hypertensive subjects. This may represent an adaptive negative feedback control mechanism to protect the cell against Ca2+ overload.

Decreased Beta-adrenoreceptor responsiveness as related to age, blood pressure, and plasma catecholamines in patients with essential hypertension.

The role of the sympathetic nervous system as it relates to adrenoreceptor-mediated hemodynamic responses was investigated in patients with essential hypertension and in normal subjects of similar age. An age-related increase in plasma norepinephrine (PNE) concentrations observed in 36 recumbent normal subjects (r = 0.623, p less than 0.001) was not found in 56 patients; the latter included some young patients with high values. Sympathetic overactivity in patients (n = 24) as compared with normotensive subjects (n = 20) was suggested by a greater increase in PNE upon standing (242 +/- 34 vs 155 +/- 25 pg/ml (SEM), p less than 0.05) and persistently higher plasma epinephrine (PE) concentrations at rest and during equieffective exercise (p less than 0.05). In patients, PNE was directly related to systolic (r = 0.57, p less than 0.01) and diastolic (r = 0.53, p less than 0.01) blood pressure. Older age was associated with diminished exercise tachycardia and increased blood pressure responses to exercise, which were both more pronounced in hypertensive patients. This higher pressure/lower heart rate pattern was paralleled by an age-related decrease in isoproterenol sensitivity in normal subjects (0.97 +/- 0.15 in six below age 34 years, 1.31 +/- 0.30 in eight between 35--49 years, and 1.82 +/- 0.12 microgra/m2 in six above 50 years), which was also more pronounced (p less than 0.05) in hypertensive patients (1.20 +/- 1.18 in seven below age 34 years, 2.42 +/- 0.30 in nine between 35--49 years, and 6.73 +/- 2.44 micrograms/m2 in eight above 50 years). Thus, an increase in the patients' blood pressure and age is associated with a progressive reduction in beta-adrenoreceptor sensitivity and/or reactivity. Defective beta-adrenoreceptor-mediated responses may result in unopposed alpha-adrenoreceptor-mediated vasoconstriction and thereby contribute to the development of hypertension.

Enhanced alpha-adrenoreceptor-mediated vasoconstriction in essential hypertension.

Forearm blood flow (FAF) has been determined using venous occlusion plethysmography in 24 patients with essential hypertension (EHT) and in 16 age-matched normotensive subjects (NT) under basal resting conditions, following nonspecific vasodilatation with sodium nitroprusside and after intraarterial infusion of the postjunctional alpha-blocking drug, prazosin. Under basal conditions, FAF was significantly higher in EHT than in NT. Infusion of sodium nitroprusside produced a similar absolute increase in FAF in both groups, whereas postjunctional alpha-blockade with prazosin led to a significantly greater increase in FAF in EHT than in NT. A positive correlation was found between plasma epinephrine concentration and prazosin-induced FAF in EHT but not in NT. These results suggest an enhanced postjunctional alpha-adrenoreceptor-mediated vasoconstrictor component in established EHT.

Activation of endothelial L-arginine pathway in resistance arteries. Effect of age and hypertension.

In conduit arteries, nitric oxide is formed from L-arginine in the endothelium and released after stimulation with acetylcholine. The contribution of the L-arginine pathway and the effects of age and hypertension on endothelium-dependent vascular regulation were studied, using a video dimension analyzer, in pressurized and perfused mesenteric resistance arteries of 8- and 16-20-week-old Wistar-Kyoto and spontaneously hypertensive rats. Norepinephrine and phenylephrine caused contractions, which were similarly augmented after removal of the endothelium. NG-Monomethyl-L-arginine, an inhibitor of nitric oxide formation, augmented the contraction, but less than endothelial removal. Acetylcholine caused endothelium-dependent relaxations that were much more pronounced with intraluminal than with extraluminal application. NG-Monomethyl-L-arginine, methylene blue, and hemoglobin only partially inhibited the response. With aging, the endothelium-dependent inhibition of the response to norepinephrine decreased in Wistar-Kyoto rats; in spontaneously hypertensive rats this inhibition was smaller as compared with age-matched Wistar-Kyoto rats. In Wistar-Kyoto rats, the difference between intraluminal and extraluminal activation became more pronounced in adult rats. In the adult but not the young spontaneously hypertensive rats, the response to intraluminal but not extraluminal acetylcholine was reduced as compared with Wistar-Kyoto rats. Thus, in mesenteric resistance arteries of the rat, nitric oxide is released from L-arginine under basal conditions and after stimulation with acetylcholine but only in part accounts for endothelium-dependent responses. With aging and hypertension, the inhibitory effects of the endothelium against norepinephrine-induced contractions decrease. In hypertension, the intraluminal but not extraluminal activation of the release of endothelium-derived relaxing factors is impaired.

