Membrane mechanisms in arterial hypertension

Altered neural and vascular mechanisms in hypertension

Essential hypertension is a multifactorial disorder which belongs to the main risk factors responsible for renal and cardiovascular complications. This review is focused on the experimental research of neural and vascular mechanisms involved in the high blood pressure control. The attention is paid to the abnormalities in the regulation of sympathetic nervous system activity and adrenoceptor alterations as well as the changes of membrane and intracellular processes in the vascular smooth muscle cells of spontaneously hypertensive rats. These abnormalities lead to increased vascular tone arising from altered regulation of calcium influx through L-VDCC channels, which has a crucial role for excitation-contraction coupling, as well as for so-called "calcium sensitization" mediated by the RhoA/Rho-kinase pathway. Regulation of both pathways is dependent on the complex interplay of various vasodilator and vasoconstrictor stimuli. Two major antagonistic players in the regulation of blood pressure, i.e. sympathetic nervous system (by stimulation of adrenoceptors coupled to stimulatory and inhibitory G proteins) and nitric oxide (by cGMP signaling pathway), elicit their actions via the control of calcium influx through L-VDCC. However, L-type calcium current can also be regulated by the changes in membrane potential elicited by the activation of potassium channels, the impaired function of which was detected in hypertensive animals. The dominant role of enhanced calcium influx in the pathogenesis of high blood pressure of genetically hypertensive animals is confirmed not only by therapeutic efficacy of calcium antagonists but especially by the absence of hypertension in animals in which L-type calcium current was diminished by pertussis toxin-induced inactivation of inhibitory G proteins. Although there is considerable information on the complex neural and vascular alterations in rats with established hypertension, the detailed description of their appearance during the induction of hypertension is still missing.

SPONTANEOUS OSCILLATORY CONTRACTIONS IN AORTAS OF RATS WITH ARTERIAL PRESSURE LABILITY CAUSED BY SINOAORTIC DENERVATION

1. The spontaneous variation of blood pressure is defined as arterial pressure lability. Sinoaortic denervation (SAD) is characterized by arterial pressure lability without sustained hypertension. 2. The phenomenon of spontaneous oscillatory contractions (SOCs) occurs more frequently in the vascular beds of hypertensive animals. In large arteries, such as the aorta, SOCs occur only occasionally or they can be initiated by application of chemical stimuli. 3. In the present study, we investigated whether the arterial pressure lability evoked by SAD could be related to the emergence of SOCs in the aorta of rats submitted to SAD compared with sham-operated rats (SO). Three days after surgery (SAD or SO), aortic rings were placed in an organ chamber and the incidence (percentage of rats presenting SOCs), frequency (number of SOCs in 10 min) and amplitude (mN) of SOCs were measured. The participation of external Ca(2+) and K(+) channels in the maintenance of SOCs was also verified. 4. The incidence and frequency of SOCs were higher in endothelium-denuded aortas from SAD rats (82% and 38 +/- 4 SOCs/10 min, respectively) than in aortas from SO rats (40% and 14 +/- 2 SOCs/10 min, respectively). In aortas from SAD rats, verapamil (0.2 micromol/L), pinacidil (0.3 micromol/L) and tetraethylammonium (TEA; 5 mmol/L) totally inhibited SOCs, whereas increasing the CaCl(2) concentration to 2.0 and 2.5 mmol/L increased the frequency of SOCs. Interestingly, increasing the concentration of CaCl(2) to 3.5 mmol/L inhibited these contractions in aortas from SAD rats. 5. These results show that although SAD rats did not become hypertensive, their aortas were capable of initiating SOCs without the application of any chemical stimuli. The SOCs seem to be dependent on Ca(2+) influx sensitive to verapamil and also involve K(+) channels sensitive to pinacidil and TEA.

Tension oscillation in arteries and its abnormality in hypertensive animals

1. The mechanisms of oscillatory contraction of arterial smooth muscle in vitro are discussed. 2. The membrane potential and cytoplasmic free Ca2+ concentration in smooth muscle cells oscillate in the presence of agonists. 3. The oscillatory change in the membrane potential of smooth muscle cells is related to Ca2+ release from intracellular stores. 4. Gap junctions between smooth muscle cells play important roles in the synchronized oscillation of the cytoplasmic free Ca2+ concentration in this population of cells. 5. Endothelial cells may increase or decrease the tension oscillation of smooth muscle cells. 6. In arteries from hypertensive rats, an increase in membrane excitability and the number of gap junctions between smooth muscle cells and impaired endothelial function are the main factors responsible for the modulation of tension oscillation.

