Calcium accumulation and enzymatic activities of subcellular fractions from aortas and ventricles of genetically hypertensive rats

Hydrogen sulfide regulates cardiac sarcoplasmic reticulum Ca(2+) uptake via K(ATP) channel and PI3K/Akt pathway

AIMS

To investigate the effects of hydrogen sulfide (H(2)S) on calcium uptake activity of the rat cardiac sarcoplasmic reticulum (SR) and possible signaling.

MAIN METHODS

Crude SR was isolated after treatment with H(2)S, then SR Ca(2+) uptake and SR Ca(2+)-ATPase (SERCA) activity was measured by the isotopic tracer method. The possible roles of the K(ATP) channel and PI3K/Akt and SR-membrane protein phospholamban (PLB) pathway were analyzed by specific blockers, and target protein activation was assayed by measuring protein phosphorylation.

KEY FINDINGS

Exogenous H(2)S lowered Ca(2+) uptake into the SR time or concentration dependently, which was associated with decreased SERCA activity. Inhibiting endogenous H(2)S production by DL-propargylglycine increased SR Ca(2+) uptake and SERCA activity. H(2)S inhibition of PLB phosphorylation was through SERCA activity and was reversed by two PI3K inhibitors, wortmannin and LY294002. Glibenclamide (a K(ATP) channel blocker) blocked the inhibitory effects of H(2)S on PLB and Akt phosphorylation. Pinacidil (a K(ATP) channel opener) reduced the phosphorylation of PLB and reversed the effects of DL-propargylglycine. H(2)S preconditioning increased PLB phosphorylation but did not affect SERCA activity.

SIGNIFICANCE

Endogenous H(2)S transiently and reversibly inhibits SR Ca(2+) uptake in rat heart SR because of downregulated SERCA activity associated with PLB phosphorylation by the PI3K/Akt or K(ATP) channel. The transient negative regulation of SR Ca(2+) uptake and the L-type Ca(2+) channel contributes to Ca(2+) cycle homeostasis, which might be an important molecular mechanism in ischemic diseases.

Sarcoplasmic reticulum Ca2+ uptake is not decreased in aorta from deoxycorticosterone acetate hypertensive rats: functional assessment with cyclopiazonic acid

Ca2+ plays a major role in vascular contraction, and a defect in intracellular Ca2+ regulation has been associated with increased vascular reactivity in hypertension. To test the hypothesis that the sarcoplasmic reticulum does not adequately buffer Ca2+ in deoxycorticosterone acetate (DOCA) hypertension, contractile experiments were performed with a specific inhibitor of the sarcoplasmic reticulum Ca2+ ATPase, cyclopiazonic acid (CPA). Contractile force in aortic strips from DOCA and control rats was measured, using standard muscle bath procedures, to evaluate (i) Ca2+ handling, assessing caffeine and serotonin (5HT) induced contractions in Ca(2+)-free buffer and (ii) relaxation rate after 5HT washout. Contractile responses elicited with 5HT (3 x 10(-6) mol/L) and caffeine (20 mmol/L) were greater in DOCA than in control arteries. CPA (1 x 10(-7) to 3 x 10(-5) mol/L) reduced phasic contractions to 5HT and caffeine in DOCA and control aorta, and no differences in the IC50 values were observed. Aortae from DOCA rats contracted when placed in normal buffer, subsequent to treatment with Ca(2+)-free buffer, but control aortae did not. CPA potentiated these responses in DOCA aorta and only caused a modest contraction in control aorta. CPA-induced contraction did not occur in Ca(2+)-free buffer, and it was inhibited by nifedipine (IC50 = 4 x 10(-9) mol/L). The relaxation rate, after 5HT washout (3 x 10(-6) mol/L), was increased in DOCA aorta (2.6 +/- 0.3 min) compared with control (1.7 +/- 0.2 min), and CPA (10(-5) mol/L) increased the relaxation rate in both groups. The results support the hypothesis of defective Ca2+ handling in DOCA hypertension. However, an increased Ca2+ influx, and not a decreased buffering ability of the sarcoplasmic reticulum, contributes to the enhanced vascular reactivity observed in DOCA hypertension.

Diminished brain synaptic plasma membrane Ca(2+)-ATPase activity in spontaneously hypertensive rats: association with reduced anesthetic requirements

We have recently reported that plasma membrane Ca(2+)-ATPase (PMCA) pumping activity in rat brain synaptic plasma membranes (SPM) was reduced by in vitro or prior in vivo exposure to inhalation anesthetics (IA). In addition, rats with streptozocin-induced diabetes were found to have diminished brain synaptic PMCA pumping and a decrease in the partial pressures of several IA required to prevent movement in response to stimulation, defined as the minimum effective dose or MED. Diminished PMCA activity in erythrocytes of spontaneously hypertensive rats (SHR) has been noted. Because PMCA is ubiquitous, it seemed possible that PMCA pumping might be decreased in the brain of SHR and perhaps associated with decreased IA requirement. Eighteen SHR and 18 control, normotensive Wistar-Kyoto rats (WKY) were studied. PMCA activity was assessed by measurement of Ca2+ uptake into synaptic plasma membrane vesicles prepared from cerebrum and diencephalon-mesencephalon (D-M) in WKY and SHR. Ca2+ pumping was significantly less in SHR than in WKY, 85% of control in the cerebrum and 90% in the D-M (p < 0.01). The MEDs for halothane, isoflurane and desflurane were also lower in SHR than in WKY, 91%, 90% and 89%, respectively, of control (p < 0.05). Thus, an animal model of primary hypertension (SHR) manifested diminished brain synaptic PMCA activity and reduced MED for several volatile anesthetics. These findings provide further evidence for a role for PMCA in anesthetic action.

