Effect of age on the prejunctional α-adrenoceptor-mediated feedback in the heart of spontaneously hypertensive rats and normotensive Wistar Kyoto rats

Summary The prejunctional ₂-adrenoceptor-mediated feed-back in the heart of pithed young and adult spontaneously hypertensive rats (SHR) and corresponding normotensive Wistar Kyoto rats (WKY) was studied. After electrical stimulation of the sympathetic outflow from the spinal cord to the heart, B-HT 920 induced an inhibition of the cardiac response, which was significant at stimulation frequencies up to 1 Hz in young SHR and WKY and up to 2 Hz in the adult animals. Rauwolscine produced a potentiation of the cardiac response to electrical stimulation in SHR, which was significant from 0.2–10 Hz in young SHR and from 0.1–10 Hz in adult SHR. In young WKY, rauwolscine did not potentiate the increase in heart rate to sympathetic nerve stimulation, whereas in adult WKY ₂-adrenoceptor blockade by rauwolscine significantly potentiated the cardiac response to electrical stimulation at frequencies in the range of 0.2–10 Hz. In SHR the potentiation of the cardiac response to sympathetic nerve stimulation by rauwolscine was much stronger than in WKY.These results suggest that in adult animals the prejunctional ₂-adrenoceptor mediated feedback is more developed than in young rats. In contrast with young WKY, a significant endogenous feedback can be demonstrated in adult WKY. In SHR, however, the physiological role of prejunctional ₂-adrenoceptors is much more important.     

Exercise and heart disease. Epidemiology of the "exercise hypothesis"

The "exercise hypothesis" states that exercise protects against coronary heart disease. Reviewed herein is the epidemiologic evidence for and against the "exercise hypothesis." The weight of evidence supports the view that exercisers have a lower risk of coronary disease, but that vigorous exercise cannot always prevent progression of coronary atherosclerosis and does increase the risk of sudden death in persons with advanced coronary atherosclerosis. It is concluded that the "exercise hypothesis" is plausible, even likely, but still unproved.     

Health for all Nigerians: some revolutionary management dimensions

Health For All by the Year 2000, usually referred to as HFA/2000, is a goal that has been accepted by most nations of the world. PHC is the strategy designed to achieve this laudable goal. In Nigeria, there is no doubt that remarkable strides have been made to achieve the goal of HFA/2000 in recent years. However, in order to fully realise this goal, some further revolutionary management steps are needed. This article first examines the progress on HFA made so far in Nigeria; and then, discusses some management initiatives that need to be taken or accelerated to be able to reach that desirable destination.     

Effects of α2-adrenoceptor agonists on locus coeruleus firing rate and brain noradrenaline turnover in N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)-treated rats

Summary Previous studies have shown that a low dose of the alkylating compound N-ethoxycarbonyl-2-ethoxy1,2-dihydroquinoline (EEDQ) reduces the density of ₂-adrenoceptors in rat cerebral cortex and antagonizes the effects of an ₂-adrenoceptor agonist on noradrenaline release in rat cortical slices. In the present study, a corresponding dose of EEDQ (1 mg/kg, s. c., 24 h) was shown to reduce the effect of the ₂-adrenoceptor agonists clonidine and guanfacine on noradrenaline turnover in rat brain while not affecting the inhibitory effect of clonidine on locus coeruleus (LC) cell firing. When considerably higher doses of EEDQ were administered (10 and 20 mg/kg, s. c., 24 h) not only the biochemical but also the electrophysiological effects of clonidine were markedly reduced (or even reversed). The data support the notion that EEDQ decreases the responsiveness of brain ₂-adrenergic receptors; moreover, they indicate that ₂-adrenoceptors regulating LC activity are characterized by a larger receptor reserve or are less sensitive to the influence of alkylation than are the population of ₂-adrenoceptors regulating noradrenaline utilization. Send offprint requests to G. Engberg, at the above address     

Pre- and postsynaptic dopaminergic activities of U-86170F

Summary The biochemical, endocrine, receptor binding, and behavioral effects of the putative dopamine autoreceptor agonist, U-86170F, were evaluated in various in vivo and in vitro models. U-86170F and apomorphine were shown to cause a significant reversal of the effects of -butyrolactone (GBL) on dopamine accumulation in mouse striata. In contrast to apomorphine, U-86170F had a ceiling effect on the extent of the reversal of GBL effects (55%), whereas apomorphine had an 82% reversal. The effect on striatal homovanillic acid (HVA) levels was also monitored, and both compounds exerted a similar and significant reduction in striatal HVA. A comparison was made between the effects of intraperitoneal (i.p.) and oral administration of U-86170F in the -methyl-p-tyrosine (-MPT)/prolactin model in rats. When administered by the i.p. route, U-86170F suppressed the effects of -MPT on prolactin level increase, having an ED₅₀ of about 0.03 mg/kg, and when administered by the oral route, its ED₅₀ was approximately 0.1 mg/kg. U-86170F has been shown to be a potent dopamine autoreceptor agonist in the GBL, prolactin, and HVA models, with an effective i.p. dose of approximately 0.03 mg/kg. When evaluated for postsynaptic dopaminergic activity in the reserpinized mouse model, and compared to apomorphine, U-86170F was found to increase locomotor activity, but its maximum effect was only 65% of that attained with apomorphine. Higher doses were needed for postsynaptic effects.In receptor binding studies using cloned D2 receptor preparations, U-86170F was found to exhibit agonist binding properties similar to dopamine as demonstrated by their inhibition of 3H-raclopride binding. Both compounds exhibited biphasic inhibition curves, with U-86170F having K_i values of 7.5 nM and 250 nM, and for dopamine the K_i values were 34.7 nM and 1031 nM. Binding studies conducted in the presence of GTP yielded only one site with Kis of 289 nM and 670 nM, for both U-86170F and dopamine, respectively.The results presented in this report demonstrated that U-86170F is a potent dopamine autoreceptor agonist, with limited activity at the postsynaptic receptor. Send offprint requests to R.A. Lahti at the above address     