Electrocardiographic changes during antihypertensive therapy in the International Prospective Primary Prevention Study in Hypertension.

In the International Prospective Primary Prevention Study in Hypertension, electrocardiographic changes before and during 3- to 5-year antihypertensive treatment were investigated in a cohort of 5819 men and women aged 40 to 64 years with entry diastolic blood pressures of 100 to 125 mm Hg. They were randomly allocated to treatment regimens that either included or excluded the slow-release beta-blocker oxprenolol. Electrocardiograms (ECGs) were assessed using the Minnesota Code and assigned to groups of normal ECGs or ECGs with pressure-related, ischemic, "intermediate," or "other" abnormalities. Antihypertensive treatment was associated with a decrease (mainly in men) of pressure-related and (mainly in women) of intermediate abnormalities. Ischemic abnormalities increased, particularly in men. Inclusion of the beta-blocker resulted in a greater reduction in intermediate abnormalities and in a lesser increase in ischemic abnormalities. Better blood pressure control was associated with a lesser increase in ischemic abnormalities and in a regression of pressure-related abnormalities. The presence of ST segment depression and of a complete left bundle branch block in the entry ECG was associated with a significant risk for sudden death and myocardial infarction. Optimal blood pressure control prevents pressure-induced cardiac target organ damage and, hence, heart failure, and may delay the progression of ischemic abnormalities. This tallies with the lower critical cardiac event rate associated with lower blood pressure that was observed in the same study.

Endothelium-dependent responses in carotid and renal arteries of normotensive and hypertensive rats.

Endothelium-dependent relaxations are impaired in the aorta of various models of hypertension, but no data are available regarding the cerebral or renal circulation. Endothelium-dependent relaxations were studied in the carotid and renal artery of Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Rings with and without endothelium were suspended in organ chambers for isometric tension recording. Acetylcholine and adenosine 5'-diphosphate (ADP) caused endothelium-dependent relaxations in both arteries that were impaired in the carotid, but not in the renal artery, of the SHR, similar to those to the endothelium-independent vasodilator sodium nitroprusside. Indomethacin did not affect relaxations to acetylcholine in the carotid artery, but it significantly augmented them in the renal artery. This finding suggests that an impaired vascular responsiveness to endothelium-derived relaxing factor is responsible for the decreased relaxations in the carotid artery of the SHR. In the renal artery, acetylcholine appears to release both endothelium-derived relaxing factor and a vasoconstrictor prostanoid. Carotid arteries of SHR were more sensitive to the constrictor effects of serotonin than were those of WKY. Endothelium removal caused a twofold to eightfold increase in sensitivity to serotonin in both strains. Thus, endothelium-dependent relaxations to acetylcholine and ADP are reduced and constrictions to serotonin are enhanced in the carotid, but not in the renal, artery of the SHR.

Vascular smooth muscle cell calcium fluxes. Regulation by angiotensin II and lipoproteins.

This study examined 45Ca uptake, 45Ca efflux, and the distribution of exchangeable 45Ca in confluent, quiescent cultures of aortic smooth muscle cells (VSMCs) from normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs). These parameters were investigated under basal conditions and after addition of angiotensin II (Ang II) and low (LDL) and high (HDL) density lipoproteins. Basal 45Ca uptake was approximately 50% greater in VSMCs from SHRs (p < 0.005 versus WKY). Calcium antagonists (diltiazem or nifedipine) abolished this difference. The 45Ca uptake response to Ang II was approximately twofold greater in SHR than in WKY VSMCs (p < 0.05), and Ang II-induced increments of 45Ca uptake were weakly inhibited (by approximately 15-25%) by calcium antagonists. Lipoproteins also stimulated 45Ca uptake in VSMCs, and the apparent affinity of this process was approximately fivefold greater for LDL than for HDL. Calcium antagonists did not inhibit either LDL- or HDL-induced 45Ca uptake. SHR and WKY VSMCs did not differ with respect to 45Ca uptake induced by either LDL or HDL. The initial size of the slowly exchangeable pool of intracellular Ca2+ was approximately 35% greater in SHR VSMCs (p < 0.05 versus WKY). Ang II-induced mobilization of intracellular calcium (measured as the decrease in 45Ca content of the slowly exchangeable pool) was threefold greater in SHR VSMCs (p < 0.005 versus WKY). LDL and HDL marginally stimulated 45Ca efflux from this pool (< or = 20% above control) and to comparable extents in both SHR and WKY VSMCs.(ABSTRACT TRUNCATED AT 250 WORDS)

Epidermal growth factor responsiveness in smooth muscle cells from hypertensive and normotensive rats.