Measurement of Ca2+ pump-mediated efflux in hypertension

Ca2+ homeostasis has been a prominent research area in the study of hypertension. There is convincing evidence that hypertension in spontaneously hypertensive rats is characterized by enhanced Ca2+ influx in various cell types. It is, however, still unclear whether hypertension is associated with reduced or enhanced Ca2+ efflux. Reduced Ca2+ efflux would augment the effects of enhanced Ca2+ influx. However, enhanced Ca2+ extrusion may occur as an adaptive process to minimize the effects of Ca2+ overload. This question remains unanswered because of inconsistent results obtained using a variety of experimental techniques. In this article we have reviewed the research findings and discuss existing and possible new techniques to assess Ca2+ efflux in hypertension, with particular attention to vascular smooth muscle. We have focused mainly on studies using the spontaneously hypertensive rat and discuss its appropriateness as a model for essential hypertension.

Effects of ethanol on brain monoamine content of spontaneously hypertensive rats (SHR)

Spontaneously hypertensive rats (SHR) were administered either 2.4 g/kg ethanol or an isocaloric glucose daily for 4 weeks and the levels of norepinephrine (NE), epinephrine (EP), dopamine (DA), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in different brain regions were determined. Results indicated a 3-fold increase in NE level in brain stem and hypothalamus and more than 2-fold increase in DA in corpus striatum in alcohol-treated rats as compared to controls. There was a significant increase in the level of DA in the corpus striatum but the levels in cerebral cortex, brain stem and hippocampus were decreased instead. Decreases in 5-HT levels were found in hypothalamus, brain stem, cortex and cerebellum of alcohol-treated brain as compared to untreated controls. These results indicate alterations of the biogenic amine contents in different regions of the SHR brain after chronic ethanol ingestion. Since stimulated release of biogenic amines in the SHR brain has been implicated in the regulation of blood pressure, changes due to ethanol ingestion may be a risk factor in hypertensive patients.

Detection and localization of triadin in rat ventricular muscle

Dyads (transverse tubule--junctional sarcoplasmic reticulum complexes) were enriched from rat ventricle microsomes by continuous sucrose gradients. The major vesicle peak at 36% sucrose contained up to 90% of those membranes which possessed dihydropyridine (DHP) binding sites (markers for transverse tubules) and all membranes which possessed ryanodine receptors and the putative junctional foot protein (markers for junctional sarcoplasmic reticulum). In addition, the 36% sucrose peak contained half of the vesicles with muscarine receptors. Vesicles derived from the nonjunctional plasma membrane as defined by a low content of dihydropyridine binding sites per muscarine receptor and from the free sarcoplasmic reticulum as defined by the M(r) 102K Ca2+ ATPase were associated with a diffuse protein band (22-30% sucrose) in the lighter region of the gradient. These organelles were recovered in low yield. Putative dyads were not broken by French press treatment at 8,000 psi and only partially disrupted at 14,000 psi. The monoclonal antibody GE4.90 against skeletal muscle triadin, a protein which links the DHP receptor to the junctional foot protein in skeletal muscle triad junctions, cross-reacted with a protein in rat dyads of the same M(r) as triadin. Western blots of muscle microsomes from preparations which had been treated with 100 mM iodoacetamide throughout the isolation procedure showed that cardiac triadin consisted predominantly of a band of M(r) 95 kD. Higher molecular weight polymers were detectable but low in content, in contrast with the ladder of oligomeric forms in rat psoas muscle microsomes. Cardiac triadin was not dissolved from the microsomes by hypertonic salt or Triton X-100, indicating that it, as well as skeletal muscle triadin, was an integral protein of the junctional SR. The cardiac epitope was localized to the junctional SR by comparison of its distribution with that of organelle markers in both total microsome and in French press disrupted dyad preparations. Immunofluorescence localization of triadin using mAb GE4.90 revealed that intact rat ventricular muscle tissue was stained following a well-defined pattern of bands every sarcomere. This spacing of bands was consistent with the interpretation that triadin was present in the dyadic junctional regions.

The use of calcium channel blockers in obstetrics and gynecology; a review

Since their introduction, calcium channel blockers gained wide use in the treatment in various fields of medicine. Calcium channel blockers were shown to inhibit contractions of the pregnant and nonpregnant uterus, and lately their properties have made it, theoretically, an ideal candidate for the use in severe pregnancy-induced hypertension: it is effective in lowering blood pressure, has a fast and long-standing action, may be conveniently administered via the oral route, and it is not associated with serious side effects.

