Nitrendipine Binding Research Articles

Allosteric interaction of trimebutine maleate with dihydropyridine binding sites

The effects of trimebutine maleate on [ 3H]nitrendipine binding to guinea-pig ileal smooth muscle membranes and Ca 2+-induced contraction of the taenia cecam were studied. Specific binding of [ 3H]nitrendipine to smooth muscle membranes was saturable, with a K D value and maximum number of binding sites (B max) of 0.16 nM and 1070 fmol/mg protein, respectively. Trimebutine inhibited [ 3H]nitrendipine binding in a concentration-dependent manner with a K i value of 9.3 μM. In the presence of trimebutine (10 μM). Scatchard analysis indicated a competitive-like inhibition with a decrease in the binding affinity (0.31 nM) without a chane in B max(1059 fmol/mg protein). However, a dissociation experiment using trimebutine (10 or 100 μM) showed that the decreased affinity was due to an increase of the dissociation rate constant of [ 3H]nitrendipine binding to the membrane. In mechanical experiments using the taenia cecum, trimebutine (3–30 μM) caused a parallel rightward shift of the dose-response curve for the contractile response to a higher concentration range of Ca 2+ under high-K + conditions in a noncompetitive manner. These results suggest that trimebutine has negative allosteric interactions with 1,4-dihydropyridine binding sites on voltage-dependent Ca 2+ channels and antagonizes Ca 2+ influx, consequently inhibiting contractions of intestinal smooth muscle.

Effect of repetitive electroconvulsive treatment on sensitivity to pain and on [3H]nitrendipine binding sites in cortical and hippocampal membranes

The effect of electroconvulsive shock (ECS) on the responsiveness to pain (measured by the hot-plate test) and on the characteristics of L-type calcium channels (measured as [3H]nitrendipine binding sites) in the cortex and hippocampus was tested on the Wistar rat. In animals receiving a single ECS, the calcium channel density and affinity 24 h after treatment did not differ from the controls; the response to pain was also at the control level. Repeated ECS (eight once-daily shocks) resulted in an increased responsiveness to pain (shortening of response latency) and in an increase in the density of cortical, but not hippocampal, calcium channels. The KD value for [3H]nitrendipine binding sites in either brain region remained unaltered by ECS. The calcium channel antagonist nifedipine, which by itself did not significantly alter the response to pain, prevented the enhancement of pain sensitivity brought about by ECS. The results suggest activation of calcium-dependent mechanisms by repeated ECS and confirm the involvement of calcium channels in pain mechanisms.

Dihydropyridine binding to the L-type Ca 2+ channel in rabbit heart sarcolemma and skeletal muscle transverse-tubules: Role of disulfide, sulfhydryl and phosphate groups

The dihydropyridine receptor is associated with the L-type Ca 2+ channel in the cell membrane. In this study we have examined the effects of group-specific modification on dihydropyridine binding in heart sarcolemmal membranes isolated from the rabbit. Specifically, dithiothreitol and glutathione were employed to assess the possible role of disulfide (-SS-) bonds in the binding of [ 3H]dihydropyridines. NEM, PCMS and iodoacetamide were employed to examine the effect of blocking free sulfhydryl groups (-SH) on the binding of [ 3H]dihydropyridines to their receptor in heart sarcolemma. Glutathione inhibited [ 3H]PN200-110 binding to sarcolemmal membranes 100%, with an IC 50 value of 50 μM, while DTT inhibited maximally by 75% with an IC 50 value in the millimolar range. Alkylation of free sulfhydryl groups by NEM or iodoacetamide inhibited binding of [ 3H]PN200-110 binding in cardiac sarcolemma approx. 40–60%. Blocking of free sulfhydryl groups by PCMS completely inhibited [ 3H]PN200-110 binding to their receptor in sarcolemmal membranes in a dose-dependent manner with an IC 50 value of 20 μM. These results suggest the involvement of disulfide bonds and free sulfhydryl groups in DHP binding to the L-type Ca 2+ channel in heart muscle. We also examined the effect of membrane phosphorylation on the specific binding of the dihydropyridine [ 3H]nitrendipine to its receptor. Phosphorylation was studied in cardiac sarcolemmal as well as skeletal muscle transverse-tubule membranes. Phosphorylation due to endogenous protein kinase and cAMP-dependent protein kinase was without effect on [ 3H]nitrendipine binding in both cardiac sarcolemmal and skeletal muscle membranes. Addition of exogenous calmodulin under conditions known to promote Ca 2+/calmodulin-dependent phosphorylation increased [ 3H]nitrendipine binding 20% with no alteration in K D in both types of membrane preparation. These results suggest a role for calmodylin in dihydropyridine binding to L-type Ca 2+ channels.

