Free Calcium Concentration In Platelets Research Articles

Essential hypertension in adolescents and children: Recent advances in causative mechanisms

Essential hypertension is the most common form of hypertension in adults, and it is recognized more often in adolescents than in younger children. It is well known that the probability of a diagnosis of essential hypertension increases with age from birth onward. The initiation of high blood pressure burden starts in childhood and continues through adolescence to persist in the remaining phases of life. The genesis of essential hypertension is likely to be multifactorial. Obesity, insulin resistance, activation of sympathetic nervous system, sodium homeostasis, renin-angiotensin system, vascular smooth muscle structure and reactivity, serum uric acid levels, genetic factors and fetal programming have been implicated in this disorder. In addition, erythrocyte sodium transport, the free calcium concentration in platelets and leukocytes, urine kallikrein excretion, and sympathetic nervous system receptors have also been investigated as other possible mechanisms. Obesity in children appears to be the lead contributor of essential hypertension prevalence in children and adolescents. Suggested mechanisms of obesity-related hypertension include insulin resistance, sodium retention, increased sympathetic nervous system activity, activation of renin-angiotensin-aldosterone, and altered vascular function. The etiopathogenesis of essential hypertension in children and adolescents appears to closely resemble that of adults. The minor variations seen could probably be due to the evolving nature of this condition. Many of the established mechanisms that are confirmed in adult population need to be replicated in the pediatric age group by means of definitive research for a better understanding of this condition in future.

Identifying pathways involved in leptin-dependent aggregation of human platelets.

To investigate the role of phospholipase C (PLC), phospholipase A(2) (PLA(2)), calcium, and protein kinase C (PKC) in mediating leptin-enhanced aggregation of human platelets. In vitro, ex vivo study. Outpatient's Service for Prevention and Treatment of Obesity at the University Hospital of Messina, Italy. In total, 14 healthy normal-weight male (age 31.4+/-1.9 y; body mass index 22.7+/-0.6 kg/m2) subjects. Adenosine diphosphate-(ADP-) induced platelet aggregation and platelet free calcium were measured after incubation of platelets with leptin alone (5-500 ng/ml), or leptin (50 and 100 ng/ml) in combination with anti-human leptin receptor long form antibody (anti-ObRb-Ab, 1:800-1:100 dilutions), PLC inhibitor U73122 (3.125-25 microM), PLA(2) inhibitor AACOCF3 (1.25-10 microM), or PKC inhibitor Ro31-8220 (1.25-10 microM). Platelet stimulation with leptin leads to a significant and dose-dependent increase in ADP-induced platelet aggregation and platelet free calcium concentrations. Leptin effects on both platelet aggregation and calcium mobilization were completely abated by the co-incubation with leptin and anti-ObRb-Ab. Leptin-induced platelet aggregation was dose-dependently inhibited by U73122, AACOCF3, or Ro31-8220. The effect of leptin on intracellular calcium was inhibited in a dose-dependent manner by incubation with U73122 and AACOCF3, but not with Ro31-8220. Our study confirms that leptin is able to enhance ADP-induced aggregation of human platelets, and raise the possibility that PLC, PKC, PLA(2), and calcium could play a relevant role in mediating the proaggregating action of leptin.

Effect of Regular Exercise on Platelet Cytoplasmic Calcium during Treatmill Exercise in Healthy Young Males

Background and Objectives：Regular aerobic exercise plays a role in the prevention of cardiovascular diseases, but the risk of primary cardiac arrest transiently increases during vigorous exercise, particularly in sedentary men. There has been a controversy regarding the effect of exercise on platelet functional behavior, which is related to the pathogenesis of coronary ischemia. We evaluated the cytoplasmic free calcium concentration of platelets in healthy men after treadmill exercise. Materials and Method：Five physically active (group I；age, 24.2± 2.3 years and five sedentary men (group II；age, 22.4±1.4 yearswere included in this study. Platelet calcium was measured with fluorescent dye, quin2 before and after treadmill exercise. Results：Platelet calcium levels were increased from 86.8±11.8 nM to 128.8±15.0 nM in group I (p<0.05and from 102.6±14.4 nM to 162.4 ±26.5 nM in group II (p<0.05immediately after exercise. Thereafter, platelet calcium levels were decreased in group I, but continuously increased in group II. At fifteen minutes after exercise, platelet calcium concentrations were significantly higher than baseline values in group II (205.8±53.9 nM vs 102.6±14.4 nM；p<0.05, but not in group I (115.2±15.7 nM vs 86.8±11.8 nM. Conclusion：Cytoplasmic free calcium concentration in platelets were continuosly increased after treadmill exercise in sedentary men but not in physically active men. (Korean Circulation J 1999;29(10 :1112-1120