Ouabain-induced contraction of vascular smooth muscle in spontaneously hypertensive rats and the effect of hydralazine

The effect of ouabain (10(-3) M) on contractile responses of SHR (spontaneously hypertensive rat) and WKY (Wistar-Kyoto rat) aortas and mesenteric arteries was studied. Ouabain addition caused a rapid contraction of aortic strips with a steeper rate of rise and a larger maximal force development in strips from SHR than WKY. This difference in contractile response is known to occur in the prehypertensive period of SHR (4-week-old). Phentolamine (10(-6) M) pretreatment had no effect on the ouabain-induced contraction but partially suppressed it in both SHR and WKY aortas when diltiazem (10(-5) M) was also added. The difference in the ouabain-induced contractions of SHR and WKY aortas was more apparent in the residual contraction during suppression by diltiazem. The 45Ca uptake in the presence of ouabain was significantly larger in the early period of incubation in SHR aorta than in WKY aorta. The ouabain-induced contraction of hydralazine-treated SHR aorta from the prehypertensive period was very similar to that of non-treated WKY aorta. These results suggested that the abnormality of the ouabain-induced contraction in SHR arterial smooth muscle could have arisen from an increased Ca2+ movement due to Ca2+ leakage when ouabain inhibited the Na+-pump in the membrane. This abnormality seems to start during the prehypertensive period and continue in the hypertensive stage.

Abnormality of calmodulin activity in hypertension. Evidence of the presence of an activator

An apparent increase of calmodulin (CaM) activity was previously observed in the heart and kidney but not in the liver of spontaneously-hypertensive rats (SHR) and mice compared with their corresponding normotensive controls. As this change was due to an elevated recovery of CaM in the organs of the hypertensive animals, the present study was designed to evaluate its activity in hypertension. A CaM activator, detected in heart and kidney supernatants from hypertensive animals, was found to be responsible for this enhanced recovery. Similar results were obtained with passaged, cultured aortic smooth muscle cells from SHR, indicating that the anomaly was not a mere consequence of elevated blood pressure but rather a genetic expression of cells of hypertensive origin. The activator was heat stable, nondialyzable, and recovered in the fraction precipitated with 30-50% ammonium sulfate. Preliminary extraction studies suggest that the activator is contained in a glycolipid fraction. The stimulation of phosphodiesterase by this activator was calcium and CaM dependent. The activator appears to affect the affinity of the phosphodiesterase for CaM rather than the maximal stimulation. The activator was also present at a low concentration in the heart and kidney of normotensive animals. These findings indicate that at least some of the calcium abnormalities implicated in the pathogenesis of hypertension could be the result of interactions between CaM, calcium, and this activator.

Calcium channel blockers in systemic hypertension

Alterations in transmembrane flux of calcium ions may be playing a role in the pathophysiology of systemic hypertension. Calcium channel blockers have been shown to be effective antihypertensive drugs with excellent safety profiles. They are efficacious in the long term treatment of systemic hypertension in all population subgroups, and have special applicability for treating patients with hypertensive urgencies and individuals with concomitant diseases such as angina pectoris and arrhythmias.

Platelet abnormalities and the pathophysiology of essential hypertension

The mechanisms whereby intracellular calcium concentration is controlled are briefly reviewed. With the current knowledge of both calcium homeostasis and the function and properties of cellular Ca2+-target proteins/signal transduction systems, a dysfunction of cellular calcium metabolism is considered in relation to the pathogenesis of hypertension. Although the enhanced peripheral vascular resistance characteristic of hypertension is ultimately a function of Ca2+ availability for smooth muscle cell contraction, the platelet possesses many parallel biochemical and physiological properties. Therefore, we have utilized the platelet as the cell-model for investigating the role of Ca2+ in hypertension disorders. An overview of Ca2+-linked platelet processes altered in essential hypertension is presented, and an attempt is made to integrate these multiple aberrations in a fundamental membrane lesion.

A role of calcium in altered sodium ion transport of hypertensives?

Research on the etiology of essential hypertension has led to many reports of altered ion transport in cells from hypertensive patients and animal models. Abnormalities in sodium and calcium ion gradients and transport in vascular smooth muscle, neuronal tissue, cardiac muscle as well as erythrocytes have been extensively investigated. It is not clear whether these abnormalities are of primary or secondary nature. The current knowledge of sodium and calcium ion transport in essential hypertension is briefly reviewed here. Furthermore, evidence is presented which suggests a role of calcium in the regulation of sodium transport activity.

Acid and alkaline phosphatases activities in vascular smooth muscle: species differences and subcellular distribution

1. Acid and alkaline phosphatase activities were studied in rat and dog aortic muscle using p-nitrophenyl phosphate (p-NPP) as the substrate. Alkaline phosphatase activity was quite comparable to acid phosphatase activity in rat aortic microsomes as well as further purified plasma membranes, but considerably lower than acid phosphatase activity in dog aortic membranes. 2. Subcellular distribution of acid and alkaline phosphatase activities in these vascular muscles indicated that alkaline phosphatases and a large portion of acid phosphatase activities were primarily associated with plasma membranes and the distribution of acid phosphatase showed little resemblance to that of N-acetyl-beta-glucosaminidase, a lysosomal marker enzyme. 3. The rat aortic plasmalemmal acid and alkaline phosphatase activities responded very differently to magnesium, fluoride, vanadate and EDTA. The alkaline phosphatase was more susceptible to heat inactivation than acid phosphatase. 4. These results suggest that these two phosphatases are likely to be two different enzymes in the smooth muscle plasma membranes. The implication of the present findings is discussed in relation to the alteration of these phosphatases in hypertensive vascular diseases.