Cortical dihydropyridine binding sites and a behavioral syndrome in morphine-abstinent rats

The density of cortical [ 3H]nitrendipine binding sites was elevated by over 40% in rats rendered morphine-abstinent by administration of naloxone after chronic treatment with morphine. The morphine-abstinent rats had significantly shortened response latencies in the hot-plate test. Nifedipine treatment abolished the signs of abstinence and normalized the hot-plate latencies in morphine-dependent, naloxone-treated rats. The results indicate that the symptoms of abstinence are related to a functional state of cortical dihydropyridine-sensitive calcium channels.

The effects of strychnine on the regulation of voltage-dependent calcium channels by dihydropyridines in brain and heart

The effects of strychnine (STR) were investigated on K +-stimulated 45Ca 2+-uptake into mouse brain neurons, the contractile activity of spontaneously beating rat atria and on [ 3H]nitrendipine and [ 3H]BAY K 8644 binding to dihydropyridine calcium channel antagonist and agonist binding sites on brain and cardiac membranes. STR (10 −6−10 −4 M) had no effect on neuronal 45Ca 2+-uptake. When combined at equimolar concentrations (10 −5 M), STR and nifedipine produced a potent (nM) inhibition (40%) of neuronal 45Ca 2+-uptake. In the spontaneously beating rat atria, STR produced a dose-dependent (10 −7−3 × 10 −4 M) decrease in chronotropy but did not affect inotropy. STR (10 −4 M) completely inhibited the positive chronotropic, but did not affect the positive inotropic effects of (−)-S-BAY K 8644. [ 3H]Nitrendipine and [ 3H]BAY K 8644 binding to brain and cardiac membranes was enhanced by STR in a concentration-dependent manner (EC 50 8 × 10 −6 M). Scatchard analysis revealed that STR increased the affinity (decreased the K d) of [ 3H]BAY K 8644 to a greater degree than that of [ 3H]nitrendipine for dihydropyridine binding sites. STR decreased the K d of [ 3H]nitrendipine binding by increasing and decreasing the microassociation and microdissociation constants respectively. STR enhanced [ 3H]nitrendipine binding to the same extent in the cerebral cortex, striatum, hippocampus, cerebellum, brainstem and spinal cord. The enhancement of [ 3H]nitrendipine binding in brain was completely inhibited by Ca 2+ and partially inhibited by Na + in a concentration-dependent manner. Glycine (10 −2 M) did not affect the STR enhancement of [ 3H]nitrendipine binding. Extensive rinsing of brain membranes increased (∼two-fold) the affinity of STR for enhancement of [ 3H]nitrendipine binding. The effects of STR on [ 3H]nitrendipine binding both resembled and were unique to those of phencyclidine. These findings suggest that STR interacts specifically with calcium channels in brain and cardiac tissues at a site similar, but not identical to that of phencyclidine and which is distinct from the dihydropyridine calcium channel regulatory site.