Increased sodium influx and calcium uptake in erythrocytes in hyperthyroidism: role of abnormal membrane lipid levels

The study was designed to examine the effects of thyroid hormones on red blood cell (RBC) membrane phospholipids and ion transport. We demonstrated that in untreated Graves' disease, an alteration in the phospholipid pattern is present at cellular levels, with a concomitant derangement in membrane permeability defined as (22)Na influx and (45)Ca uptake. Thionamide therapy replaced the normal membrane permeability, presumably as a consequence of restoring the normal phospholipid membrane composition. We conclude that thyroid hormones are able to induce a quick breakdown of a large number of membrane components such as membrane phospholipids.

Alterations in sodium metabolism as an etiological model for hypertension

An adequate matching for race, sex, stage of the menstrual cycle, family history of hypertension, and the amount of sodium and other electrolytes in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium in essential hypertensive patients in comparison with normal subjects. Alterations in intracellular sodium concentration and transmembrane sodium transport systems as causes of essential hypertension are postulated. This review article describes how this abnormal sodium and calcium metabolism translates into increased systemic vascular resistance through altered vasoactive responses and/or vasculature structural changes.

Role of Ca(+)-dependent K-channels in the membrane potential and contractility of aorta from spontaneously hypertensive rats

1. Contractile responses to KCl and membrane potentials were determined in aortic rings from spontaneously hypertensive rats (SHR), normotensive Wistar rats (NWR) and Wistar Kyoto rats (WKY) both in the absence and in the presence of the Ca(2+)-dependent K-channel blockers, apamin and tetraethylammonium (TEA). 2. Compared to NWR, aortic rings from WKY and SHR were less reactive and their Ca2+ uptake after stimulation with K+ was decreased. 3. Smooth muscle cell membrane potentials were higher in aortae from SHR and WKY than in NWR aortae, whereas SHR had higher K+ and lower Na+ intracellular activities than WKY and NWR, suggesting overactivity of the Na+/K+ pump in the hypertensive animals. 4. Treatment with apamin caused depolarization of WKY and SHR aortae, and increased their contractile responses to the same level as those of the NWR. Treatment with TEA also caused depolarization of aortae from WKY and SHR, but in the SHR the depolarization induced by TEA was smaller than that produced by apamin and the contractile responses to KCl did not reach the level of those of aortae from NWR. 5. It is concluded that overactivity of Ca(2+)-dependent K-channels in aortae of WKY and SHR contributes to their higher membrane potentials and lower responsiveness to vasoconstrictor stimuli. In SHR, an overactive Na+/K+ pump is also present, and the contribution of apamin-sensitive Ca(2+)-dependent K-channels to the membrane potential and reactivity appears to be more relevant than that of TEA-sensitive channels.

Voltage-dependent Ca2+ channels in resistance arteries from spontaneously hypertensive rats

Alterations in voltage-dependent Ca2+ channels in the arterial smooth muscle cells of spontaneously hypertensive rats (SHR) were investigated using the whole-cell voltage clamp and compared with Wistar-Kyoto (WKY) rats. Single cells were freshly isolated from resistance mesenteric arteries from 4- to 5-week-old (young) and 16- to 18-week-old (adult) SHR. Elevated blood pressure was only evident in adult SHR, not in young SHR. In young rats, the Ca2+ channel current density (current amplitude normalized by cell capacitance) was significantly higher (P < .01) in SHR than in WKY rats at the command potential of -10 mV or higher (with 50 mmol/L Ba2+): The current density at 20 mV was -16.8 +/- 1.1 pA/pF in SHR (n = 38 cells) and -11.0 +/- 0.8 pA/pF in WKY rats (n = 30 cells). In adult rats, the difference in current densities disappeared: -15.9 +/- 1.3 pA/pF in SHR (n = 25 cells) and -15.6 +/- 1.5 pA/pF in WKY rats (n = 29 cells). The ratio of maximal amplitude of T-type current to that of L-type current was low in young SHR (0.10 +/- 0.01) compared with the other three groups (0.16 to 0.20). Neither the activation curve nor the steady-state inactivation curve of SHR was different from that of age-matched WKY rats. However, the activation curves in adult rats were shifted to a hyperpolarized direction compared with those of young rats in both strains. These results suggest that the increased activity of voltage-dependent L-type Ca2+ channels of resistance arteries in young SHR may be related to the development of hypertension. The changes observed in adult rats may be due to a secondary modification of the channel during maturation and the presence of hypertension.