Comparative studies of acid and alkaline phosphatase activities in nonvascular smooth muscle membranes

Acid and alkaline phosphosphatase activities of subcellular fractions isolated from rat gastric muscle and vas deferens by differential centrifugation, sucrose density gradient and cation-induced aggregation methods were studied using p-nitrophenyl phosphate as the substrate. Alkaline phosphatase and a large portion of acid phosphatase activities were found to be of plasmalemmal origin. Acid and alkaline phosphatase activities were different in the effect of Mg2+, fluoride, vanadate, EDTA and resistance to heat inactivation suggesting that these two phosphatase activities were not expressed by the same enzyme.

Decreased Na+K+ATPase activity in the aortic endothelium and smooth muscle of the spontaneously hypertensive rats

The activity of Na+K+ ATPase in the endothelium and smooth muscle of the aortae of normotensive and hypertensive rats was investigated. The enzyme activity in the endothelium and smooth muscle of the spontaneously hypertensive rats (SHR) was 2.15 +/- 0.48 and 12.98 +/- 0.99 respectively. These values were significantly lower (P less than 0.05) than the enzyme activity in the corresponding tissues (10.10 +/- 1.78 for endothelium, 20.77 +/- 2.54 for smooth muscle) of the normotensive Wistar Kyoto (WKY) rats. However, with the low blood pressure spontaneously hypertensive rats (LBP-SHR) i.e. in those animals whose blood pressures were below 150 mm Hg, the enzyme activity in both tissues was not significantly different from those of the WKY. Since Na+ K+ ATPase is coupled to the sodium-potassium pump whose activity affects the functions of other pumps, the results indicate that the development of high blood pressure in the SHR may be related to an alteration in the transport of cations across the cell membrane.

Phospholipids and fatty acid profile of brain synaptosomal membrane from normotensive and hypertensive rats

1. The main synaptosomal membrane phospholipids and their acyl group profiles, from 3-4 month-old spontaneously hypertensive rats (SHR), were compared with those of age-matched normotensive Wistar Kyoto (WKY) rats. 2. The contents of the main or total phospholipids were not found to be significantly different between these two groups. It was also true for the membrane cholesterol contents in these two groups. 3. The acyl groups of the main phospholipids from hypertensive rats were significantly higher in the saturated fatty acids: such as palmitic acid or stearic acid, and lower in polyunsaturated fatty acids: such as undecylenic acid or docosahexaenoic acid, when compared to the corresponding normotensive controls. 4. The differences in the acyl group profile of the brain membrane phospholipids of the hypertensive rats seem to reflect an abnormality in the genetically related lipolytic process.

Influence of dietary linoleic acid on leucocyte sodium transport and blood pressure

In a randomised double blind study to determine whether an increase in the polyunsaturated fat linoleic acid might influence leucocyte membrane sodium transport 22 normotensive volunteers received an oral supplement of linoleic acid or placebo daily for four weeks. Mean total sodium efflux rose significantly during supplementation with linoleic acid compared with placebo. In addition, all components of lying and standing blood pressure fell, though only the fall in supine systolic pressure was significant. Dietary supplementation with linoleic acid may alter ion fluxes across the cell membrane, presumably through changes in its physicochemical structure. In addition, the change in fat intake may lower blood pressure, though to only a very modest extent.

Decreased content of integral membrane calcium-binding protein (IMCAL) in tissues of the spontaneously hypertensive rat

Prior studies report that plasma membranes of the spontaneously hypertensive rat (Okamoto-Aoki strain) bind less calcium than do the corresponding preparations from Wistar Kyoto controls. The possibility that the differences result from a decrease in the content of integral membrane calcium-binding protein (IMCAL) was explored by the application of immunoassays with polyclonal antisera and a mouse monoclonal antibody. IMCAL binds calcium with relatively high affinity, and its content in many rat tissues is regulated by vitamin D and the level of dietary calcium. Immunoassays of tissue IMCAL demonstrate significant reductions in content in the erythrocyte ghost, intestinal mucosa, kidney, heart, testis, and liver of the spontaneously hypertensive rat as compared to the control strains. The decreases are observed both at 4-5 weeks of age, before the onset of severe hypertension and at 8-9 weeks in the presence of severe hypertension. Moreover, the magnitude of the decrease in erythrocyte IMCAL can account for much of the decrease in the calcium-binding capacity of erythrocyte membranes reported by others. The results are especially significant because an abnormality in the membrane binding of calcium and in the regulation of cytosolic calcium ion concentration could underlie the pathogenesis of the hypertension.

Activities of transport adenosine triphosphatases in erythrocyte membranes of healthy and hypertensive subjects

The Na+, K+-ATPase activity in the erythrocyte membrane was measured in 25 untreated essential hypertensive patients and 25 age-matched healthy normotensive subjects. In addition, the Ca2+, Mg2+-ATPase activity was measured in 20 hypertensive and 25 age-matched normotensive subjects. The Na+, K+-ATPase activity of healthy Chinese measured in this study was similar to the data reported in a Dutch study. We therefore could not support a theory which speculated an ethnic influence on Na+, K+-ATPase activity. Both Na+, K+-ATPase and Ca2+, Mg2+-ATPase activities were slightly lower in hypertensive patients as compared with those in normotensive subjects, but the differences were not statistically significant. We concluded that the transport ATPase activities were not impaired in the erythrocyte membranes of hypertensive patients.