3H]Nitrendipine Binding to Rabbit Colonic Smooth Muscle

We used [3H] nitrendipine binding to isolated smooth muscle cells and isometric tension studies of muscle strips to characterize the calcium channels from rabbit proximal and distal colon. At 25°C [3H] nitrendipine binding was rapid, saturable, reversible, specific, and linearly proportional to cell number. The affinity of the ligand for its receptor was similar in proximal and distal colon (KD 129 ± 21 pM and 124 ± 17 pM, respectively). In the proximal colon there were 68,000 receptors per cell, compared to 58,000 receptors per cell in the distal colon (p > .1). The Hill coefficient for nitrendipine was close to unity, suggesting binding to a single receptor. Although nitrendipine and nifedipine competitively inhibited [3H]nitrendipine binding, verapamil did not alter [3H] nitrendipine binding, suggesting the presence of at least two discrete, noninteracting sites for the binding of drugs that block calcium channels. In studies with muscle strips nitrendipine competitively inhibited isometric tension stimulated by both bethanechol and high extracellular potassium concentration. There were no significant differences in response from proximal and distal colon. These results suggest that calcium antagonist binding characteristics to calcium channels are similar in proximal and distal colon, and do not explain previously observed differences in the function of muscle in these tissues.

Nitrendipine binding in congestive heart failure due to myocardial infarction.

Depressed cardiac pump function is the hallmark of congestive heart failure, and it is suspected that decreased influx of Ca2+ into the cardiac cell is responsible for depressed contractile function. Since Ca2+ channels in the sarcolemmal membrane are considered to be an important route for the entry of Ca2+, we examined the status of Ca2+ receptors/channels in failing rat hearts after myocardial infarction of the left ventricular free wall. For this purpose, the left coronary artery was ligated and hearts were examined 4, 8, and 16 weeks later; sham-operated animals served as controls. Hemodynamic assessment revealed decreased total mechanical energy (left ventricular systolic pressure x heart rate), increased left ventricular diastolic pressure, and decreased positive and negative dP/dt in experimental animals at 4, 8, and 16 weeks. Although accumulation of ascites in the abdominal cavity was evident at 4 weeks, other clinical signs of congestive heart failure in experimental rats were evident from the presence of lung congestion and cardiac dilatation at 8 and 16 weeks after induction of myocardial infarction. The density of Ca2+ receptors/channels in crude membranes, as assessed by [3H]nitrendipine binding assay, was found to be decreased in the uninfarcted experimental left ventricle at 8 and 16 weeks; however, no change in the affinity of nitrendipine was evident. A similar depression in the specific binding of another dihydropyridine compound, [3H]PN200-110, was also evident in failing hearts. Brain and skeletal muscle crude membrane preparations, unlike those of the right ventricle and liver, revealed a decrease in Ca2+ receptors/channels density in experimental animals at 16 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hypo‐ and hyperthyroidism on binding of [3H]‐nitrendipine to myocardial and brain membranes.

The density of calcium channel binding sites as determined by [3H]-nitrendipine binding was found to be decreased in hearts of hyperthyroid rats but hardly altered by hypothyroidism. In contrast, dihydropyridine binding sites in the cerebral cortex were unaffected by dysthyroid states. Myocardial [3H]-nitrendipine binding sites were also decreased after treatment of the animals with isoprenaline but not in spontaneously hypertensive rats. These findings suggest that myocardial hypertrophy is not necessarily accompanied by a loss of calcium channels and that thyroid hormone regulates the density of [3H]-nitrendipine binding sites in a tissue-specific manner.

Calcium antagonist binding sites in failing and nonfailing human ventricular myocardium

Studies in myopathie hamsters have described an increase in calcium antagonist binding sites, which is presumably associated with an increase in the number of calcium channels. Such an abnormality might predispose the heart to further myocardial damage from calcium overload. We tested the hypothesis that calcium antagonist binding sites are increased in human idiopathic dilated cardiomyopathy by examining [ 3H]PN 200 − 110 and [ 3H]nitrendipine binding in membranes prepared from nonfailing controls and severely failing ventricles with idiopathic dilated cardiomyopathy. Despite the fact that β receptor density was decreased by 50% in failing hearts (iodocyanopindolol B max 84.4 ± 8.9 fmol/mg protein in nonfailing hearts vs 42.9 ± 3.2 fmol/mg in failing hearts, P < 0.01), dihydropyridine calcium antagonist binding sites were not reduced significantly by heart failure. Maximum binding of [ 3 H]PN 200 − 110 was 92.9 ± 19.4 fmol/mg protein in membranes derived from failing ventricles, and 93.5 ± 17.4 fmol/mg in membranes derived from nonfailing ventricles (P = NS); values for [ 3H]nitrendipine maximum binding were similar to those for [ 3H]PN 200 − 110 and also were not reduced significantly in failing ventricles. Additionally, the dissociation constants ( K spd) for [ 3H]nitrendipine and [ 3H]PN 200 − 110 were not significantly different in failing and nonfailing heart. We conclude that dihydropyridine calcium antagonist binding sites are not altered significantly in the failing human left ventricle with idiopathic dilated cardiomyopathy.