The action of amlodipine on voltage-operated calcium channels in vascular smooth muscle

1. Amlodipine, a dihydropyridine derivative largely ionized at physiological pH, inhibited calcium channel currents in single vascular smooth muscle cells isolated from rabbit ear artery in a concentration-dependent manner. 2. Amlodipine inhibited the current-voltage relationship for calcium channel currents across the range of test potentials used. However, the effect of amlodipine was more marked on more depolarized test potentials. Amlodipine also shifted the steady-state inactivation curve for calcium channel currents in a hyperpolarized direction. 3. The potency of amlodipine as determined from the steady-state inhibition of calcium channel current induced by the drug was dependent on the holding potential of the cells. Use of a more depolarized holding potential increased the potency of amlodipine. 4. Onset of amlodipine-induced inhibition was relatively rapid at both -60 mV and -40 mV holding potential. The use of a more depolarized holding potential increased the rate of association of amlodipine. No recovery from amlodipine-induced inhibition was seen over a 20 min period following washout of the drug. 5. In addition to voltage-dependence, the action of amlodipine showed use-dependence, in that the effect of amlodipine was more marked when calcium channel currents were evoked frequently. Increasing the frequency of activation of calcium channel currents did not alter the apparent onset rate of amlodipine-induced inhibition, but increased the degree of inhibition achieved by the drug. 6. The electrophysiological properties of amlodipine, particularly its voltage-dependence are probably important determinants of its action in vivo.

Membrane transport of ions in hypertension

A variety of disturbances in transmembrane monovalent and divalent cation fluxes has been described in blood cells from hypertensive patients. Other membrane properties, such as fluidity and calcium binding, are also altered. It is now abundantly clear that some of the inconsistencies in this field are due to poor matching of patients and controls. However, even when careful matching is carried out, differences in membrane functions are still seen. It is suggested that these are due to a disturbance in the physicochemical properties of the cell membrane, related to changes in cell membrane phospholipid fluidity. This change could maintain peripheral resistance either by directly or indirectly increasing tone or by predisposing to resistance vessel hypertrophy. Recent evidence emphasizes the role of the latter rather than the former in experimental hypertension. It is postulated that overactivity of the phosphoinositide second messenger system as a result of alteration in all membrane properties predisposes genetically susceptible individuals to resistance-vessel hypertrophy and hypertension.

Chronic inhibition of angiotensin converting enzyme decreases Ca2+-dependent tone of aorta in hypertensive rats

Long-term effects of a novel angiotensin converting enzyme (ACE) inhibitor, CS-622, on Ca2+-dependent tone in aortic smooth muscles of spontaneously hypertensive rats (SHR) were examined. CS-622 (3 or 10 mg/kg/day), when orally administered to SHR for 21 weeks, exhibited a dose-dependent antihypertensive action. In Krebs-Henseleit solution, removal of Ca2+ caused much greater relaxation in aortas excised from control SHR than those from SHR treated with CS-622. Restoration of Ca2+ from zero to 2.5 mM elicited a marked contraction in aortas from control SHR but only a small contraction in aortas from both CS-622-treated SHR and normotensive Wistar-Kyoto rats. These findings suggested that myogenic tone that resulted from increased Ca2+ permeability in aortas of SHR was suppressed by long-term treatment with CS-622. The aortic tone from the individual rats correlated well with systolic blood pressure in both CS-622-treated and control SHR. The exaggerated myogenic tone in aortas of SHR was attenuated in the medium containing nicardipine but was not altered in the presence of CS-622 diacid (active form of CS-622) at a concentration high enough to fully inhibit aortic ACE. The myogenic tone in normal Ca2+ concentration was not decreased in aortas excised from SHR treated with hydralazine (5 mg/kg/day) for 21 weeks. We conclude that after prolonged administration CS-622 reduced the high vascular tension resulting from increased Ca2+ permeability of vascular smooth muscle membrane in SHR and that the restoration of normal Ca2+ permeability of vascular smooth muscles may underlie long-term antihypertensive action of ACE inhibitors.