A low dose of a calcium antagonist (nitrendipine) ameliorates cardiac and renal lesions induced by DOC in the rat

A low dose of nitrendipine (a calcium antagonist) ameliorated the percentage incidence and severity of cardiac and renal lesions induced by deoxycorticosterone (DOC) despite maintenance of the systolic blood pressure of the DOC plus nitrendipine group in the hypertensive range. The percentage mortality in the DOC-calcium antagonist group was slightly lower than that in the DOC-vehicle injected group. Nitrendipine did not reduce the DOC-induced renal hypertrophy, cardiomegaly, splenomegaly, or hepatomegaly as reflected in the absolute or relative weights of these organs. The absolute and relative weights of the thymus of the nitrendipine-DOC group did not differ significantly from those of controls although these weights decreased significantly in the group receiving DOC. No changes in relative weights of the adrenal gland were observed. The level of calcium in the serum of groups receiving DOC with or without nitrendipine was reduced significantly as compared to the comparable controls. Nitrendipine at the low dose employed separates at least in part the changes exerted by elevated blood pressure in animals receiving DOC from cardiac and renal lesions.

A Mg2+-independent high-affinity Ca2+-stimulated adenosine triphosphatase in the plasma membrane of rat stomach smooth muscle. Subcellular distribution and inhibition by Mg2+

Plasma membrane enriched fraction isolated from the fundus smooth muscle of rat stomach displayed Ca2+-stimulated ATPase activity in the absence of Mg2+. The Ca2+ dependence of such an ATPase activity can be resolved into two hyperbolic components with a high affinity (Km = 0.4 microM) and a low affinity (Km = 0.6 mM) for Ca2+. Distribution of these high-affinity and low-affinity Ca2+-ATPase activities parallels those of several plasma membrane marker enzyme activities but not those of endoplasmic reticulum and mitochondrial membrane marker enzyme activities. Mg2+ also stimulates the ATPase in the absence of Ca2+. Unlike the Mg2+-ATPase and low-affinity Ca2+-ATPase, the plasmalemmal high-affinity Ca2+-ATPase is not sensitive to the inhibitory effect of sodium azide or Triton X-100 treatment. The high-affinity Ca2+-ATPase is noncompetitively inhibited by Mg2+ with respect to Ca2+ stimulation. Such an inhibitory effect of Mg2+ is potentiated by Triton X-100 treatment of the membrane fraction. Calmodulin has little effect on the high-affinity Ca2+-ATPase activity of the plasma membrane enriched fraction with or without EDTA pretreatment. Findings of this novel, Mg2+-independent, high-affinity Ca2+-ATPase activity in the rat stomach smooth muscle plasma membrane are discussed with those of Mg2+-dependent, high-affinity Ca2+-ATPase activities previously reported in other smooth muscle plasma membrane preparations in relation to the plasma membrane Ca2+-pump.

Calcium and sodium transport and vitamin D metabolism in the spontaneously hypertensive rat

Serum ionized calcium levels are lower and immunoreactive parathyroid hormone levels are higher in the spontaneously hypertensive (SH) rat than in the normotensive Wistar-Kyoto (WKy) control. We postulated that there is either a defect in the regulation of vitamin D metabolism by parathyroid hormone or that the gut target organ for vitamin D in the SH rat is unresponsive. To test these hypotheses we measured serum concentrations of vitamin D metabolites and intestinal transport of calcium and sodium. Compared with that of WKy controls, in vitro calcium transport by duodenal sacs of the SH rat was decreased (P less than 0.001) at 5 wk, before the development of hypertension, and at 12 wk, after hypertension was well established. When measured in vivo in the most proximal 20 cm of small intestine, maximum velocity (Vmax) for calcium transport was decreased (P less than 0.05) and net absorption of sodium and water was increased (P less than 0.05) in SH rats as compared with WKy rats. Vmax for calcium transport was also decreased (P less than 0.05) in the most distal 20 cm of small intestine of SH rats, but net sodium and water transport were the same in SH and WKy rats. At 12 wk, serum concentration of 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] was the same in both SH and WKy groups, but its precursor, 25-hydroxycholecalciferol, was increased (P less than 0.05) in the SH rat. We conclude that in the SH rat: (a) the concentration of 1,25-(OH)2D3 is inappropriately low in relation to the elevated immunoreactive parathyroid hormone and the depressed calcium absorption, suggesting a defect in the regulation of vitamin D metabolism; and (b) the depressed calcium absorption, in the setting of normal concentrations of [1,25-(OH)2D3], demonstrates unresponsiveness of the gut to vitamin D and may explain in part the low serum ionized calcium found in earlier studies. The presence of these abnormalities before we found a significant difference in blood pressure suggests that they may be causal, not secondary, to the hypertension.

Stimulation of Na+,K+-ATPase of isolated smooth muscle membranes by the Ca2+ channel inhibitors, nimodipine and nitrendipine

Nimodipine (0.015 to 1.5 microM) increased Na+, K+-ATPase activity by 70-120% in isolated smooth muscle membranes. At 0.015 microM, nitrendipine, but not nifedipine, verapamil or diltiazem, also activated this enzyme. Nimodipine stimulated this Na+, K+ATPase three times more than nitrendipine at 15 nM. Marked stimulation of Na+,K+-ATPase by nimodipine was seen in membranes from rat and guinea pig aorta and rat vas deferens, but not in membranes from guinea pig heart or brain. Although it is not known whether these results are applicable to intact cells, the results are consistent with the hypothesis that vasodilation produced by nimodipine and nitrendipine may be due not only to inhibition of Ca2+ entry but also to the stimulation of the Na+ pump.