Developmental changes in β‐adrenoceptors, muscarinic cholinoceptors and Ca2+ channels in rat ventricular muscles

1. In an attempt to explain the previous electrophysiological data on the ontogeny of beta-adrenergic and muscarinic cholinergic interactions on cardiac Ca2+ current, biochemical studies were performed on the ontogeny of beta-adrenoceptors, muscarinic cholinoceptors and Ca2+ channels in cardiac muscle of developing rats: 16-20 days old foetuses, 0-20 days old neonates, and 2-3 months old adults. 2. Developmental changes in cardiac beta-adrenoceptors, muscarinic cholinoceptors, and Ca2+ channels were determined with the use of specific radioligands, [3H]-dihydroalprenolol (DNA), [3H]-quinuclidinyl benzilate (QNB), and [3H]-nitrendipine (NTD), respectively. 3. The Bmax value (fmol mg-1 tissue) for [3H]-DNA binding started to increase on post-gestation day 20, reached almost its maximum level on neonatal day 6, kept almost the same level until neonatal day 20, and then decreased slightly to its adult level. 4. The Bmax value (fmol mg-1 tissue) for [3H]-QNB binding started to increase on post-gestation day 16, reached almost its maximum level on neonatal day 0, remained almost constant until neonatal day 15, and then decreased to its adult level. 5. The Bmax value (fmol mg-1 tissue) for [3H]-NTD binding increased with age between post-gestation day 18 and neonatal day 15, stayed almost constant until neonatal day 20, and then decreased to its adult level. 6. The Kd values for [3H]-DHA, [3H]-QNB, and [3H]-NTD bindings remained almost constant during the developmental period examined. 7. Isoprenaline (Iso) increased the kx of slow action potentials (APs) from post-gestation day 18, and the adult level was reached at about 2 weeks after birth; this developmental time course is similar to that of Ca2+ channels. The number of beta-adrenoceptors also started to increase a few days before birth, but attained its peak about one week earlier than did the Pax of slow APs or the number of Ca2 + channels. 8. Acetylcholine (ACh) almost completely abolished the Iso-induced increase in m,,ax observed from postgestation day 18 to neonatal day 20; this developmental time course for the ACh effect is consistent with the finding that the number of muscarinic cholinoceptors started to increase on post-gestation day 16 and reached a peak on the day of birth. 9. Previous electrophysiological and the present biochemical findings strongly suggest that the functional coupling between muscarinic cholinoceptors and Ca2+ current is already established when the coupling between beta-adrenoceptors and Ca2 + current starts to operate in developing rat hearts.

Smooth muscle contractility and calcium channel density in hibernating and nonhibernating animals

Hibernating animals consistently survive prolonged periods of cold with body temperatures near the freezing point. Previous studies have suggested that regulation of calcium influx may be a fundamental cellular mechanism for cold tolerance in hibernating species. The present study was undertaken to compare (i) the calcium dependence of contractility and (ii) [3H]nitrendipine binding in homogenates of ileal longitudinal smooth muscle from the nonhibernating guinea pig (Cavia porcellus) and a hibernator, the ground squirrel (Spermophilus richardsonii). The contractility studies indicate that both the activation threshold for calcium and the concentration-response curve were shifted to the right in ground squirrel when compared with guinea pig. The binding site density in ground squirrel muscle was about an order of magnitude less than in guinea pig (Bmax = 10 +/- 2 (n = 12) and 86 +/- 6 fmol/mg protein (n = 5), respectively). These results indicate that ground squirrel tissues are less sensitive to external calcium and clearly have fewer calcium channels than the smooth muscle of the non-hibernator. The results continue to support the hypothesis that cold tolerance in hibernating species involves calcium homeostatic control mechanisms.