Distribution of proteins in erythrocyte membranes from patients with hypertension

The distribution of characteristic proteins in erythrocyte membranes was studied in patients with essential hypertension (EH) (n = 44), secondary hypertension of renal genesis (n = 42), and healthy persons (n = 44). Densitograms of gels analyzed after electrophoresis of erythrocyte ghosts showed a twofold increase in the amount of band 4.5 (Mw = 52-59 kD) and band 6 (Mw = 35 kD) polypeptides in EH patients as compared to that in healthy persons. Radioimmunoassay with monoclonal antibodies obtained to the sarcoplasmic reticulum (SR) of heart muscle has demonstrated that the amount of the antibodies bound to fragmented erythrocyte membranes from EH persons is greater by at least 28% than that in healthy people. Patients with secondary arterial hypertension of renal genesis did not reveal a significant difference in binding of monoclonal antibodies as compared to the control group. Thus, erythrocyte membranes from EH subjects are different from those taken from the blood of healthy people by the increased amount of bands 4.5 and 6 proteins.

Hypertensive mechanisms associated with centrally administered aldosterone in dogs

The mechanism by which intracerebroventricularly administered aldosterone increases arterial pressure was investigated in trained, conscious dogs with cannulas chronically implanted in a lateral cerebral ventricle. In salt-replete and salt-depleted dogs, artificial cerebrospinal fluid with or without aldosterone (0.05 microgram/kg/hr) was infused intracerebroventricularly for 12 days by an osmotic minipump. A similar dose of aldosterone was infused subcutaneously for 12 days. Aldosterone infused intracerebroventricularly increased blood pressure significantly in both salt-replete and salt-depleted dogs. In salt-replete animals the hypertension was associated with increased total peripheral resistance without concomitant changes in blood volume, cardiac output, or in any of the neurohumoral parameters measured. We conclude that this type of hypertension is resistance-mediated from its outset and appears to be relatively independent of salt and water retention. The mechanism by which intracerebroventricularly administered aldosterone increases vascular resistance remains to be determined.

Ion channel effects of pinacidil in vascular muscle

Cellular investigation of the vasodilator, pinacidil, was carried out to determine the membrane mechanisms leading to vascular muscle relaxation. Patch clamp recording of whole cell currents from isolated vascular muscle cells of rat azygos vein demonstrated a significant increase in K+ currents when 1 to 50 mumol/L pinacidil was added. The increases of K+ current by pinacidil were small (number of openings increased by 2%) when the recording pipette (and intracellular solution) contained 10 mmol/L EGTA, but were relatively large (50-300% increase in number of openings) when calcium in the intracellular solution was greater than 300 mumol/L. These observations suggest that pinacidil causes vasodilatation by increasing potassium conductance, primarily or completely via the large (200pS), calcium-dependent K+ channel.

Vascular muscle membrane cation mechanisms and total peripheral resistance

Passive and active carrier-mediated transport of sodium across vascular muscle membranes has been suggested to be more important in the increased total peripheral resistance found in genetic hypertension. Using manipulations of ion gradients and recordings of ion currents, membrane potentials, and tension, I have found evidence for calcium regulation as the central pathophysiological mechanism in spontaneously hypertensive rats. Increased sodium pump activity, which may be a partial compensation for the increased sodium influx in hypertension, may thus be secondary to altered calcium channel regulation in hypertension. The calcium channel, and the membrane potentials governing it, seem to be the most immediately important membrane mechanisms for hypertension research.

Pathophysiological role of cation transport and natriuretic factors in hypertension

This review considers in some detail the hypothetical relationships between sodium fluxes, both active and passive, across the cell membrane, and intracellular sodium concentration in vascular smooth muscle in the animal models of hypertension. It appears that two basic types of transport defects, increased cell membrane permeability to sodium and decreased active pumping of sodium at a given internal sodium concentration, can exist in vascular smooth muscle in experimental hypertension, and that sometimes the two defects coexist, further increasing internal sodium concentration. It is possible that eventually we may find similar transport defects in vascular smooth muscle in humans with arterial hypertension. Decreased active pumping at a given internal sodium concentration appears to result from a humoral sodium pump inhibitor. Future directions for research in the area are also considered. First priority should be given efforts to determine the chemical structure of the sodium pump inhibitor(s). High priority should also be given to attempts to measure passive and active sodium fluxes and intracellular sodium concentration in vascular smooth muscle cells in vivo, and to determine the role of atrial natriuretic factor in the genesis and maintenance of hypertension.