Hemodynamic and humoral responses to enalapril and nifedipine in the rat

The hemodynamic and humoral effects of enalapril, an angiotensin converting enzyme (ACE) inhibitor, and nifedipine, a calcium-entry blocker, were evaluated in conscious spontaneously hypertensive rats (SHR). Rats received enalapril (5 mg/kg, po) or vehicle, followed in one hour by nifedipine (2.5 mg/kg, ip) or its vehicle. After treatment with enalapril alone, systolic blood pressure (BP) declined over a 3 hour period from 204 +/- 4 (mean +/- SE) to 168 +/- 6 mm Hg and remained suppressed for an additional 4 hours. Heart rate (HR) did not change. Plasma renin activity (PRA) increased approximately 9-fold and serum ACE was maximally inhibited. BP response to nifedipine was more rapid, greater in magnitude (-59 +/- 6 mm Hg) and shorter in duration; heart rate increased and remained elevated for 2.5 hours. PRA only rose two-fold. After the drugs in combination, BP declined as rapidly and to the same degree as after nifedipine and remained reduced for a longer duration. Treatment with enalapril attenuated the reflex tachycardia observed after nifedipine. These data suggest that coadministration of an ACE inhibitor and calcium-entry blocker may provide better blood pressure control than either drug class alone and at the same time prevent the reflex tachycardia frequently observed after nifedipine.

The sympathetic nervous system in hypertension

Sympathetic drive can contribute not only to the acute adjustment to circulatory stresses but also to sustained elevation in vascular resistance and arterial pressure. In hypertensive animals and humans excessive sympathetic activity coupled with a possible genetic factor may result in defects in electrogenic ion transport of vascular muscle or in excessive vascular hypertrophy. Increased sympathetic activity and release of the neurotransmitter may occur as a result of defects of the efferent, afferent or central components of the sympathetic system. Several studies in various animal models and in humans are described to support the notion that the mosaic concept in hypertension may be embodied in a dual theory that includes a vascular muscle defect coupled with excessive sympathetic drive both of which may be of genetic or acquired origin. Almost regardless of the cause of hypertension, interruption of sympathetic efferent activity lowers arterial pressure.

Isolation of a plasma-membrane fraction from gastric smooth muscle. Comparison of the calcium uptake with that in endoplasmic reticulum

1. A plasma-membrane fraction was isolated from the smooth muscle of the pig stomach by using differential and sucrose-density-gradient centrifugations. When the centrifugation was carried out after preloading the crude microsomal fraction with Ca2+ in the presence of oxalate, the contamination of the plasma-membrane fraction by endoplasmic reticulum was decreased and a fraction enriched in endoplasmic reticulum vesicles filled with calcium oxalate crystals was obtained. 2. The plasmalemmal and endoplasmic-reticulum membranes could be distinguished by differences in the activity of marker enzymes and in the cholesterol content and by their different permeability to oxalate and phosphate. Oxalate and phosphate stimulated the Ca2+ uptake in the endoplasmic reticulum much more than in the plasmalemmal vesicles. In the plasma-membrane vesicles 40 mM-phosphate was more effective for stimulating the Ca2+ uptake than was 5 mM-oxalate, but the reverse was seen in the endoplasmic reticulum. 3. The high cholesterol/phospholipid ratio of the crude microsomal fraction are of the majority of the vesicles present in the crude microsomal fraction are of plasmalemmal origin. 4. The Ca2+ pump of the plasmalemmal and endoplasmic-reticulum vesicles could be differentiated by their different sensitivities to calmodulin. However, the two Ca2+-transport ATPases did not differ by their sensitivity to vanadate nor by the energization of the Ca2+ transport by different nucleoside triphosphates.

Calcium antagonists. Clinical use in the treatment of systemic hypertension

Increased peripheral vascular resistance is the cause of elevated systemic blood pressure in most patients with long standing hypertension. The desired haemodynamic effect in antihypertensive therapy is dilation of the constricted arterioles by a drug that acts directly on the vascular smooth muscle while not affecting the heart or the venous return. Hydralazine, diazoxide and minoxidil act directly on vascular smooth muscle to produce vasodilatation and have been used with variable degrees of success in the long term treatment of hypertension. Their cellular mechanism of dilation is not understood fully, but the ability to chelate certain trace metals required for smooth muscle contraction has been proposed as a possible mechanism of action for these drugs. The calcium antagonists (calcium entry blocking drugs) are a distinct group of compounds that interfere with the normal transmembrane flux of extracellular calcium ions on which vascular tissue depends for contraction or impulse generation. Thus, calcium anti-agonists can reduce the contractile activity of the heart, and promote coronary and systemic vasodilatation. These effects provide the clinical rationale for the use of calcium antagonists in the management of ischaemic heart disease and hypertrophic cardiomyopathy. Since systemic vasodilatation can be expected to reduce elevated arterial blood pressure, interest has focused recently on calcium antagonists in the medical management of systemic hypertension. All the calcium antagonists are able, in low concentrations, to relax the smooth muscle vasculature from coronary, cerebral, mesenteric, and renal arteries. The effects on the myocardium, cardiac impulse tissue, and vascular smooth muscle are different in magnitude, however, depending on the individual agent that is used. Clinical experience in the treatment of hypertension with this class of agents is confined to verapamil, nifedipine, and diltiazem. In this article, the scientific rationale for using calcium antagonists in the treatment of arterial hypertension is explored and the clinical experiences with the different calcium antagonists used in hypertension are reviewed.

