Binding of [3H]isradipine (PN 200-110) on smooth muscle cell membranes from different bovine arteries

The binding of [3H]isradipine [( 3H]PN 200-110), a new dihydropyridine (DHP) calcium blocker on smooth muscle cell (SMC) membranes from different bovine arteries was saturable with comparable high affinities but different binding site densities (Bmax). The data were fitted to a model that provided a common estimation for the dissociation constant (Kd = 0.46 nM, SD = 0.03) but different Bmax values. Two groups of arteries could be distinguished, large-sized with high Bmax (aorta, 149 fmol/mg, SD = 4; intrapulmonary, 134 fmol/mg, SD = 4) and medium-sized with lower Bmax (mesenteric, 67 fmol/mg, SD = 2; internal carotid, 50 fmol/mg, SD = 2; renal artery, 29 fmol/mg, SD = 2). The Kd values were similar to those previously reported, but the Bmax value on aorta SMC was higher than usually reported with other DHPs, showing that isradipine was a high full antagonist of calcium channel. Our results also suggest that the increase in arterial compliance induced by DHPs will probably be more important on large-sized arteries than on medium-sized arteries because of higher DHP binding.

Comparative binding of two closely related dihydropyridines (isradipine and darodipine) to serum proteins and erythrocytes

The binding of the two drugs isradipine and darodipine, chemically related to dihydropyridines and potent calcium channel blockers, was studied in vitro to isolated plasma proteins, erythrocytes and human serum. The two drugs were strongly bound to serum proteins (up to 97%), mainly to human serum albumin (HSA), alpha 1-glycoprotein (AAG) and lipoproteins (VLDL, LDL and HDL). Their bindings to AAG were saturable with high affinity constants (isradipine 498,000 M-1, darodipine = 155,000 M-1; n = 1). The binding of these drugs to HSA, VLDL and HDL was unsaturable, but it was saturable on LDL. In blood the drugs partitioned in erythrocytes, 16% for isradipine and 14.8% for darodipine.

Effect of the binding of isradipine and darodipine to different plasma proteins on their transfer through the rat blood-brain barrier. Drug binding to lipoproteins does not limit the transfer of drug

Brain extraction of two calcium channel antagonists, isradipine (PN 200-110) and darodipine (PY 108-068) was investigated using the carotid injection technique in rats. An inhibitor effect of binding to plasma proteins on the brain extraction was also investigated. Equilibrium dialysis at 37 degrees C showed that both drugs were highly bound to human serum proteins, including albumin, alpha-1 acid glycoprotein and lipoproteins. The free dialyzable drug fraction was inversely related to the protein concentration. The brain extraction of the drugs in the presence of either albumin or alpha-1 acid glycoprotein was inversely related to the protein concentrations in the presence of either albumin or alpha-1 acid glycoprotein, but it was higher than expected from the in vitro measurement of the dialyzable fraction. Despite a significant degree of binding to lipoproteins, no significant reduction in the brain extraction of the drugs was observed, regardless of the class or the concentration of lipoproteins. These data indicate that the amount of circulating darodipine or isradipine available for entry in a peripheral tissue such as brain exceeds the dialyzable fraction of drug. However, the in vivo exchangeable drug fraction still parallels the dialyzable fraction, except if the drug is lipoprotein bound.

Binding of (+)-PN 200-110 to rat pituitaries and to normal and adenomatous human pituitaries

Endocrine cells possess voltage-sensitive Ca2+ channels involved in the modulation of hormonal secretion. Using the dihydropyridine, (+)-PN 200-110, we have investigated the binding characteristics of this ligand to pituitary membrane Ca2+ channels from normal rat, normal and adenomatous human pituitaries. [3H]PN 200-110 binds specifically to rat pituitary membranes to one class of sites (Kd = 0.41 +/- 0.10 mM; Bmax = 39 +/- 1.3 fmol/mg protein). At 37 degrees C, equilibrium is reached in 45 min and half-life of the binding is 13 min. No significant changes were observed for either the Kd or Bmax values between normal rat and human pituitaries or between the different types of adenomas (GH- and PRL-secreting and non-secreting). As the secretory activity of the pituitary adenomas, involving Ca2+ mobilization, varies from one adenoma to another, our results could indicate that, if there is a modified regulation of Ca2+ entry in the adenomas, it may not be related to a varying number of calcium channels, at least the channels labeled by the dihydropyridine (+)-PN 200-110.