Aortic smooth muscle cells from spontaneously hypertensive rats (SHR) exhibit inappropriate proliferation characteristics in culture that suggest a modified response to serum mitogens or growth factors. The present study compares vascular smooth muscle cells from SHR and normotensive Wistar-Kyoto (WKY) rats with respect to their proliferative and functional response to growth factors. Specific attention was focused on the interaction of these vascular smooth muscle cells with epidermal growth factor. An increased growth rate of vascular smooth muscle cells from SHR (vs. WKY rats) was observed when cells were cultured in the presence of serum (10% and 0.5%), but not under serum-free conditions. The additional presence of low serum concentrations (0.5%) was required for epidermal growth factor to elicit a proliferative response, whereupon smooth muscle cells from SHR displayed an increased (vs. WKY rats) growth rate. Saturation binding of [125I]epidermal growth factor to intact smooth muscle cells indicated a twofold increase in receptor density in SHR-derived cells (p less than 0.001 vs. WKY rats) without an alteration in affinity for the growth factor. Cells derived from SHR also exhibited greater functional responsiveness to epidermal growth factor when compared with smooth muscle cells from WKY rats as evidenced by amplifications of both S6 kinase activation, phosphoinositide catabolism, elevation of intracellular pH, and DNA synthesis (nuclear labeling). We conclude that increased responsiveness of SHR-derived smooth muscle cells to epidermal growth factor could contribute to alterations in vascular smooth muscle growth and tone that may be fundamental to the pathogenesis of hypertension and atherosclerosis.

Enhanced responsiveness to angiotensin II in vascular smooth muscle cells from spontaneously hypertensive rats is not associated with alterations in protein kinase C.

This study compares vascular smooth muscle cells from spontaneously hypertensive and normotensive Wistar-Kyoto rats with respect to protein kinase C and intracellular responses to angiotensin II (Ang II). Ang II-induced degradation of polyphosphoinositides and accumulation of inositol di- and tris-phosphates was enhanced (approximately twofold) in hypertensive-derived cells, without a change (vs. normotensive-derived cells) in half-maximally effective concentrations of Ang II. Intracellular pH (approximately 6.6) was comparable between both cell isolates at quiescence, but alkalinization induced by Ang II, serum, or phorbol ester was greater (delta 0.1-0.2 pH units) for hypertensive-derived cells. For both cell types, the intracellular pH response to these agonists was prevented in the presence of Na+-H+ exchange inhibitors. S6 kinase activation induced by Ang II was enhanced (approximately twofold) in hypertensive-derived cells, whereas activation in response to serum or 12-O-tetradecanoylphorbol 13-acetate did not differ significantly between the two cell types. Quantitation of protein kinase C by immunoblotting and [3H]phorbol dibutyrate binding procedures revealed no differences between the two smooth muscle cell isolates (at quiescence or in the presence of serum) with respect to either total amounts or subcellular distribution. Sensitivity of protein kinase C to phorbol ester was apparently also not different between the two cell types, as assessed from dose-dependent (phorbol ester) S6 kinase activation profiles. Phorbol ester caused a similar subcellular redistribution of [3H]phorbol dibutyrate binding in the two cell isolates, but for both, minimal (10%) translocation occurred in response to Ang II. The data suggest that enhanced agonist responsiveness in vascular smooth muscle cells is unlikely to involve alterations in protein kinase C.

Endothelin stimulated by angiotensin II augments contractility of spontaneously hypertensive rat resistance arteries.