Unlabeled hemoglobin adducts of 4,4′-methylenebis (2-chloroaniline) in rats and guinea pigs

The capacity of N-oxidized metabolites of 4,4-methylenebis(2-chloroaniline) (MBOCA) to form hemoglobin (Hb) adducts was determined in vitro, and the formation of Hb adducts following in vivo administration of MBOCA was assessed with or without prior induction of cytochrome P-450 enzymes with phenobarbital or -naphthoflavone. Hb adduct formation was determined by electron-capture GLC of MBOCA as the heptafluorobutyryl derivative following mild acid hydrolysis of protein-bound MBOCA. The method was confirmed by gas chromatography-mass spectrometry with selected ion monitoring. N-hydroxy- and mononitroso-MBOCA, but not MBOCA itself, formed adducts to rat and human Hb in vitro in a dose-related manner. Binding was inhibited by cysteine and glutathione but not oxidized glutathione or methionine. Intravenous administration of as little as 0.04 mol/kg N-hydroxy-MBOCA to rats resulted in measurable formation of MBOCA-Hb adducts (0.9 ng/50 mg Hb). Intraperitoneal administration of 0.5–50 mg/kg MBOCA to rats, and subcutaneous administration of 5–500 mg/kg MBOCA to rats and 4–100 mg/kg to guinea pigs resulted in dose-related formation of Hb adducts. MBOCA-Hb remained elevated in blood for greater than 10 weeks following a single subcutaneous dose in guinea pigs. Pretreatment of rats with phenobarbital induced microsomal benzphetamine N-demethylase (BND) activity and resulted in a small increase in in vitro N- andortho- hydroxylation of MBOCA, but did not increase in vivo Hb adducts. Pretreatment of rats with -naphthoflavone induced microsomal aryl hydrocarbon hydroxylase as well as ethoxyresorufin-O-deethylase, and increased in vitro N- but notortho-hydroxylation of MBOCA. -Naphthoflavone pretreatment increased the formation of MBOCA-Hb adducts when rats were dosed with MBOCA at 100 and 500 mg/kg, but not 20 mg/kg subcutaneously.     

Inhibition of Ca2+ transport associated with cAMP-dependent protein phosphorylation in rat cardiac sarcoplasmic reticulum by triorganotins

Organotin compounds have been shown to interfere with cardiovascular system. We have studied the in vitro and in vivo effects of tributyltin bromide (TBT), triethyltin bromide (TET) and trimethyltin chloride (TMT) on the cardiac SR Ca²⁺ pump, as well as on protein phosphorylation of SR proteins, in order to understand the relative potency of these tin compounds. All the three tin compounds inhibited cardiac SR⁴⁵Ca uptake and Ca²⁺-ATPase in vitro in a concentration-dependent manner. The order of potency for Ca²⁺-ATPase as determined by IC₅₀, is TBT (2 M) > TET (63 M) > TMT (280 M). For⁴⁵Ca uptake, it followed the same order i.e., TBT (0.35 M) > TET (10 M) > TMT (440 M). In agreement with the in vitro results, both SR Ca²⁺-ATPase and⁴⁵Ca uptake were significantly inhibited in rats treated with these tin compounds, indicating that these tin compounds inhibit cardiac SR Ca²⁺ transport. cAMP significantly elevated (70–80%) the³²P-binding to SR proteins in vitro in the absence of any organotin. In the presence of organotins, cAMP-stimulated³²P-binding to proteins was significantly reduced, but the decrease was concentration dependent only at lower concentrations. The order of potency is TBT > TET > TMT. In agreement with in vitro studies, cAMP-dependent³²P bound to proteins was significantly reduced in rats treated with TBT, TET and TMT. SDS-polyacrylamide gel electrophoresis of the cardiac SR revealed at least 30 Coomassie blue stainable bands ranging from 9 to 120 kDa. Autoradiographs from samples incubated in the presence of cAMP indicated³²P incorporation in seven bands. Of these, the band corresponding to about 24 kDa molecular weight protein decreased in its intensity with the treatment of organotins. These results suggest that triorganotins may be affecting Ca²⁺ pumping mechanisms through the alteration of phosphorylation of specific proteins in rat cardiac SR.This work has been presented in part at the Annual meeting of Society of Toxicology, 1990 at Miami Beach, FL. The Toxicologist 10: 35 & 108 (1990).     