Phenobarbital-mediated modulation of gene expression in rat liver. Analysis of cDNA clones

Phenobarbital evokes a pleiotypic response in the liver characterized by cell hypertrophy and mono-oxygenase induction. These phenomena arise through complex modulation mechanisms changing the pattern of protein synthesis, distinct from those triggered by other well known inducers, like steroid hormones or polycyclic hydrocarbons. To investigate the mechanisms involved in regulating the expression of the phenobarbital-inducible tissue-specific genes, we constructed two libraries of recombinant bacterial plasmids pBR322 in Escherichia coli. Each library contains cDNA copies of polysomal poly(A)-rich RNA obtained from control and 16-h phenobarbital-induced rat liver. A thousand cloned sequences from each library were screened by double-cross colony hybridization using [32P]cDNA prepared from homologous and heterologous poly(A)-rich RNAs as the probes. The statistical analysis of the results revealed that phenobarbital treatment significantly changes the relative abundance of different polysomal mRNA classes in rat liver. Clones corresponding to mRNAs clearly induced following phenobarbital treatment have been further selected by plasmid DNA dot hybridization, and used as probes for measuring the changes in each mRNA concentration in the whole cell and in the polysomal RNAs from rat livers, at different times after phenobarbital treatment. The fact that changes in the concentration of each specific mRNA in the polysomes does not parallel the variation of its total concentration in the cell indicates that the induced modulation of protein synthesis in the liver is brought about by mechanisms involving both transcriptional and translational regulation, since besides the increases in whole cellular mRNA concentration a marked mobilization of mRNA into active polysomes could be demonstrated during the onset of the adaptive response to phenobarbital.

Molecular cloning of cDNA for rat L-type pyruvate kinase and aldolase B

Two double-stranded cDNA recombinant pBR322 plasmid libraries were constructed starting from high carbohydrate diet rat liver poly(A)+ mRNA, either fractionated by denaturing sucrose gradient centrifugation for the cloning of L-type pyruvate kinase cDNA, or nonfractionated for aldolase B. Both libraries were screened with single-stranded cDNA probes reverse transcribed from fasted or high carbohydrate diet rat liver mRNAs. mRNAs from fasted animals were also fractionated by sucrose gradient centrifugation and mRNAs from the fed animals were, in addition, further purified by high performance liquid gel filtration chromatography. Those clones hybridizing with the "positive" probe (from animals fed the high carbohydrate diet) and not with the "negative" one (from fasted animals) were preselected and their plasmid DNA was purified and analyzed by positive hybridization-selection. Thirty of 4500 bacteria colonies transformed by recombinant plasmids were preselected by differential screening for pyruvate kinase, and 8 of 864 colonies for aldolase B. Twenty-two recombinant plasmids for pyruvate kinase and two for aldolase B were shown to contain specific cDNA inserts by positive hybridization-selection. Plasmids DNAs of some pyruvate kinase and aldolase B clones (whose inserts ranged from 700 to 1050 bases in length) were labeled by nick translation and used as probes for Northern blot hybridization. The pyruvate kinase cDNA probes recognized mainly a 3400-base RNA species which was detected in high carbohydrate diet rat liver, but not in fasted rat liver and in tissues which do not synthesize L-type pyruvate kinase. In addition, some pyruvate kinase probes hybridized with minor RNA species of about 2000 bases in length, only observed after carbohydrate diet. For aldolase B, the recombinant plasmid DNA hybridized with a single RNA species of 1750 bases. This RNA, detected in kidney, small intestine and liver, was induced by a high carbohydrate diet and increased with liver development. The rat probe cross-hybridized with human aldolase B messenger RNA.

Altered PGE2 and PGF2 alpha production by glomeruli and papilla of sodium-depleted and sodium-loaded rats

Prostaglandin (PG) E2 and F2 alpha synthesis by isolated glomeruli and papillary homogenates prepared from control, salt-loaded, and salt-depleted rats was measured in vitro with and without added arachidonic acid using specific radioimmunoassays. Glomeruli from salt-depleted rats synthesized less PGE2 and more PGF2 alpha than glomeruli from control rats under both conditions. The effect of sodium restriction could be attributed to stimulation of glomerular 9-keto-PGE2 reductase activity unrelated to a change in the concentration of this enzyme. High salt diet had no effect on PG synthesis by glomeruli. Papillary homogenates prepared from salt-loaded rats synthesized more PGE2 than those from control rats both with and without added arachidonic acid. This finding suggests an effect of high salt diet at a stage further than phospholipid deacylation. Low salt diet had no effect on PG synthesis by papillary homogenates. The physiological control of PG synthesis in response to changes in the NaCl content of the diet is, therefore, different for the glomeruli and the papilla.