Insulin sensitivity, sodium-lithium countertransport and platelet free calcium concentrations in normotensive men with a family history of hypertension.

Peripheral glucose disposal (assessed with the euglycaemic-hyperinsulinaemic clamp technique), Na+-Li+ countertransport in erythrocytes and the cytosolic free Ca2+ concentration in platelets were determined in 36 men from families with essential hypertension in at least two close relatives and in 28 age- and weight-matched men from families without hypertension. All had diastolic blood pressure consistently below 90 mm Hg and a normal oral glucose tolerance test. The mean age of the study population as a whole was 37 years (range 24-46). Insulin sensitivity index values (glucose disposal/serum insulin concentration during the clamp) were lower in the familial hypertension group than in the control group, but the two groups did not differ in Na+-Li+-countertransport, or in the platelet cytosolic free Ca2+ concentration which was correlated to the waist: hip circumference ratio in both groups (r = 0.38 and r = 0.49, respectively). The present findings do not support the hypothesis that an increase in Na+-Li+ countertransport activity or the platelet cytosolic free Ca2+ concentration is a marker of insulin resistance in hypertension prone men.

Noradrenergic blood pressure dysregulation and cytosolic calcium in primary hyperparathyroidism.

The purpose of this study was to characterize the changes in noradrenergic blood pressure regulation and the causes of this dysregulation in patients with histologically proven primary hyperparathyroidism before and after parathyroidectomy. In untreated hypertensive patients with primary hyperparathyroidism slightly higher plasma levels of norepinephrine, enhanced cardiovascular reactivity to norepinephrine (p<0.05) and increased cytosolic free calcium concentration in platelets (p<0.05) were found. Parathyroidectomy resulted in the correction of noradrenergic blood pressure dysregulation and in the reduction of cytosolic free calcium and led to normotension. In vitro incubation of platelets from normal subjects with heterologous plasma ultrafiltrates from either hypertensive patients with primary hyperparathyroidism or normotensive uremic patients with secondary hyperparathyroidism led to raised platelet calcium in primary hyperparathyroidism but not in secondary hyperparathyroidism, although in both conditions intact parathormone levels were elevated equally. Furthermore, when platelets of normotensive subjects were preincubated with commercially available parathormone, no effect on resting platelet cytosolic calcium was observed. Our results suggest that hypertension in primary hyperparathyroidism may be caused in part by noradrenergic blood pressure dysregulation and seems to be causally linked to the secretion of a parathyroid hypertensive factor, which increases cytosolic calcium and alters vascular tone and reactivity.

Treatment of non-insulin-dependent diabetic hypertensive patients with Ca 2+ channel blockers is associated with increased platelet sensitivity to insulin