Prodrugs of Peptides. 9. Bioreversible N-α-Hydroxyalkylation of the Peptide Bond to Effect Protection Against Carboxypeptidase or Other Proteolytic Enzymes

Various N--hydroxyalkyl derivatives of N-acyl amino acids and di- and tripeptides were prepared by hydrolysis or aminolysis of N-acyl 5-oxazolidinones. The stability of these derivatives was studied in aqueous solution as a function of pH. The compounds were all degraded quantitatively to their parent N-acylated amino acid or peptide and aldehyde but with vastly different rates. At pH 7.4 and 37°C the half-lives of decomposition ranged from 4 min to 1500 hr. The structural factors influencing the stability included both steric and polar effects within the acyl and N--hydroxyalkyl moieties as well as within the amino acid attached to the N--hydroxyalkylated N-acyl amino acid. Whereas the N-benzyloxycarbonyl (Z) derivatives of the dipeptides Gly-L-Leu and Gly-L-Ala were readily hydrolyzed by carboxypeptidase A, the N-hydroxymethylated compounds, i.e., Z-Gly(CH₂OH)-Leu and Z-Gly(CH₂OH)-Ala, were resistant to cleavage by the enzyme as revealed by their similar rates of decomposition in the presence or absence of the enzyme at pH 7.4 and 37°C. The results suggest that N--hydroxyalkylation of a peptide bond protects not only this bond but also an adjacent peptide bond against proteolytic cleavage. Since the N--hydroxyalkyl derivatives are readily bioreversible, undergoing spontaneous hydrolysis at physiological pH, this prodrug approach promises to overcome the enzymatic barrier to absorption of various peptides.     

Solubilization and Stabilization of a Benzylpenicillin Chemical Delivery System by 2-Hydroxypropyl-β-cyclodextrin

A dihydropyridine pyridinium salt redox carrier-based chemical delivery system for benzylpenicillin (1) was complexed with 2-hydroxypropyl--cyclodextrin (HPCD). The solubility of the lipophilic 1, which is incompatible with aqueous formulations, was dramatically increased and showed a linear dependency on the HPCD concentration. The degree of incorporation was 20 mg of 1 per g of complex. The stability study of 1 in various pH buffers indicated the base-catalyzed hydrolysis of the acyloxyalkyl linkage and the hydration of the 5,6 double bond of the dihydropyridine as the main degradation processes. The overall loss of 1, which follows first-order kinetics, was not influenced by changes in ionic strength and elimination of oxygen from the reaction medium. The HPCD complex of 1, which has a stability constant of 720–940 M ^–1, stabilized the chemical delivery system. The influence of the temperature on the stability of 1 is also discussed.     

2-Nitroimidazole potentiation of nitrosourea induced cytotoxicity in subcutaneous implants of rat 9L brain tumor cells

Summary To determine if the 2-nitroimidazole (2-NI) and the nitrosourea (NU) in a brain tumor chemopotentiation trial should be selected on the basis of known structure-activity relationships (electron affinity, lipophilicity, alkylating activity, carbamoylating activity), s.c. implants of rat 9L brain tumor cells were treated with combinations of misonidazole (MISO) or etanidazole (SR-2508) administered under oxic and hypoxic conditions, and BCNU, CCNU or chlorozotocin (CLZ) administered under oxic conditions. Cell kill was assessed by an in vivo to in vitro colony formation assay. To mimic the preincubation effect, the 2-NI was injected i.p., and 30min later the tumor was clamped. After 2hr, the clamp was released, and the NU administered immediately. MISO (2.5 mmole/kg) and SR-2508 (3.75 mmole/kg) reached the same peak tumor concentration in 30 min. Both 2-NIs were metabolized at the same rate in the clamped tumors; however, metabolism of the 2-NIs by hypoxic cells over the 2hr clamping period did not produce any measurable s.c. 9L cell kill. The relative effectiveness of the NUs for killing oxic s.c. 9L tumor cells was: BCNU > CCNU > CLZ. Clamping the tumor prior to NU administration did not change the NU cytotoxicity. No potentiation of the NU cytotoxicity by the 2-NIs was observed in oxic tumors. Although metabolism of MISO by hypoxic cells did not result in potentiation of CLZ cytotoxicity at any dose, it resulted in potentiation of BCNU cytotoxicity at all doses and CCNU cytotoxicity at high doses. Metabolism of SR-2508 by hypoxic cells did not result in potentiation of BCNU, CCNU or CLZ cytotoxicity at any dose. These in situ data indicate that, 1) MISO is superior to SR-2508 for potentiating NU cytotoxicity, and 2) the NU in a brain tumor chemopotentiation trial should be selected on the basis that it is the most effective potentiable NU against a particular type of brain tumor.