In cultured endothelial cells, endothelin is produced after stimulation with angiotensin II. The effects of angiotensin II and endothelin-1 on vascular sensitivity to norepinephrine were studied in perfused rat mesenteric resistance arteries. Expression of endothelin messenger RNA (mRNA) was determined in endothelial cells obtained from the mesenteric circulation. Perfusion (5 hours) of the arteries with angiotensin II (10(-7) M) potentiated contractions in arteries with endothelium induced by norepinephrine in spontaneously hypertensive rats but not Wistar-Kyoto rats. The potentiation was inhibited by phosphoramidon and an endothelin antibody. Short-term stimulation (1 hour) with angiotensin II did not cause the potentiation. Stimulation with angiotensin I (10(-7) M; 5 hours) caused a potentiation prevented by captopril. In endothelial cells collected from the mesenteric arterial bed of spontaneously hypertensive rats, endothelin-specific mRNA was constitutively expressed, and the level of endothelin transcripts was increased by angiotensin II (10(-7) M). Threshold concentrations of exogenous endothelin-1 potentiated contractions induced by norepinephrine in arteries with and without endothelium of spontaneously hypertensive rats but not Wistar-Kyoto rats. Thus, angiotensin II stimulates the endothelial production of endothelin in situ and therapy potentiates contractions to norepinephrine in mesenteric resistance arteries of spontaneously hypertensive rats. This suggests that vascular endothelin production acts as an amplifier of the pressor effects of the renin-angiotensin system that may play an important role in hypertension.

Free calcium concentration in platelets closely relates to blood pressure in normal and essentially hypertensive subjects.

Calcium has been implicated in smooth muscle contraction, arterial resistance, and the pathophysiology of essential hypertension. Using the intracellularly trapped fluorescent dye quin2 , the free calcium concentration in platelets was found to be elevated in patients with borderline (n = 8, p less than 0.01) and established essential hypertension (n = 23, p less than 0.001) when compared with normotensive subjects (n = 30). There was a close correlation between intracellular free calcium and systolic blood pressure (n = 61, r = 0.882, p less than 0.001) as well as diastolic blood pressure (n = 61, r = 0.950, p less than 0.001). The slopes of the regression lines did not differ between the groups.

Enhanced vasodilatation in essential hypertension by calcium channel blockade with verapamil.

The dependency of arteriolar tone on calcium influx was studied in 11 patients with essential hypertension (EH) and compared to 11 age-matched normotensive subjects (NT) by measuring the forearm blood flow response to intraarterial infusion of the calcium channel blocker verapamil (Verap) and the non-specific vasodilator sodium nitroprusside (Nip) using venous occlusion plethysmography. Verap in incremental dosages from 1 to 75 micrograms/100 ml forearm tissue induced a greater increase in forearm blood flow ( delta FAF) in EH then in NT, whereas there was no significant difference in delta FAF following Nip 1,2 micrograms/100 ml tissue. Delta FAF to Verap as adjusted for delta FAF to Nip was still greater in EH than in NT. Delta FAF to all dosages of Verap correlated positively with basal plasma epinephrine concentration in EH. At the two highest dosages of Verap, systemic blood pressure fell in EH, and the Verap-induced vasodilator (as adjusted for the response to Nip) correlated negatively to plasma renin activity or plasma angiotensin II concentration. These findings support the concept of an increased dependency of arteriolar tone on calcium influx in EH, which is related to the activity of the sympathetic nervous system. This association may be due to a common underlying derangement in transmembranous ionic fluxes in smooth muscle cells and sympathetic neurons in EH.

Psychosomatic factors in borderline hypertensive subjects and offspring of hypertensive parents.

Psychosomatic factors, sympathoneural and sympathoadrenal as well as cardiovascular mechanisms, were studied in 24 patients 18-24 years of age with borderline hypertension, 50 age-matched normotensive offspring of hypertensive parents, and 49 controls with no family history of hypertension. They were compared by projective and questionnaire-based psychological tests and their circulatory and neurohormonal reactivity to mental (Stroop color-word conflict test and arithmetic test) and physical stressors (orthostasis and bicycle ergometry test) were measured. Borderline hypertensive subjects externalized aggression less (p less than 0.05) but internalized it more (p less than 0.05) and were more submissive (p less than 0.05) when compared with controls. Offspring of hypertensive parents showed a similar but weaker pattern. Both risk groups reported more positive interactions with their parents (genetic risk subjects versus controls, p less than 0.05; borderline hypertensive patients versus controls, p = 0.08) and had higher state-anxiety levels (p less than 0.05). There were more subjective symptoms of beta-adrenergic receptor-mediated functions (e.g., tachycardia, tremor) in borderline hypertensive subjects and offspring of hypertensive parents, elevated heart rates (analysis of repeated measures, p less than 0.001), and enhanced plasma norepinephrine concentrations (p less than 0.05) when compared with controls. These findings in subjects at risk for the development of hypertension suggest that psychosomatic factors and sympathetic overactivity are involved in the early phase of hypertension.