Insulin influences platelet function by reducing platelet responsiveness to vasoactive agonists. In hypertension these insulin-related inhibitory effects are blunted. The aim of this study was to determine whether platelet sensitivity to insulin is affected after 12 weeks of antihypertensive treatment with calcium channel blockers. Eleven patients with essential hypertension and non-insulin-dependent diabetes were treated with either isradipine or diltiazem. Platelet free calcium concentration was measured spectrofluorometrically in nonstimulated and in angiotensin II (1 nmol/L)-stimulated platelets that had been pre-exposed to insulin (70 microU/mL). Platelets were studied before therapy and after 12 weeks of treatment. Systolic blood pressure and mean arterial pressure decreased significantly during treatment. Both calcium channel blockers equally decreased fasting serum insulin levels (from 138 +/- 2.7 pmol/L before therapy to 106 +/- 9.9 pmol/L after therapy). Results with both calcium channel antagonists were similar and have been pooled. In the pretreatment phase, insulin pre-incubation had no effect on angiotensin II-stimulated intracellular free calcium and responses to angiotensin II were similar in the absence (265 +/- 10.5 nmol/L) and presence of insulin (233 +/- 12.1 nmol/L). After treatment with calcium channel blockers, insulin significantly (P < .001) reduced angiotensin II-induced rise of intracellular free calcium from 214 +/- 7.7 nmol/L (absence of insulin) to 153 +/- 9.3 nmol/L (presence of insulin). Whether these effects are due specifically to calcium channel blockers or whether they are the result of lowering of blood pressure remains unclear. Serum insulin levels were positively correlated with angiotensin II-stimulated increase in intracellular free calcium in platelets pre-exposed to insulin, (r = 0.46; P < .05) suggesting that with increasing levels of insulin resistance, the inhibitory effects of insulin are reduced. This study shows that treatment of non-insulin-dependent diabetic hypertensive patients with these calcium channel blockers reduces hyperinsulinemia and increases sensitivity of platelets to the inhibitory effect of insulin on angiotensin II-stimulated calcium responses.

Blunted inhibition by insulin of agonist-stimulated calcium, pH and aggregatory responses in platelets from hypertensive patients

Insulin may influence platelet function by modulating platelet responsiveness to vasoactive agonists. In essential hypertension platelet hyperactivity might be related to altered modulation by insulin. The present study examined the effects of physiological concentrations of insulin on agonist-stimulated platelet responses [free calcium concentration, intracellular pH (pHi) and thrombin-induced aggregation] in platelets from 30 normotensive subjects and 53 untreated essential hypertensive patients. Platelet free calcium concentration and pHi were measured spectrofluorometrically using specific fluorescent dyes [Fura-2/acetoxymethyl ester and 2',7'-bis-(carboxyethyl)-5(6)-carboxyfluorescein/acetoxymethyl ester, respectively] in unstimulated and in angiotensin II (Ang II; 1 nmol/l)- and endothelin-1 (1 nmol/l)-stimulated platelets that had been pre-exposed to insulin (70 microU/ml). Aggregatory responses were measured turbidometrically in platelets stimulated by thrombin (0.05 U/ml), alone or in combination with angiotensin II or endothelin-1 in the absence and presence of insulin pre-incubation. Blood pressure and serum glucose and insulin levels were significantly elevated in the hypertensive patients. Basal and agonist-stimulated free calcium concentration, pHi and aggregation were significantly higher in the hypertensive than in the normotensive group. In platelets pre-exposed to insulin for 5 min, agonist-induced responses were significantly reduced (by 40-60%) in the normotensive group but not in the hypertensive group. Serum insulin was positively correlated with Ang II-stimulated cytosolic free calcium concentration (r = 0.47, P < 0.01) and pHi (r = 0.26, P = 0.047) in platelets pre-exposed to insulin. This study demonstrates that insulin inhibits Ang II- and endothelin-1-stimulated platelet free calcium concentration and pHi, and thrombin-induced aggregation. In hypertension these insulin-related inhibitory effects are blunted. Attenuated inhibition of agonist-stimulated platelet responses by insulin could be a manifestation of peripheral insulin resistance which may contribute to platelet hyperactivity in essential hypertension.

Effect of recombinant human erythropoietin on cytosolic free calcium concentration in platelets

Intracellular free calcium concentration ([Ca2+]i) was examined in the platelets of 15 control subjects (NT), 6 predialysis patients with chronic renal failure (CRF), 17 patients on hemodialysis (HD), 20 patients on continuous ambulatory peritoneal dialysis (CAPD), 10 normotensive persons with genetic hypertension (GHT) and 8 essential hypertensive patients (EHT). Levels of [Ca2+] i in the platelets were measured by the fluorescent calcium indicator Fura-2. Resting [Ca2+] i in CRF and HD patients was higher than the value in NT and that in CAPD patients was similar to NT. rHuEPO significantly increased the level of [Ca2+] i in CRF and HD patients compared to those in NT. Under resting and EPO-stimulated conditions, the levels of [Ca2+] i in GHT and EHT were higher than those in NT. rHuEPO increased the levels of [Ca2+] i in the absence of extracellular calcium in NT, GHT and EHT. In addition, EPO-stimulated calcium influx in GHT and EHT was greater than that in NT. Thus, it appears that the mechanism of rHuEPO-induced hypertension may be mainly due to elevation of [Ca2+] i. EPO may contribute to the development of genetic hypertension.