The effect of alkaline pH and transmural pressure on arterial constriction and membrane potential of hypertensive cerebral arteries

These studies were undertaken to examine the effect of alkalosis to modify "pressure-induced" activation of isolated cerebral arteries from spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto (WKY) controls. At pH 7.4 and PCO2 of 34 torr elevation of transmural pressure from 0-140 mm Hg resulted in myogenic activation preceded by membrane depolarization in both SHR and WKY. The degree of developed myogenic tone in SHR was elevated above WKY. Alkalosis (pH 7.4-7.7) depolarized and activated SHR cerebral arteries to a greater extent than WKY. Furthermore, both the electrical and mechanical responses to elevation in transmural pressure were exaggerated in SHR compared to WKY at pH 7.7 (PCO2 constant at 34 torr). Manipulation of PCO2 at constant pH of 7.4 had similar effects on "pressure-induced" myogenic tone in both SHR and WKY. Thus, cerebral arteries from both SHR and WKY depolarize and develop myogenic tone in response to increasing transmural pressure. This response is augmented in SHR, but to a much greater extent upon elevation of extracellular pH, while PCO2 is maintained within normal limits. The implications of these findings are discussed.

Membrane ATPase mechanism of K+-return relaxation in arterial muscles of stroke-prone SHR and WKY

These studies compared the importance of electrogenic Na+-K+ active (ATP driven) transport, changes in K+ conductance, and passive Ca2+-Na+ countertransport in the large relaxation that occurs in the rat caudal and basilar artery on return to K+ from K+-free solutions. Furthermore, we compared the importance of these three membrane electrical mechanisms in stroke-prone spontaneously hypertensive rats (SP-SHR) versus their normotensive Wistar-Kyoto control rats (WKY) in basilar (cerebral) and caudal arteries. We found that in both basilar and caudal arteries the hyperpolarization and relaxation that occurred on return to K+ after exposure to a 0 K+ (extracellular) solution was consistently greater in SP-SHR than in WKY. The change in membrane potential occurring on transition to 0 K+ in arteries maintained at low temperature (16 degrees C), used as an estimate of the change in K+ conductance during the K+ transition, was not different in either basilar or caudal arteries between SP-SHR and WKY. Thus the hyperpolarization on return to K+ at body temperature would depend primarily on the level of activity of the membrane ATPase, referred to as the Na+ pump. We also sought to compare the passive (but electrogenic) Ca2+-Na+ countertransport mechanism between strains for both arteries, but we were unable to detect any evidence of the predicted hyperpolarization-contraction on transition from 145 to 10 mM extracellular Na+. Furthermore, the return to extracellular Na+ solution failed to show the depolarization-relaxation predicted by the Ca2+-Na+ countertransport mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Small vessel membrane potential, sympathetic input, and electrogenic pump rate in SHR

Comparative measurements of transmembrane potential (Em) were made in situ in vascular smooth muscle cells (VSM) of mesenteric small principal arteries and veins with innervation and circulation intact. Vessels were in an externalized, topically suffused jejunal loop in 4- to 5-wk-old (initial hypertension) and 12- to 15-wk-old (established hypertension) anesthetized, spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive control rats. Comparable in vitro measurements of Em were also made in VSM of isolated intact small mesenteric vessel segments (from the 12- to 15-wk-old animals) maintained at their in situ lengths and suffused with physiological salt solution (PSS). During suffusion in situ with control PSS, VSM of both small veins and arteries in older (but not younger)SHR were less polarized than in WKY. Local chemical sympathetic denervation in situ (with 6-hydroxydopamine) hyperpolarized VSM of both vessel types in older (but not younger) SHR to the same Em levels measured in situ in respective WKY vessels. After local denervation, VSM of small arteries (but not veins) of both SHR and WKY remained less polarized in situ than in vitro, suggesting the presence of one or more circulating factors with a specific depolarizing action on the arterial side in both animal types. In vitro, VSM of both small arteries and veins from WKY but not SHR were depolarized immediately by 10(-3) M ouabain. In contrast, reduction of the PSS suffusate temperature to 16 degrees C caused a significantly greater depolarization in VSM of SHR vessels.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of high and low sodium diets on the resistance vessels and their adrenergic vasoconstrictor fibre control in normotensive (WKY) and hypertensive (SHR) rats