Identification of ω‐Conotoxin Binding Sites on Adrenal Medullary Membranes: Possibility of Multiple Calcium Channels in Chromaffin Cells

Binding of 125I-omega-conotoxin GVIA and [3H]nitrendipine to membranes from bovine adrenal medulla was investigated to test for the presence of N- and L-type Ca2+ channels in adrenal chromaffin cells. Saturable, high-affinity binding sites for 125I-omega-conotoxin and [3H]nitrendipine were detected in a membrane fraction from adrenal medulla. [3H]Nitrendipine binding sites were found to have a KD of 500 +/- 170 pM and a Bmax of 26 +/- 11 pmol/g of protein. 125I-omega-Conotoxin binding sites had a KD of 215 +/- 56 pM and a Bmax of 105 +/- 18 pmol/g of protein, about four times the number of sites found for [3H]nitrendipine. 125I-omega-Conotoxin binding was potently inhibited by unlabeled toxin and Ca2+ but was unaffected by dihydropyridines, verapamil, and diltiazem. [3H]Nitrendipine binding was not affected by omega-conotoxin, whereas it was inhibited by other dihydropyridines. Bay K 8644 potentiated K+-evoked cytosolic Ca2+ transients measured by fura-2 fluorescence, and this potentiation was completely blocked by nifedipine. In contrast, omega-conotoxin had no effect on Bay K 8644-evoked Ca2+ transients. Thus, the binding sites for omega-conotoxin and for nitrendipine appear to be different. The results confirm the presence of L-type Ca2+ channels and open the possibility of N-type Ca2+ channels as the omega-conotoxin binding sites in chromaffin cell membranes.

Selective inhibition of [3H]nitrendipine binding to brain and cardiac membranes by bacitracin

The effects of bacitracin were investigated on [3H]nitrendipine binding to rat brain and cardiac membranes in a low ionic strength (5 mM Tris-HCl) buffer. Bacitracin inhibited [3H]nitrendipine binding to rat brain and cardiac membranes with IC50 values of 400 +/- 100 and 4600 +/- 400 micrograms/mL, respectively. Scatchard analysis in brain membranes revealed that bacitracin inhibited [3H]nitrendipine binding primarily by reducing the Bmax but also by producing a small increase in the Kd. In brain membranes, Na+ (100 mM) and Ca2+ (2 mM) reduced the potency of bacitracin to inhibit [3H]nitrendipine binding by approximately sixfold with IC50 values of 2600 +/- 300 and 2100 +/- 400 micrograms/mL observed for bacitracin in the presence of 100 mM Na+ and 2 mM Ca2+, respectively. The EC50 values for the effects of Na+ and Ca2+ were 800 +/- 200 microM and 25 +/- 5 mM. K+, Mg2+, choline, and increasing the assay buffer of Tris-HCl to 50 mM also decreased the inhibition of [3H]nitrendipine binding by bacitracin. These results suggest that bacitracin specifically modulates [3H]nitrendipine binding in a cation-dependent manner and that brain and cardiac dihydropyridine binding sites are either biochemically different or exist in a different membrane environment.

Ligand-binding characteristics of [ 3H]QNB, [ 3H]prazosin, [ 3H]rauwolscine, [ 3H]TCP and [ 3H]nitrendipine to cerebral cortical and hippocampal membranes of senescence accelerated mouse

The senescence accelerated mouse (SAM) is known as a murine model of aging and memory dysfunction. In the cerebral cortical membranes of male 9-month-old SAM mice, the B max values of [ 3H]rauwolscine and [ 3H]nitrendipine binding, and the values of both K d and B max of [ 3H]TCP binding in the accelerated aging strain SAM-P 8 were significantly increased compared with the values in the control strain SAM-R 1 . In hippocampal membranes, however, the B max values of [ 3H]quinuclidinyl benzilate and [ 3H]nitrendipine binding were significantly decreased in SAM-P 8 compared with those in SAM-R 1 . These results suggest that muscarinic acetylcholine receptors, α 2-adrenoceptors, N- methyl- d-aspartate receptor channels and L-type Ca 2+ channels are changed in cerebral cortex and hippocampus in SAM-P 8 at 9 months.