Quinapril treatment and arterial smooth muscle responses in spontaneously hypertensive rats

1 The effects of long-term angiotensin-converting enzyme inhibition with quinapril on arterial function were studied in spontaneously hypertensive rats, Wistar-Kyoto rats serving as normotensive controls. 2 Adult hypertensive animals were treated with quinapril (10 mg kg-1 day-1) for 15 weeks, which reduced their blood pressure and the concentrations of atrial natriuretic peptide in plasma and ventricular tissue to a level comparable with that in normotensive rats. 3 Responses of mesenteric arterial rings in vitro were examined at the end of the study. Compared with normotensive and untreated hypertensive rats, responses to noradrenaline were attenuated in hypertensive animals on quinapril, both force of contraction and sensitivity being reduced. Quinapril also attenuated maximal contractions but not sensitivity to potassium chloride. Nifedipine less effectively inhibited vascular contractions in normotensive and quinapril-treated than in untreated hypertensive rats. 4 Arterial relaxation responses by endothelium-dependent (acetylcholine) and endothelium-independent (sodium nitrite, isoprenaline) mechanisms were similar in normotensive and quinapril-treated rats and more pronounced than in untreated hypertensive rats. 5 Cell membrane permeability to ions was evaluated by means of potassium-free solution-induced contractions of endothelium-denuded denervated arterial rings. These responses were comparable in normotensive and quinapril-treated rats and less marked than in untreated hypertensive rats. 6 Intracellular free calcium concentrations in platelets and lymphocytes, measured by the fluorescent indicator quin-2, were similar in normotensive and quinapril-treated rats and lower than in untreated hypertensive rats. 7 In conclusion, quinapril treatment improved relaxation responses and attenuated contractions in arterial smooth muscle of hypertensive rats. These changes may be explained by diminished cytosolic free calcium concentration, reduced cell membrane permeability, and alterations in dihydropyridine-sensitive calcium channels following long-term angiotensin-converting enzyme inhibition.

Increased internal calcium mobilization in platelets of patients with chronic renal failure.

Platelet free calcium concentrations ([Ca2+]i) were measured with Fura-2 to elucidate the intracellular calcium kinetics in patients with renal disease. There were no significant differences of the resting [Ca2+]i among the control subjects (C) (n = 12), patients with chronic glomerulonephritis (CGN) (n = 8), and patients with chronic renal failure (CRF) (n = 12). In all groups, platelets [Ca2+]i were significantly increased by agonists (thrombin, adenosine diphosphate) compared with their respective basal level. Thrombin-induced [Ca2+]i rise was significantly higher in CRF (840 +/- 265 nM) than in C (600 +/- 163) and CGN (562 +/- 137). Also adenosine diphosphate elicited similar responses. In the presence of calcium chelator in the incubation buffer, the elevation of [Ca2+]i after thrombin stimulation was statistically higher in CRF (469 +/- 85 nM) than in C (275 +/- 60) and CGN (301 +/- 41). These findings suggest that platelets of CRF were capable of increasing [Ca2+]i in response to agonists, through further mobilization of calcium from the intracellular pool rather than the elevation of transmembrane calcium influx.

Correlation of Blood Pressure in End-Stage Renal Disease with Platelet Cytosolic Free Calcium Concentration during Treatment of Renal Anemia with Recombinant Human Erythropoietin