Intracellular localization of the calcium antagonist propyl-methylenedioxyindene in cardiac tissue

Propyl-methylenedioxyindene (pr-MDI) is a coronary dilating and antiarrhythmic calcium antagonist with an intracellular site of action in cardiac tissue, probably involving inhibition of calcium mobilization from the sarcoplasmic reticulum and preservation of mitochondrial structural and functional integrity. Perfusion of isolated guinea-pig hearts with [14C]pr-MDI resulted in negative inotropism, with the time-course of this effect paralleling the uptake of the drug in the whole heart and the time-course of its accumulation in atrial and ventricular tissue. Tissue-to-medium ratios of pr-MDI for both atria and ventricles were high, and washout of the drug from the heart was biexponential. Pr-MDI associated poorly with a cardiac subcellular fraction exhibiting the highest specific activity of sarcolemmal marker enzyme. The highest specific activity of pr-MDI occurred in the cardiac mitochondrial fraction and in a fraction enriched with a putative sarcoplasmic reticular marker, and the accumulation of drug in these fractions was temporally correlated with its negative inotropic action. The results support an intracellular site of action of pr-MDI in cardiac tissue.

Clinical applications of slow channel blocking compounds

The slow-channel blockers constitute a structurally diverse group of drugs with varying mechanisms of action, propensities for site of greatest cardiovascular activity, and clinical efficacy. They share however the property of blocking the slow inward channel in heart muscle and of inhibiting calcium fluxes in smooth muscle. Their in vivo and in vitro actions must be distinguished. The overall actions represent a balance of direct and autonomically-mediated reflex actions interacting with the compounds' varying degrees of intrinsic non-competitive sympathetic antagonism. A knowledge of the pharmacodynamic differences between these drugs allows the physician to select the most appropriate agent for a given clinical situation. The central role of calcium in the cellular processes in the heart and the vascular system forms the basis for the utility of this class of drugs in a wide variety of cardiovascular disorders. Current intensive experimental and clinical investigations are likely to further define the roles of nifedipine, verapamil and diltiazem and their congeners in cardiovascular therapeutics. The prospect of development of newer compounds with greater selectivity of action is real. As pointed out by Braunwald (1982 a,b), with further clarification of the mechanisms of actions of these compounds and elucidation of the role of calcium fluxes throughout the body, more specific and potent agents may be developed. The apparent efficacy of the nifedipine congener nimodipine, in the treatment of cerebral vasospasm associated with subarachnoid hemorrhage (Allen et al., 1983) may simply be the first of a large number of 'specific' or targeted slow channel blockers. The development of such compounds may offer further therapeutic possibilities in the control of a variety of cardiocirculatory diseases.

Ion transport in hypertension

A wide range of abnormalities of membrane sodium and potassium transport can be demonstrated in patients with essential hypertension, and in rats with genetic hypertension and with some forms of experimental hypertension. In the human red cell increased permeability to sodium and potassium, increased ouabain-sensitive sodium pumping, lithium-sodium counter-transport, and frusemide-sensitive co-transport have been described; by contrast, in the human leucocyte sodium pumping is reduced. In the spontaneously hypertensive rat and the rat with mineralocorticoid-induced hypertension, increased permeability to sodium and potassium, with increased ouabain-sensitive pumping, is shared by the red cell and the arterial smooth muscle. This abnormality is associated with decreased cell-membrane affinity for calcium and increased cell-membrane viscosity. It is proposed that in essential hypertension the decreased membrane affinity for calcium is a primary pathogenetic change giving rise to secondary changes in sodium and potassium transport.

Altered distribution and properties of cAMP-dependent protein kinase isozymes in spontaneously hypertensive rat aorta

cAMP-dependent protein kinase activity was reduced in the cytosol fraction of spontaneously hypertensive rat (SHR) aorta compared to that of the Kyoto Wistar control rat (WKY). Two major peaks, isozymes I and II, of soluble cyclic AMP-dependent protein kinase activity could be separated by DEAE-cellulose chromatography. The distributions of isozymes I and II were 40 and 60%, respectively, in WKY compared to 26 and 74% in SHR. Isozyme I of SHR eluted at a conductance of 2-3 mmhos compared to 5-6 mmhos in WKY. In addition, activity under the peak of isozyme I of SHR was reduced by approximately 55% compared to WKY. The half-life of thermal denaturation of isozyme I at 50 degrees was 21 min in WKY compared to 84 min in SHR. On the other hand, for isozyme II no significant differences were observed between WKY and SHR in elution pattern, total activity under the peak, or thermal denaturation of enzyme activity. These data suggest that specific changes had occurred in isozyme I of SHR.

Influence of ATP on sarcoplasmic reticulum function of vascular smooth muscle

A vesicular fraction isolated from bovine aorta and enriched in fragmented sarcoplasmic reticulum (FSR) exhibited active calcium transport and ATPase activity. By use of a hypotonic NaHCO3 extraction solution, an active preparation was isolated that retained activity for up to 4 days. A small but significant (P less than 0.05) Ca2+-stimulated, Mg2+-dependent ATPase associated with calcium transport was demonstrated with a specific activity of 0.33 mumol inorganic phosphate (Pi).mg-1.min-1. The basal Mg2+ ATPase demonstrated Michaelis-Menten kinetics [Km(Mg2+-ATP) = 0.44 +/- 0.01 X 10(-3) M; Vmax = 2.22 +/- 0.01 mumolPi.mg-1.min-1]. The Ca2+-stimulated, Mg2+-ATPase demonstrated apparent substrate inhibition (Ks approximately 10 mM) with no evidence for end-product (ADP) or excess added Ca2+ contributing to this inhibition. Oxalate-supported active calcium uptake velocities also exhibited quantitatively similar substrate inhibition. These results suggest that FSR from vascular smooth muscle contains either two enzymes or one enzyme with two isomeric forms, one of which is associated with the calcium uptake activity of this structure and the other of unknown function.