As part of our studies in normotensive (WKY) and hypertensive (SHR) rats concerning the cardiovascular effects of 240-fold variations in sodium (Na) intake, the present experiments explore how vascular design, smooth-muscle sensitivity to noradrenaline and adrenergic vasoconstrictor fibre function are affected. In vitro comparisons were performed on pair-perfused hindquarter vascular beds and on paired small mesenteric arteries (diameter 150-200 micron), using a two-vessel Mulvany-Halpern myograph. Preparations were taken from WKY and SHR which between 5 and 12-13 weeks of age were on 'low' (LNa, 0.5), 'control' (CNa, 5), 'high' (HNa, 50) or 'very high' (vHNa, 120 mmol Na 100 g-1 food) sodium diets. Structural vascular adaptation occurred only when arterial pressure was altered (only in LNa SHR). In both preparations smooth-muscle sensitivity and dose-response curves to noradrenaline remained unaffected by the Na diets. However, in both LNa groups the frequency-response curves to vasoconstrictor fibre stimulation in the small arteries were displaced to the right of the CNa one, with generally attenuated responses, while the curves of particularly the vHNa arteries were displaced to the left, with enhanced responses. Inhibition of NaKATPase by ouabain particularly enhanced the neurogenic responses, but to similar extents in all Na groups. Thus, low sodium intake apparently reduces the transmitter release/impulse in adrenergic neurons, while it increases the transmitter stores. High sodium intake has the opposite effects. These adaptations of adrenergic neuronal function may be one of the most important long-term consequences of altered sodium intake.

Differences in norepinephrine activation and diltiazem inhibition of calcium channels in isolated rabbit aorta and mesenteric resistance vessels

The mechanisms of norepinephrine stimulation of calcium ion entry in isolated rabbit aorta and mesenteric resistance vessels were studied through measurements of effects on calcium-45 influx, tension, and membrane potential. The resistance vessels were considerably less sensitive to norepinephrine than the aorta. The aorta exhibited complex dose-response curves for norepinephrine-stimulated calcium influx and contraction, whereas these were simple in the arterioles. Both vessels were depolarized with increasing concentrations of potassium. Norepinephrine did not depolarize the aorta, whereas it did depolarize the mesenteric resistance vessels. This result supports the contention that norepinephrine opens receptor-operated channels to induce calcium entry in the aorta, while it may activate potential sensitive calcium channels in the mesenteric resistance vessels. However, the maximum depolarization with norepinephrine (10(-4) M) in the arterioles was completely blocked by 10(-5) M diltiazem, whereas that induced by 80 mM potassium was unaltered by the diltiazem. Furthermore, 10(-4) M norepinephrine was able to stimulate virtually the same contraction and calcium influx in 80 mM potassium-depolarized arterioles as in normal polarized tissues. These results are consistent with norepinephrine opening of receptor-operated channels to allow calcium entry in the rabbit mesenteric resistance vessels. That the behavior of norepinephrine-activated channels in the aorta is more complex than in the arterioles is further illustrated by a dramatically decreasing sensitivity of norepinephrine-stimulated calcium influx to diltiazem with increasing norepinephrine in the aorta but not in the arterioles.(ABSTRACT TRUNCATED AT 250 WORDS)

Vascular smooth muscle function and its changes in hypertension

The contractile state of vascular smooth muscle influences arterial blood pressure and regulates organ blood flow. Current evidence suggests that the contractile apparatus of vascular smooth muscle is composed of thin and thick filaments, and that force generated between these two filaments provides the mechanism for cell shortening. The molecular events that initiate the interaction between these filaments are dependent upon the free sarcoplasmic concentration of activator calcium, which is regulated by the cell membrane and at subcellular sites. Changes in electrical activity of the cell membrane and interaction of pharmacologic agents with membrane receptors alter the cell, causing either a decrease or increase in sarcoplasmic calcium concentration and thus changing the contractile state of the vascular smooth muscle cell. Alterations in the cellular mechanisms that regulate intracellular calcium concentration may contribute to abnormal vascular function in pathologic states. In this brief review, the normal mechanism of vascular smooth muscle contraction is described, and the evidence that indicates that components of the contractile process change in hypertension is examined.