The hemodynamic hallmark of hypertension complicating the treatment of renal anemia with recombinant human erythropoietin (rHu-EPO) is increased total peripheral vascular resistance, but the mechanisms underlying the arteriolar vasoconstriction are still an enigma. We studied body fluid volumes, plasma renin activity, plasma norepinephrine, and calcium metabolism in platelets in 40 previously normotensive hemodialysis patients before and after 12 weeks of rHu-EPO treatment. Partial correction of anemia caused a rise in arterial pressure (94 +/- 6 mmHg vs 124 +/- 7 mmHg, p less than 0.05) and in platelet cytosolic calcium concentration (113 +/- 5 nM vs 171 +/- 18 nM, p less than 0.05) in eight patients. Hypertensive patients had significantly higher plasma noradrenaline concentrations, but they did not differ significantly in body fluid volumes and plasma renin activities. There was a close correlation between free calcium concentration in platelets and mean arterial pressure in patients developing rHu-EPO-induced-hypertension (r = 0.95). Short-term antihypertensive treatment resulted in a reduction of free calcium concentrations in platelets and a concomitant fall in blood pressure. The main results of the present studies suggest that rHu-EPO-induced hypertension might be associated with altered cellular calcium homeostasis and hyperactivity of the sympathetic nervous system. If rHu-EPO therapy induces alterations of pressor factors or the hormone itself raises the cytosolic calcium not only in platelets but also in vascular smooth muscle cells, altered cellular calcium influx may contribute to the arteriolar vasoconstriction.

Modulation of platelet Ca2+ homeostasis by hypertensive plasma factor(s) derived from patients with early-stage renal disease.

To determine whether blood-borne factors in hypertension accompanying early-stage kidney disease might be responsible for altered cellular calcium homeostasis, we measured changes in cytosolic calcium before and after incubating platelets in plasma ultrafiltrates from normotensive and hypertensive renal patients. With the use of the chelating agent quin 2, we found the free-calcium concentrations in platelets to be higher in the hypertensive than in the normotensive group. When both groups of participants were combined, a direct correlation was found between arterial pressure and cytosolic calcium. The cytosolic calcium concentration in platelets of normotensive renal patients increased after incubation with plasma from patients with untreated renal hypertension, but it was unchanged after incubation with plasma from normotensive subjects. These data indicate that the total cell burden of calcium is increased in platelets of hypertensive patients with early-stage renal disease, and that plasma from these patients contains a substance that is capable of increasing the cytosolic calcium concentration in platelets. If the plasma factor (or factors) acts not only on platelets, but also on vascular smooth muscle cells, it may contribute to the increased peripheral vascular resistance associated with hypertension of renal origin.

Increased intracellular calcium mobilization in platelets from patients with Type 2 (non-insulin-dependent) diabetes mellitus

Enhanced platelet functions have been reported in patients with diabetes mellitus. Our recent study demonstrated that phosphoinositide turnover is increased in platelets from diabetic patients. In the present study, we evaluated the abnormality in platelet intracellular calcium mobilization in patients with Type 2 (non-insulin-dependent) diabetes mellitus using fura-2, a fluorescent calcium indicator. Washed platelets were prepared from six diabetic patients with increased platelet aggregation rates (DM-A group), seven diabetic patients with normal platelet aggregation rates (DM-B group), and eight age-matched healthy control subjects. The basal intracellular free calcium concentrations in platelets were similar among the three groups. Thrombin (0.025-0.1 U/ml) induced a dose-dependent increase in intracellular calcium in both the presence and the absence of extracellular calcium. This increase in the presence of extracellular calcium, which depends on calcium influx and release, was significantly higher in the DM-A group than in the DM-B and control groups. However, there was no significant difference between the control group and the DM-B group. In the absence of extracellular calcium, thrombin-induced calcium increase, which depends only on calcium release, was also significantly enhanced in the DM-A group. Furthermore, the calcium increase stimulated by platelet-activating factor (10 nmol/l) with and without extracellular calcium was significantly higher in the DM-A group than in the other groups. Additionally, calcium ionophore A23187 (100 nmol/l) caused a significantly higher calcium increase in the DM-A group with extracellular calcium, while the calcium increase without extracellular calcium showed no significant difference among the three groups. These observations suggest that enhanced intracellular calcium mobilization due to increased calcium influx and release may be closely related to platelet hyperfunctions in diabetes mellitus.