Nifedipine in the treatment of cardiovascular disease

Nifedipine, a calcium channel blocking agent, has been shown safe and effective in the treatment of various cardiac disorders. In Prinzmetal's variant angina, nifedipine relieves the acute spasm of a large coronary artery, and thereby reverses the sudden decrease in myocardial oxygen supply. In chronic stable angina, the efficacy of nifedipine stems from an interplay of direct and reflex effects. A decrease in systemic vascular resistance, and thus, myocardial oxygen demand, is considered the most important effect. Although this is partially offset by baroreceptor-mediated reflex increases in blood pressure and heart rate, the net effect is a decrease in myocardial oxygen consumption. The importance of nifedipine-induced increases in coronary blood flow in relieving stable angina remains controversial. Numerous clinical trials in patients with chronic angina have demonstrated that nifedipine increases exercise tolerance by lowering the heart rate-blood pressure product (an approximation of myocardial oxygen consumption). The maximal double product during exercise, however, does not increase, suggesting that the drug does not importantly improve myocardial oxygen supply. Nifedipine may also be of value in the therapy of unstable angina, acute myocardial infarction, and hypertension, as well as in the protection of the myocardium at risk during open heart surgery. However, definite recommendations on the use of the drug for these indications must await results of ongoing randomized studies. The effects of nifedipine are dose-related. Larger doses have been shown more effective than smaller doses, as long as an appropriate blood pressure is maintained. The side effects of nifedipine are mild and related to its vasodilating action.

A calmodulin-stimulated Ca2+ pump in rat aorta plasma membranes

An ATP-driven Ca2+-transport system has been characterized in a microsomal fraction from rat aorta. Calmodulin enhanced 2.5-fold 45Ca accumulation by EGTA-treated microsomes incubated with 10 microM Ca2+ (in the absence of oxalate) by increasing markedly the apparent affinity of the transport system for Ca2+. The ionophore A23187 induced a rapid release of the sequestered 45Ca. The vesicles that took up 45Ca were distributed like plasmalemmal marker enzymes when the microsomal fraction was subfractionated by density gradient centrifugation. In particular, these vesicles were markedly shifted towards higher equilibrium densities after addition to the microsomes of 0.2 mg digitonin/mg protein before isopycnic centrifugation. We conclude that the calmodulin-stimulated Ca2+ pump associated with the microsomal fraction is located in plasmalemmal elements.

Properties of acyl-CoA:cholesterol O-acyltransferase in aortic microsomes from atherosclerotic rabbits

Acyl-CoA:cholesterol O-acyltransferase (EC 2.3.1.26) was in microsomal fractions from atherosclerotic rabbit aortic tissue. Activity was increased over 70-fold following 8-11 weeks of cholesterol feeding. Comparison of the lipid composition of aortic microsomal fractions from control and cholesterol-fed animals showed a 2-fold increase in the molar ratio of unesterified cholesterol to phospholipid in the cholesterol-fed group, although no change in phospholipid content or composition was found. Aortic microsomes were fractionated by sucrose density gradient centrifugation. Acyl-CoA:cholesterol O-acyltransferase was localized in fractions containing neutral alpha-glucoside activity but was clearly separated from 5'-nucleotidase activity. The cholesteryl ester formed during in vitro incubation of incubations of microsomes with either [1-14C]-oleoyl CoA or [7-3H]cholesterol was localized in the same region of the density gradient as acyl-CoA:cholesterol O-acyltransferase. The studies indicate that the increased acyl-CoA:cholesterol O-acyltransferase activity found in cells from aortic atherosclerotic lesions is due to both an increased amount of enzyme and to an expanded pool of microsomal cholesterol which is available for esterification.

Demonstration of a (Ca2+ + Mg2+)-ATPase activity probably related to Ca2+ transport in the microsomal fraction of porcine coronary artery smooth muscle

A (Ca2+ + Mg2+)-ATPase activity is demonstrated in the microsomal fraction of porcine coronary artery. The characteristics of the ATPase activity are compared with those of the Ca2+ transport, both measured in similar solutions. It is concluded that the (Ca2+ + Mg2+)-ATPase is related to the Ca2+ transport because: 1. Both transport and ATPase have similar low Km values for Ca2+ as well as comparable Hill coefficients. The Km values are respectively 0.34 +/- 0.03 microM [4] and 1.17 +/- 0.15 microM [6]. The Hill coefficients are n = 1.69 +/- 0.09 [4] and n = 1.23 +/- 0.17 [6]. 2. Ionophores A23187 and X537A stimulate (Ca2+ + Mg2+)-ATPase activity while they inhibit net Ca2+ accumulation by increasing the Ca2+ permeability of the membranes. 3. The V values for Ca2+ accumulation and for (Ca2+ + Mg2+)-ATPase are comparable.