Changes in [3H]nitrendipine binding and gamma-aminobutyric acid release in rat hippocampus following repeated morphine administration.

An antagonistic effect of calcium on the action of morphine was studied in rat hippocampal slices. The effect of repeated administration of morphine on gamma-aminobutyric acid (GABA) release and binding of [3H]nitrendipine, a calcium antagonist, was also examined. (1) In rat brain hippocampal slices, morphine enlarged the amplitude of the field potentials evoked in pyramidal neurons, disinhibiting them through basket cells. When the calcium concentration was elevated, potentiation of the field potentials by morphine was reduced. Decrease of the calcium concentration, on the other hand, enhanced the potentiating effect of morphine. Following repeated administration of morphine, its enhancing effect on the field potentials in slices was not observed. (2) In hippocampal membrane fractions obtained from rats repeatedly treated with morphine, enhancement of [3H]nitrendipine binding was observed. (3) In hippocampal slice preparations from rats receiving morphine repeatedly, K+ (45 mM)-stimulated [3H]GABA efflux was enhanced. The above results indicate that morphine antagonizes calcium, thereby reducing the release of transmitters. Furthermore, increase in calcium channels following repeated treatment of rats with morphine may explain the mechanism underlying development of tolerance.

Pharmacological effects and binding studies of new methylxanthine thioderivatives

The effects of two methylxanthine derivatives, 6-thiocaffeine (TC) and 6-thiotheophylline (TT), were investigated in different in vitro and in vivo conditions. On guinea-pig isolated trachea, both TC and TT showed a relaxant effect (EC 50 50 μ m and 60 μ m, respectively), more potent than theophylline (300 μ m). In guinea-pig isolated atria TC (30–50 μm) was able to antagonize R-PIA (a stable agonist on adenosine receptors) negative effect in not a clearly competitive way. Higher concentration (100 μ m) began to reverse that inhibitory effect. In vitro K i of TC and TT for A 1 and A 2 adenosine receptors was intermediate in comparison to caffeine and theophylline. On the contrary, the two thioderivatives showed a higher affinity for [ 3H]-nitrendipine binding sites, in comparison to the original methylxanthines. All these data suggest a complex mechanism of action, probably at the level of adenosine extracellular receptors and L-type Ca 2+ channels. In vivo experiments in mice provided evidence for a lack of CNS stimulant effects, but a loss of motor coordination was observed. Both thioderivatives showed a reduced acute toxicity. These data can be useful for the development of drugs for the therapy of asthma with reduced side effects.

Reduction of calcium channel antagonist binding sites by oxygen free radicals in rat heart

In view of the importance of Ca 2+-channels in controlling the entry of Ca 2+ into the myocardium, this study was undertaken to examine the effects of oxygen free radicals on the binding of Ca 2+-channel antagonists in rat heart by employing [ 3H]-nitrendipine as a ligand. Isolated heart membranes were incubated with xanthine + xanthine oxidase (a superoxide anion radicals generating system), hydrogen peroxide (an activated species of oxygen), or hydrogen peroxide + Fe 2+ (a hydroxyl radicals generating system). The assay of the [ 3H]-nitrendipine binding activity revealed that the maximal number of binding sites ( B max) were reduced in a time-dependent manner by superoxide radicals without any changes in the binding constant ( K d); a significant reduction of B max was seen after incubating membranes with xanthine + xanthine oxidase for a 10-min-period. Superoxide dismutase showed a protective effect on the superoxide radicals induced reduction in B max. Both hydrogen peroxide and hydroxyl radicals also depressed the B max for [ 3H]-nitrendipine binding without any significant change in K d; catalase and mannitol showed protective effects on hydrogen peroxide or hydroxyl radicals induced depression in B max, respectively. These results indicate that oxygen free radicals may reduce the number of Ca 2+-channels in the cell membrane and this change may contribute towards decreasing the voltage-dependent Ca 2+ influx in the cardiac cell.