Changes in Free Calcium Concentrations in Platelets of SHRSP and WKY: Its Relationship to ATP Releasing Potencies and Platelet Aggregation Activities Upon Stimulation of Several Reagents

To clarify the relationships between free calcium levels, ATP release and aggregation potencies of SHRSP platelets, we examined the platelets from 9-month-old SHRSP and WKY using fura-2AM and luciferine-luciferase. In the absence of extracellular Ca2+, each reagent elevated the free calcium level to the same extent in the samples of both SHRSP and WKY. With regard to ATP release, thrombin and collagen less potentiated the platelet action in SHRSP than WKY, and ATP release was not affected by extracellular Ca2+. Collagen and ADP induced aggregations showed lower activities in SHRSP than WKY. TPA caused higher Ca2+ influx and aggregation activity in SHRSP than WKY in the presence of extracellular Ca2+. These results indicate that Ca2+ release must be followed by ATP release, and ATP release may be less potentiated, while thrombin and TPA induced aggregation is likely to be stimulated in SHRSP platelets, because protein kinase C activity in SHRSP platelets appears to be high.

Correlation of blood pressure in end-stage renal disease with platelet cytosolic free-calcium concentration

The fluorescent probe Quin-2 showed the cytosolic free-calcium concentration in platelets to be elevated in 12 hypertensive hemodialysis patients, who were compared with 12 normotensive hemodialysis patients. There was a close correlation between the free-calcium level of platelets and mean arterial pressure (r = 0.88). Short-term antihypertensive treatment resulted in a reduction of free-calcium concentration and a concomitant fall in blood pressure. These results suggest an integrative contributory role for calcium in the pathophysiology of hypertension accompanying end-stage renal disease.

Relationship between the inhibition of receptor-induced increase in cytosolic free calcium concentration and the vasodilator effects of nitrates in patients with congestive heart failure

Nitroglycerin, isosorbide dinitrate and sodium nitroprusside, like nifedipine, were found to inhibit the receptor-provoked increase of cytosolic free calcium concentration in human platelets loaded with 2-[(2-amino-5-methylphenoxy)methyl]-6-methoxy-8-aminoquinoline- N,N,N′,N′-tetraacetate. Sodium nitroprus-side and nitroglycerin induced elevation of cyclic guanosine 3′,5′-monophosphate content in platelets which correlated with their calcium-blocking activity. Methylene blue and epinephrine decreased the calcium-blocking effect and the influence of nitroglycerin on cyclic guanosine 3′-5′-monophosphate content, but failed to suppress the inhibitory effect of sodium nitroprusside. Ascorbic acid increased the calcium blocking effect of sodium nitroprusside and its influence on cyclic guanosine 3′,5′-monophosphate content, but did not alter the inhibitory effect of nitroglycerin. In order to evaluate the relationship between the mode of action of nitrates at cellular level and their vasodilatory effectiveness, we studied the circulatory response of the forearm to isosorbide dinitrate and the influence of nitroglycerin on free calcium concentration in the platelets in 10 patients with chronic heart failure. We established a significant positive correlation between the basal values for venous tone and its peak decrease after administration of the 10-mg dose of isosorbide dinitrate. A correlation was also found between the deviation of maximal decrease of venous tone by this dose of isosorbide dinitrate from the regression line (the relationship between the basal venous tone and its lowering by the drug) and mean inhibitory concentration values for nitroglycerin in blocking that proportion of the rise of calcium ion concentration in platelets due to blocking of the platelet-activating factor. Thus, nitrates, like calcium antagonists, inhibit the receptor-provoked calcium supply to the contractile system of the cells so neutralizing the effects of increased concentrations of vasoconstrictors. This suggests that the effectiveness of nitrates appears to be positively related to the contribution of receptor-induced increase of cytosolic free calcium concentration in vasoconstriction together with their capacity to raise cyclic guanosine 3′,5′-monophosphate.

Recombinant Human Erythropoietin Increases Blood Pressure, Platelet Aggregability and Platelet Free Calcium Mobilisation in Uraemic Children: A Possible Link?

Recombinant human erythropoietin was administered to 10 uraemic children on chronic haemodialysis, all of whom responded by correcting their haemoglobin. In addition, they showed an increase in blood pressure; platelet aggregations, subnormal before therapy, improved during treatment. The intracellular free calcium concentration in platelets after thrombin stimulation also increased significantly during erythropoietin administration. We hypothesize that the effect of erythropoietin on platelet aggregability and on blood pressure may be due to an increase in the intracellular free calcium mobilisation in platelets and possibly in smooth muscle cells respectively.