5-Hydroxytryptamine kinetics and activation of blood platelets in patients with essential hypertension.

To investigate possible alterations in 5-hydroxytryptamine (5HT) kinetics and sensitivity of blood platelets in patients with essential hypertension, 45 essential hypertensive patients and 45 normotensive healthy subjects matched in pairs for age, sex, and smoking status were compared. There were 18 women and 27 men in each group, ranging from 30 to 73 years of age. Results of essential hypertensive patients differed in several ways from those of normotensive subjects. In essential hypertensive patients, maximal 5HT uptake velocity (Vmax) decreased with increasing blood pressure and age and was reduced the most in older men. Vmax was positively related to the EC50 of 5HT for inducing a shape change reaction. In essential hypertensive patients, both Vmax of 5HT uptake and the EC50 of 5HT for shape change showed positive correlations with the 5HT content in platelets; the former relation was different between the essential hypertensive and normotensive groups (F = 5.53; p = 0.02). These results indicate reduced uptake of 5HT by blood platelets and suggest enhanced 5HT plasma concentrations in local areas, especially vascular lesions in essential hypertensive patients. Increased periplatelet concentrations of 5HT may lead to preactivation of platelets and possibly stimulation of vascular smooth muscle via their 5HT2-receptors. These changes are likely to be involved in the pathogenesis of increased thromboembolic complications in essential hypertensive patients, particularly in older men.

Alpha 2 adrenoceptor-mediated vasoconstriction of arteries.

We investigated the possibility that adrenoceptors of the alpha 2 subtype mediate vasoconstriction of arteries in response to administered catecholamines. Clonidine, which in vitro, stimulates alpha 2-adrenoceptors was infused into a brachial artery in 12 subjects (0.48 micrograms/min/100 ml tissue for 3 min). Afterward, prazosin was infused intraarterially in the first six subjects (0.5 micrograms/min/100 ml for 10 min) and in the remaining subjects, phentolamine was infused (0.12 micrograms/min/100 ml) for 10 min. Subsequently, the clonidine infusion was repeated. Clonidine decreased forearm blood flow from 3.5 +/- 0.52 to 1.8 +/- 0.32 in the first six subjects and from 4.2 +/- 0.84 to 2.7 +/- 0.61 ml/min/100 ml in the other subjects. Alpha 1-Adrenoceptor blockade by prazosin increased forearm blood flow by 122.7 +/- 33.8% and combined alpha 1 and alpha 2 blockade by phentolamine by 127.2 +/- 29.9%, indicating much the same degree of postjunctional alpha-adrenoceptor blockade. Alpha 2-Adrenoceptor-mediated vasoconstriction by clonidine was abolished after phentolamine (9.1 +/- 2.29 and 9 +/- 2.51 ml/min/100 ml) but was still present after prazosin (7.8 +/- 1.7 and 4.8 +/- 1.6 ml/min/100 ml). The results suggest that, apart from the classical alpha 1 adrenoceptor, there is a second type of adrenergic receptor on smooth muscle cells that can mediate vasoconstriction, resembling the alpha 2-adrenoceptor pharmacologically.

Antihypertensive mechanism of amlodipine in essential hypertension: role of pressor reactivity to norepinephrine and angiotensin II.

Calcium entry blockade may affect the pressor reactivity to vasoconstrictors. The pressor response to norepinephrine and angiotensin II, as well as several other blood pressure modulating factors, were studied in normal subjects (n = 9) and patients with essential hypertension (n = 10) before and after 8 weeks of treatment with the long-acting dihydropyridine amlodipine. In control subjects, calcium entry blockade did not modify blood pressure, the pressor and aldosterone response to angiotensin II, the activity of the renin-angiotensin and sympathetic nervous systems, or urinary dinoprostone (prostaglandin E2) excretion; however, the pressor response to norepinephrine was significantly decreased (p less than 0.01). In patients with hypertension, amlodipine decreased blood pressure (p less than 0.01) and the pressor response to both norepinephrine and angiotensin II (p less than 0.01), without changes in body weight, plasma renin, angiotensin II and catecholamine levels, dinoprostone excretion, or aldosterone responsiveness to angiotensin II. These findings suggest that calcium entry blockade modifies sympathetic-dependent vasoconstriction in both normal subjects and in patients with hypertension. Angiotensin II pressor response may be selectively decreased in essential hypertension.