Pharmacokinetics of nitrendipine in terminal renal failure.

The pharmacokinetics and plasma protein binding of nitrendipine in patients with terminal renal failure have been compared with those in subjects with normal renal function. Kinetic parameters were calculated after a single 40 mg oral dose, an i.v. injection of 3 mg and after a 15 mg i.v. infusion of nitrendipine. Steady-state plasma levels were determined after 5 days of oral treatment with 20 mg b.d. Pharmacokinetic parameters and steady-state plasma levels in patients with renal failure did not differ from those in subjects with normal renal function. Nitrendipine was as highly bound to plasma proteins in patients with renal failure, as in subjects with normal renal function. The plasma protein did not differ between the two. The dosage of nitrendipine need not be modified for kinetic reasons in patients with renal failure.

Sarcolemmal Ca2+ transport activities in cardiac hypertrophy caused by pressure overload

To examine Ca2+ transport activities in sarcolemmal membrane in cardiac hypertrophy caused by pressure overload, rats were subjected to aortic banding for 28 days. Heart-to-body weight ratio was increased by 46% in aortic-banded animals in comparison with the sham-operated rats. Ouabain-sensitive Na+-K+-ATPase activity in sarcolemma (SL) from hypertrophied hearts was not different from that in the control preparation. The initial rate of Na+-dependent Ca2+ uptake in SL vesicles from the hypertrophied hearts was stimulated by 53% compared with the control vesicles. ATP-dependent Ca2+ uptake and Ca2+-stimulated adenosinetriphosphatase (ATPase) activities in SL from hypertrophied hearts were increased by 35%. The number of Ca2+ channels estimated by [5-methyl-3H]nitrendipine binding was decreased by 33% in SL from hypertrophied hearts. Total and individual phospholipid contents in the hypertrophied heart preparations were not different from those in the control, except that phosphatidylcholine and phosphatidylethanolamine contents were significantly increased. Sarcolemmal preparations from hypertrophied hearts from the 22-wk-old spontaneously hypertensive rats exhibited changes in Na+-Ca2+ exchange and Ca2+-pump activities (similar to those observed in banded hearts), whereas the Na+-K+-ATPase activity decreased, [3H]nitrendipine binding increased, and phospholipid contents were not different. Thus, although differences were defined between two types of hypertrophy, these results suggest alterations in the sarcolemmal Ca2+ transport activities that may serve as an adaptive mechanism for the removal of Ca2+ from the myocardial cells during the development of cardiac hypertrophy.

Lactamimides: A novel chemical class of calcium antagonists with diltiazem-like properties

The effects of a series of lactamimides on [ 3 H]d-cis- diltiazem binding to rat brain membranes, on [ 3H]nitrendipine binding to cardiac membranes, and on calcium-induced contractions in depolarized guinea pig taenia and ileum preparations were examined. Several of the lactamimides examined displaced [ 3 H]d-cis- diltiazem binding and antagonized, in a competitive fashion, calcium-induced contractions. Over the series of lactamimides, there was a highly significant, positive linear correlation ( r = 0.87, P < 0.001) between their potency to displace [ 3 H]d-cis- diltiazem and their potency to antagonize calcium-induced contractions in the depolarized taenia and ileum preparations. Of the lactamimides examined, MDL 16,582A [ N-(2,2-diphenylpentyl)azacyclotridecan-2-imine · hydrochloride] had potency equivalent to d- cis-diltiazem with p A 2 values of 7.27 and 7.38, respectively, against calcium-induced contractions in the guinea pig ileum. These lactamimides are a novel chemical class displaying diltiazem-like calcium antagonist properties.