Platelet free calcium concentrations measured with fura-2 are influenced by the transmembrane sodium gradient

The influence of the transmembrane Na + gradient on the intracellular free calcium concentration, [Ca 2+] i, was studied in Sepharose gel-filtered platelets from healthy human subjects, using the Ca-sensitive fluorescent dye, fura-2. Raising the internal Na + concentration, [Na +] i, by Na + pump inhibition with 0.05 mM ouabain, without changing external Na + did not cause a significant increase in [Ca 2+] i. Substitution of extracellular Na + by iso-osmolar sucrose induced a rapid (half-time about 2 min) and significant rise in [Ca 2+] i; this effect was ampilfied in Na-loaded platelets. Partial restitution of external Na + in these cells with increased [Ca 2+] i promoted a significant and rapid Na + concentration-dependent fall in [Ca 2+] i; little decline in [Ca 2+] i was observed if K + was used instead of Na +. These observations represent in vitro evidence for the existence of a Na Ca exchange mechanism in human platelets that may, in vivo, participate in the control of [Ca 2+] i.

Does cytosolic free calcium concentration in platelets reflect tone and structural changes of resistance vessels?

To investigate tone and vascular changes of resistance vessels as related to cytosolic free calcium concentration [( Ca2+]i) in platelets, we measured forearm vascular resistance and free calcium concentration in platelets from 10 essential hypertensives and 15 normotensives. The [Ca2+]i in platelets was significantly higher in essential hypertensives than in normotensives (184 +/- 43 versus 146 +/- 23 nmol/l, P less than 0.01). The [Ca2+]i levels in platelets were significantly correlated with both systolic (r = 0.50, P less than 0.05) and diastolic blood pressures (r = 0.57, P less than 0.01). Resting vascular resistance and minimal vascular resistance were significantly higher in essential hypertensives than in normotensives (30.6 +/- 12.6 and 2.57 +/- 1.30 versus 16.3 +/- 8.0 and 1.47 +/- 0.63 mmHg/ml per min per 100 ml, P less than 0.05 and P less than 0.01, respectively). Both resting and minimal vascular resistance were significantly correlated with [Ca2+]i in platelets (r = 0.40 and P less than 0.05, r = 0.55 and P less than 0.01, respectively). These results suggest that [Ca2+]i in platelets reflects [Ca2+]i in vascular smooth muscle cells and may be a significant determinant of not only tone but also structural changes of resistance vessels.

The role of antihypertensive drugs in counteracting adverse influence on large arteries

Studies in humans have shown that chronic elevation of blood pressure induces early alterations in the large arteries, consisting mainly of increased diameter, increased pulse wave velocity, and decreased arterial compliance. Acute reduction of blood pressure, per se, does not necessarily correct these alterations homogeneously. For instance, cadralazine reduced brachial artery diameter and urapidil decreased pulse wave velocity, without accompanying changes in forearm arterial compliance. In contrast, acebutolol, isosorbide dinitrate, nicardipine, and nitrendipine, for the same acute decrease in blood pressure, improved forearm arterial compliance by a concomitant decrease in pulse wave velocity and increase in arterial diameter. To determine whether or not these vascular effects persist with long-term acebutolol therapy, nine hypertensive patients were treated for a period of 3 months. Forearm hemodynamic and cytosolic free calcium concentrations in platelets were simultaneously assessed before and after treatment. Compared with placebo baseline values, chronic acebutolol therapy significantly decreased mean arterial pressure ( p < 0.01), pulse wave velocity ( p < 0.01), and platelet free calcium concentration ( p < 0.05); forearm arterial compliance was increased ( p < 0.01), but brachial artery diameter did not change. Platelet calcium concentration correlated closely with pulse wave velocity even at constant mean arterial pressure. These findings suggest a relaxant effect of acebutolol on the smooth musculature of large arteries, which is independent of changes in blood pressure and arterial diameter, and possibly mediated by changes in cytosolic calcium levels.